Comparison of modeling calculations with measured brine compositions during titration to higher pcH indicate additional data are required to improve the modeling of magnesium and borate concentrations in WIPP brines at pcH 9.5 and greater
       DOE calculated organic ligand concentrations as a function of the brine volume and used these concentrations to calculate baseline actinide solubilities. This approach accounts for possible dilution of the organic ligands in larger brine volumes and is adequately justified. This change in calculation of the actinide solubilities mainly affects the dissolved concentrations of +III actinides
       U(VI) concentrations in WIPP brines continue to be fixed at 1.0 x10[-3] M and no evidence has been presented that would justify changing this concentration
       The EQ3/6 geochemical modeling code has replaced the FMT code for solubility calculations, and DOE converted the FMT database for use with the EQ3/6 code. DOE added the solubility data for phase 5, a Mg-chloride-hydroxide-hydrate solid phase that forms in Salado brines to create the database DATA0.FM1, which was used for the CRA-2014 PA actinide solubility calculations
       During the CRA-2009 review, EPA expected DOE to update the +III actinide solubility data in the WIPP thermodynamic database to account for the results of Neck et al. (2009) and to update the Th(IV)-carbonate speciation in the database to account for data from Altmaier et al. (2005, 2006). DOE did not update the EQ3/6 thermodynamic database to account for these data showing the thermodynamic database continues to be deficient
       In response to EPA comments regarding recent data for borate aqueous speciation, hydromagnesite solubility and EDTA aqueous speciation, DOE created an updated datablase (DATA0.FM2) that included changes to these data as well as changes to citrate, magnesium-sulfate, and lead aqueous speciation and solubility data. EPA's preliminary review indicates more work will need to be performed on this database prior to the Agency's acceptance. Additionally, EPA's acceptance of this new database will require a thorough review of the data.
       The relatively low hydromagnesite solubility in the revised DATA0.FM2 database is inconsistent with literature data. EPA would like this value revised back to the Robie and Hemingway (1973) solubility used in previous WIPP actinide solubility calculations
       EPA found inconsistencies in DOE's selection of +III and +IV actinide solubility data used to calculate the CRA-2014 PA actinide solubility uncertainty distributions. EPA developed a defensible set of criteria for selecting data for the actinide solubility uncertainty distribution calculations and used these criteria to identify appropriate +III and +IV actinide solubility data for these calculations. Recalculating the +III and +IV actinide solubility data using the revised data set had significant effects on the uncertainty distributions and would increase mean and median +III actinide solubilities and decrease mean and median +IV actinide solubilities relative to the concentrations used in the CRA-2014 PA.

Colloidal Actinide Source Term

The Colloidal Actinide Source Term conceptual model was peer reviewed prior to the CCA PA and PAVT and found to be adequate, if somewhat conservative (Wilson et al. 1996a). This conceptual model and its implementation were unchanged for the CCA PAVT, PABC04 and PABC09. The colloidal actinide source term was unchanged from the CCA PA until the CRA-2014 PA.
Four types of colloids can form in the WIPP repository and influence the Actinide Source Term:
 Microbial colloids
 Humic colloids
 Intrinsic colloids
 Mineral fragment colloids
DOE described these four types of colloids in the CCA (DOE 1996, Appendix SOTERM, Section 6.1.2). Microbial colloids may act as substrates for actinide sorption, or they may intracellularly bioaccumulate actinides. Humic substances are relatively small organic compounds (less than 100,000 atomic mass units) and are often powerful substrates for uptake of metal cations. Intrinsic colloids are hydrophilic actinide macromolecules that may in some cases mature into larger hydrophobic mineral fragment colloids. Mineral fragment colloids are hydrophobic, hard-sphere amorphous or crystalline solids. Mineral fragment colloids may act as substrates for actinide sorption, or may consist of precipitated or coprecipitated actinide solids.
The concentrations of mineral fragment colloids and actinide intrinsic colloids used in WIPP PA are constant values (Table 7-1) and are represented by the parameters CONCMIN and CONCINT, respectively. Actinide concentrations associated with humic substances are calculated by multiplying the dissolved actinide concentration by a proportionality constant (parameters PHUMSIM for Salado brine and PHUMCIM for Castile brine), with the total humic colloid actinide concentration limited by a maximum value (parameter CAPHUM, Table 7-2). Actinide concentrations associated with microbial colloids are also based on proportionality constants (parameter PROPMIC), with assigned upper limits on the actinide concentrations associated with microbial colloids (parameter CAPMIC, Table 7-3).
Mineral Fragment Colloids

DOE (1996) did not explicitly state a size range for mineral fragment colloids, but did assume a diameter range of 1 nm to 1 μm during particle surface area calculations used to derive the mineral fragment concentrations used in PA (DOE 1996, Appendix SOTERM, Section <IP_ADDRESS>). Mineral fragment colloid parameters(CONCMIN) have not changed since the original CCA.
The concentrations of actinides associated with mineral fragment colloids, also called pseudocolloids, were determined for the CCA PA from experiments that measured their kinetic stability in WIPP-relevant brines and estimates of adsorption site densities. These experiments were carried out using a variety of minerals or materials (Papenguth 1996). The actinide concentrations from mineral fragment colloids developed for the CCA PA have been used in all WIPP PAs (Table 7-1). At the time the experiments were designed, MgO backfill was not included in the repository design, so formation of mineral fragment colloids from MgO and its reaction products was not investigated. A triangular distribution spanning one order of magnitude about the geometric mean was developed for mineral fragment colloid concentrations, but this distribution was not sampled for PA. Instead, the maximum parameter values were used (DOE 1996, Appendix SOTERM, Section <IP_ADDRESS>).
Table 7-1.	Actinide Concentrations Associated with Mineral Fragment and Intrinsic Colloids
Actinide
                        Mineral Fragment Colloids (M)
                              (Parameter CONCMIN)
                  Intrinsic Colloids (M) (Parameter CONCINT)

             CCA PAVT, PABC04, PABC09, CRA-2014 PA and CRA14_SEN4
                          CCA PAVT, PABC04 and PABC09
                          CRA-2014 PA and CRA14_SEN4
Th(IV)
                                  2.6  10[-8]
                                      0.0
                                   2  10[-8]
U(IV)
                                  2.6  10[-8]
                                      0.0
                                   2  10[-8]
U(VI)
                                  2.6  10[-8]
                                      0.0
                                   3  10[-8]
Np(IV)
                                  2.6  10[-8]
                                      0.0
                                   2  10[-8]
Np(V)
                                  2.6  10[-8]
                                      0.0
                                Not determined
Pu(III)
                                  2.6  10[-8]
                                      0.0
                                   2  10[-8]
Pu(IV)
                                  2.6  10[-8]
                                  1.0  10[-9]
                                   2  10[-8]
Am(III)
                                  2.6  10[-8]
                                      0.0
                                   4  10[-9]
          Source: DOE 1996, Appendix SOTERM, Table SOTERM-14 and DOE 2014b, Table SOTERM-21

Table 7-2.	Proportionality Constants and Maximum Concentrations for Humic  Colloids Used in All WIPP PAs
Actinide
Proportionality Constant Humic Colloids in Salado Brine[a] (Parameter PHUMSIM)
Proportionality Constant Humic Colloids in Castile Brine[b] (Parameter PHUMCIM)
                Maximum Sorbed on Humics (M) (Parameter CAPHUM)
Th(IV)
                                      6.3
                                      6.3
                                  1.1  10[-5]
U(IV)
                                      6.3
                                      6.3
                                  1.1  10[-5]
U(VI)
                                     0.12
                                     0.51
                                  1.1  10[-5]
Np(IV)
                                      6.3
                                      6.3
                                  1.1  10[-5]
Np(V)
                                  9.1  10[-4]
                                  7.4  10[-3]
                                  1.1  10[-5]
Pu(III)
                                    0.19[d]
                                    1.1[d]
                                  1.1  10[-5]
Pu(IV)
                                      6.3
                                      6.3
                                  1.1  10[-5]
Am(III)
                                    0.19[d]
                                    1.1[d]
                                  1.1  10[-5]
a  -  	Units of moles colloidal actinide per mole dissolved actinide
b  -  	Units of moles colloidal actinide per mole dissolved actinide
c  -  	Units of moles total mobile actinide per liter
d  -  	For Salado brine, the maximum value in the distribution is used for PA; for Castile brine, a cumulative distribution from 0.065 to 1.60 with a mean value of 1.1 and a median of 1.37 is sampled for PA
e  -  	Not applicable
Source: DOE 2014b, Appendix SOTERM-2014, Table SOTERM-21

Table 7-3.	Proportionality Constants and Maximum Concentrations for Microbial Colloids

           Proportionality Constant Microbes[a] (Parameter PROPMIC)
                    Maximum Sorbed by Microbes[c] (CAPMIC)

                          CCA PAVT, PABC04 and PABC09
                          CRA-2014 PA and CRA14_SEN4
                          CCA PAVT, PABC04 and PABC09
                          CRA-2014 PA and CRA14_SEN4
Th(IV)
                                      3.1
                                     1.76
                                    0.0019
                                  2.3  10[-6]
U(IV)
                                    0.0021
                                     1.76
                                    0.0021
                                  2.3  10[-6]
U(VI)
                                    0.0021
                                     1.76
                                    0.0023
                                  2.3  10[-6]
Np(IV)
                                     12.0
                                     1.76
                                    0.0027
                                  2.3  10[-6]
Np(V)
                                     12.0
                                     1.76
                                    0.0027
                                  2.3  10[-6]
Pu(III)
                                      0.3
                                     1.76
                                  6.8  10[-5]
                                  2.3  10[-6]
Pu(IV)
                                      0.3
                                     1.76
                                  6.8  10[-5]
                                  2.3  10[-6]
Am(III)
                                      3.6
                                     0.32
                                     NA[e]
                                  3.1  10[-8]
a  -  	Units of moles colloidal actinide per mole dissolved actinide
b  -  	Units of moles colloidal actinide per mole dissolved actinide
c  -  	Units of moles total mobile actinide per liter
d  -  	For Salado brine, the maximum value in the distribution is used for PA; for Castile brine, a cumulative distribution from 0.065 to 1.60 with a mean value of 1.1 and a median of 1.37 is sampled for PA
e  -  	Not applicable
Source: DOE 1996, Appendix SOTERM, Table SOTERM-14 and DOE 2014b, Appendix SOTERM-2014, Table SOTERM-21
EPA (2010b) found that the potential formation of mineral fragment colloids by MgO and its hydration and carbonation products under WIPP-relevant conditions had not been adequately evaluated and recommended that DOE investigate the possible formation of mineral fragment colloids by minerals in the MgO backfill. For example, Altmaier et al. (2004) found that mineral fragment colloids formed in MgCl2 solutions with ionic strengths from 2.5 to 4.5 M may mobilize thorium in brines suggesting actinides can form colloids with Mg. During review of the CRA-2009, EPA (2010b) determined that the colloids reported by Altmaier et al. (2004) are unlikely to form in the repository because the korshunovskite solid observed in the experiments does not form in WIPP brines.
DOE (Xiong and Kim 2011) described an investigative approach for determining the possible formation of mineral fragment colloids from MgO under WIPP repository conditions. Experiments were planned in GWB or simplified GWB, because a Mg-chloride-hydroxide-hydrate solid (phase 5) has been observed to precipitate from GWB but not from ERDA-6. Samples from long-term MgO hydration and carbonation experiments performed in GWB (Deng et al. 2009) were to be analyzed by filtration from 220 nm to 10 nm. The filtrates would be analyzed for Mg to determine if Mg-bearing colloids were present. Additional experiments were planned in 1.0 M and 1.65 M MgCl2 with Martin Marrietta MgO and 0  -  1.0 M NaCl to assess the role of Na[+] on the stabilization of phase 5. DOE did not report the results of these experiments. DOE (Reed et al. 2013) considered the magnesium colloid effects reported by Altmaier et al. (2004) and stated that they were likely transient. However, Altmaier has reiterated that there was strong evidence that Th(IV) or perhaps Th(IV) colloid was strongly sorbed on the korshunovskite solid phase (Sassani 2013). Although formation of the Mg colloids observed by Altmaier et al. (2004) remains unlikely because of differences in the brines and Mg chloride-hydroxide-hydrate solid phases between their experiments and WIPP brines, whether MgO reaction products in WIPP brines might form mineral fragment colloids capable of sorbing actinides remains unresolved. It would be appropriate for DOE to address this issue by performing experiments with WIPP brines and MgO.
DOE (Reed et al. 2013) reported the results of long-term experiments of Pu(VI) reduction in WIPP brine by iron coupons or magnetite. Reed et al. (2013) defined dissolved plutonium as the concentration passing through a 2.5 nm filter, intrinsic colloid plutonium as the concentration between 2.5 nm and 10 nm, and mineral colloid plutonium as greater than 10 nm material. After approximately six years of reaction, filtration of the brines showed that in pcH 9.1  -  9.5 ERDA-6 brine, the mineral-colloid Pu concentration ranged from 8.6 x 10[-9] M to 2.3 x 10[-8] M. Because this concentration range was bounded by the 2.6 x 10[-8] M concentration originally established for the CCA PAVT (Table 7-1), the value of CONCMIN remained unchanged for the CRA-2014 PA.
The current value of parameter CONCMIN remains consistent with the available data. However, EPA believes DOE will strengthen this parameter by addressing uncertainty regarding possible formation of mineral fragment colloids from MgO under WIPP repository conditions .
Intrinsic Colloids

The concentrations of intrinsic Pu(IV) colloids were determined for the CCA PAVT based on the results of experimental investigations carried out with Pu(IV) (DOE 1996 Appendix SOTERM Section <IP_ADDRESS>, Papenguth and Behl 1996). Intrinsic Pu(IV) colloids were detected in NaCl brines at pcH values of approximately 4 and 7, but at pcH values of approximately 8 and higher, intrinsic Pu(IV) colloid concentrations in the experiments were below the analytical detection limit of 10[-9] M (DOE 1996, Appendix SOTERM Section <IP_ADDRESS>). Because the expected pcH of the repository was approximately 9.3, the concentration of intrinsic Pu(IV) colloids used in the CCA PAVT was set equal to the analytical detection limit (Table 7-1). A literature review did not provide evidence of the formation of other actinide intrinsic colloids, so the concentrations of thorium, uranium, neptunium, Pu(III) and americium intrinsic colloids were set equal to zero (DOE 1996, Appendix SOTERM, Section <IP_ADDRESS>).
DOE has updated the intrinsic colloid concentrations (parameter CONCINT) using data from recent investigations. EPA has identified issues with the pcH ranges considered by DOE. Because of the narrow pcH ranges DOE used to develop the CONCINT parameters, many of the parameter values used in the CRA-2014 PA do not represent upper limiting values. The investigations used to develop CONCINT parameter values for Th(IV), Nd(III) (an analogue for Am(III)), Pu(III), and U(VI) are considered below.
Reed et al. (2013) examined the results of sequential filtration of samples from Nd(III), Pu(III), Th(IV) and U(VI) solubility experiments carried out in WIPP brines. Using filtration data with a size range of 2 nm to 10 nm for intrinsic colloids and a pcH range from 9 to 10, Reed et al. (2013) established revised intrinsic colloid concentrations that were used in the CRA-2014 PA and CRA14_SEN4 PA (Table 7-1). EPA noted that these concentrations did not appear to be bounding and requested that DOE demonstrate that these parameters were upper limiting values based on the available data (Edwards 2015c, Comment 3-C-9, Economy 2016).
Recent investigations have identified the formation of intrinsic Th(IV) colloids in dilute solutions and in brines. Neck et al. (2002) found that intrinsic Th(IV) colloids were present at low ionic strength based on ultracentrifugation of sampled solutions. Neck et al. (2002) determined that previous investigations of amorphous Th(IV) hydrous oxide solubility had established higher log Ksp[0] values because of incomplete separation of these colloids from solution. Bitea et al. (2003) investigated the formation and stability of Th(IV) colloids in 0.5 M NaCl and found that these colloids remained stable over the course of 400 days without exhibiting agglomeration or precipitation. Results reported by Altmaier et al. (2004) indicated that intrinsic Th(IV) colloids can form and remain stable at high ionic strength, with total mobilized Th(IV) concentrations (dissolved plus intrinsic colloids) of approximately 10[-6.3] M. EPA (2010b) reviewed the literature data, found the evidence of Th(IV) intrinsic colloids to be credible and recommended that DOE revise future calculations of Th(IV) intrinsic colloid concentrations.
Reed et al. (2013) selected Th(IV) intrinsic colloid concentration data by limiting the pcH range from 9 to 10. Limiting the pcH to this very narrow range resulted in only two data sets (duplicates from the same experiment) for Th(IV) and may be unjustified given the low pcH dependence of Th(IV) solubilities in this pcH range (Economy 2016). Three experiments with two duplicate samples each were within +- 1 pcH unit of expected repository brine pcH conditions (Reed et al. 2013, Figure 4-6, samples G8C2-1, UG9C2-2 and E10C3-1). The filtration results from these three experiments demonstrated a wide range of intrinsic colloid concentrations. However, the dissolved thorium concentrations (3 kDa, approximately 2.5 nm) in the E10C3-1 samples were fairly high (approximately 7 x 10[-7] M) compared to ThO2(am) solubilities in WIPP brine, which are on the order of 7 x 10[-8] M (Table 6-17). Consequently, experiment E10C3-1 was likely too far from equilibrium to be used to determine the intrinsic colloid concentration. The dissolved Th(IV) concentration in G8C2-1 and the predicted Th(IV) solubility were similar, so this experiment may have been close to equilibrium. The intrinsic Th(IV) colloid concentrations in the duplicate samples from G8C2-1 were approximately 3 x 10[-6] M (Borkowski 2012b), which is greater than both the 2 x 10[-8] M intrinsic Th(IV) colloid concentration used in the CRA-2014 PA and the dissolved Th(IV) concentrations calculated in WIPP brines (6 x 10[-8] M to 7 x 10[-8] M, Table 6-19), indicating that including the G8C2-1 concentrations in the intrinsic Th(IV) colloid concentration evaluation could significantly affect total mobilized Th(IV). Although the pcH of UG9C2-2 was close to pcH values expected in the WIPP repository, the Th(IV) intrinsic colloid concentrations measured in G8C2-1 were consistent with the concentrations measured in 0.5 M and 5 M NaCl by Altmaier et al. (2004). Because of the low dependence of thorium solubility on pcH in the range anticipated in the WIPP brines and because significantly higher Th(IV) intrinsic colloid concentrations were measured within a pcH range of 8.5  -  10.5, EPA recommends DOE include data obtained within this pcH range in the calculation of intrinsic Th(IV) colloid concentrations.
The Am(III) intrinsic colloid concentration was calculated to be 2 x 10[-9] M based on a single Nd(III) measurement in ERDA-6 brine at pcH 9.1, where 33% of the total Nd(III) in solution was present as intrinsic colloids (Reed et al. 2013). At slightly lower pcH values (8.53 and 8.55) in GWB, dissolved concentrations were significantly higher with widely differing percentages of intrinsic colloids (30% and 77%). Because of the much higher solubilities in the lower pcH samples, the use of only the higher pcH sample (9.1) by Reed et al. (2013) is reasonable. The Nd(III) filtration data in Figure 4-2 of Reed et al. (2013) do not appear to be documented by the cited reference (Borkowski et al. 2009) because of differing pcH and filter sizes based on a comparison with Figure 4-2 of Borkowski et al. (2009). In addition, identification of colloids in the Nd(III) experiments appears to contradict the statement by Borkowski et al. (2009, page 23): "All the neodymium concentrations determined in filtered samples and unfiltered samples (with one exception) were in good agreement and within experimental error. This indicates that colloids or a fine particle suspension were not formed during the solubility experiments."
For Pu(III) in ERDA-6 with a pcH range from 9.1 to 9.5, the intrinsic colloid concentrations ranged from 9.3 x 10[-10] to 7.4 x 10[-9] M. Although Reed et al. (2013, Attachment 2, page 96) stated that the highest concentration for each actinide or analogue species would be selected as the recommended values for intrinsic colloids, Reed et al. (2013) selected an average value of 4 x 10[-9] M for Pu(III) intrinsic colloids. Substituting the highest measured intrinsic colloid concentration of 7.4 x 10[-9] M would provide an upper-bound value, but would have little effect on total mobilized +III actinide concentrations because the dissolved concentrations are much higher (Table 6-17). The plutonium 0.45 μm, 0.22 μm, 20 nm and 10 nm filtration data in Figures 4-3 and 4-4 of Reed et al. (2013) do not appear to be documented by the cited reference (Reed et al. 2010), which reported only 20 nm filtration.
U(VI) sequential filtration results from experiments carried out in CO2-free ERDA-6 brines, with final pcH values of 9.3 and 9.8 had low intrinsic colloid concentrations (~ 0 M and 6 x 10[-8] M). Reed et al. (2013) used an average of 3 x 10[-8] M, but even the higher 6 x 10[-8] M concentration is insignificant compared to the dissolved U(VI) concentration of 1 x 10[-3] M assumed for PA.
Roselle (2013) provided a summary of the recommended intrinsic colloid enhancement parameters based on the results of Reed et al. (2013, Table 1). The U(IV) intrinsic colloid concentration (2 x 10[-8] M) used in the CRA-2014 PA was lower than the 3 x 10[-8] M concentration recommended by Roselle (2013), but is consistent with the recommended Th(IV) intrinsic colloid concentration.
Humic Colloids

Actinide concentrations associated with humic substances are based on proportionality constants (Table 7-2). Proportionality constants determined for Th(IV) were extended to U(IV), Np(IV), and Pu(IV), and proportionality constants developed for Am(III) were extended to Pu(III). Proportionality constants for Np(V) and U(VI) were based on data for these oxidation states (DOE 1996, Appendix SOTERM, Table SOTERM-12). The proportionality constants were calculated from experimentally determined humic substance solubilities in brines at various concentrations of Ca[2+] and Mg[2+]; data on site-binding capacities measured for the WIPP research program and literature values; actinide complexation factors for Am(III), Th(IV), Np(V) and U(VI) binding on humic substances; and stability constants for Ca[2+] and Mg[2+] binding to humic substances. The maximum value used in PA (CAPHUM, Table 7-2) represents the theoretical maximum concentration of actinides that can be bound by a humic substance, assuming a solubility limit of 2 mg/L, the highest site-binding capacity for fulvic acids of 5.56 meq OH[-]/g and the limiting case of a monovalent actinide species (DOE 1996, Appendix SOTERM, Section <IP_ADDRESS>).
Wall and Mathews (2005) evaluated the solubility of humic acids in GWB and ERDA-6 brines and the effects of MgO on the quantities of humic acids in WIPP brines. Wall and Mathews (2005) found that GWB and ERDA-6 brine promoted coagulation of the humic acid, although complete humic acid removal was not observed. Humic acids were completely removed from the brines in the presence of either reagent-grade MgO or Premier MgO, which was the backfill material being emplaced in WIPP at the time of the experiments (Section 4.3.1).
No new data were included in the CRA-2014 PA regarding humic colloids. The parameters used to estimate humic colloid contributions to total mobile actinide concentrations have remained unchanged since the CCA PA (Table 7-2).
Microbial Colloids

Actinide concentrations associated with microbial colloids in WIPP PA are based on proportionality constants (Table 7-3). For microbial colloids, the CCA PA proportionality constants were developed for each element. Experimental data on the mobile concentrations of microbes and bioaccumulation and toxicity experiments were used to develop the proportionality constants (Papenguth 1996; DOE 1996, Appendix SOTERM Section 6.3.4) using a bacterium isolated from WIPP environs (Halomonas sp. WIPP-1A) and an uncharacterized mixed culture which presumably contained largely archaea (BAB). Upper limits for the concentrations of actinides for microbial colloids were established based on the concentration at which no growth was observed (Table 7-3). Because the high radiation levels of Am(III) prevented increasing concentrations until toxicity effects were observed, no maximum concentration value was determined for Am(III). Because of limited data, distributions were not developed for the proportionality constants or the maximum concentrations for microbial colloids and single values were used in PA.
These parameters have been updated for the CRA-2014 based on new experiments on pure cultures of a representative bacterium and a representative archaeon, both isolated from WIPP. While these experiments provide new data and new parameters to include for +4 and +3 actinides, EPA finds many issues with the experiments and the assumptions, some of which are major changes to the previous conceptual models. These experiments are summarized below.
Reed et al. (2013) summarized experiments investigating the biosorption of Nd(III) and Th(IV) on representative halophilic bacteria (Chromohalobacter sp) and archaea (Halobacterium noricense). Chromohalobacter sp was isolated from area groundwater and Halobacterium noricense was isolated from incubations of halite in generic media and brine. Four WIPP simulated brines (Lucchini et al. 2013) were used in biosorption experiments conducted at pcH 8.5, 9.0, 9.5 and 9.7 and at 90% saturation to prevent precipitation of solids from the brine during the experiments. The Nd(III) stock solution concentration was 4 x 10[-8] M, which is less than the expected solubility of Nd(III) solids under the experimental conditions. The Th(IV) stock solution was prepared by adding 1 x 10[-][4] M Th(IV) to each pcH-specific brine, which after equilibration was filtered with a 100 kDa (~ 20 nm) filter. These stock solutions had initial concentrations ranging from 1 x 10[-6] to 1 x 10[-7] M that decreased with increasing pcH. Metal-EDTA stock solutions were similarly prepared using an excess of EDTA equal to or greater than a factor of 2 times the Nd(III) and Th(IV) concentrations. Nd(III) stock solutions were 0.22 um filtered and Th(IV) stock solutions were 20 nm filtered immediately prior to the experiments. The biosorption experiments were carried out for two hours. At the end of the experiments, the solutions were ~ 20 nm filtered to remove microorganisms and the supernatants were analyzed. Abiotic control experiments were conducted to account for all other possible causes of Nd(III) and Th(IV) losses from the solutions.
Reed et al. (2013) noted significant biosorption of Nd(III) both in the absence and presence of EDTA. Significant biosorption was not anticipated in the presence of EDTA, because Reed et al. (2013) had expected EDTA complexation of Nd(III) to out-compete biosorption. Reed et al. (2013) attributed these results to precipitation of Nd(III) as colloidal or solid Nd(III), even though the solubility of Nd(III) in WIPP brine should have been higher than the initial 4 x 10[-8] M (Table 6-17). Evaluation of biosorption test results as a function of biomass showed that hydrolysis/precipitation could account for about 35% of the Nd(III) removed from solution at pcH 9.4 without EDTA and about 12% of the Nd(III) removed from solution at pcH 8.9 in the presence of EDTA. Geochemical modeling of the brines used in the experiments showed that the initial EDTA-free brines were oversaturated with respect to Nd(III) solid and that as much as 20% of the Nd(III) was not complexed by EDTA.
Reed et al. (2013) observed significant precipitation of Th(IV) in the controls at the highest pcH values. The biosorption results showed that metastable Th(IV)-EDTA complexes formed. Plots of biosorption as a function of biomass for the archaea extrapolated to near zero at zero biomass. However, extrapolation to zero biosorption at zero biomass loading was not observed for the bacteria. Based on the observation of precipitation in the Th(IV) stock solution at high pcH and the biomass loading data, Reed et al. (2013) determined that greater reliance should be placed on the EDTA-free data, on lower pcH data and on the archaea data.
Reed et al. (2013) generated oxidation-state specific recommendations for PROPMIC parameters for archaea and for bacteria for Th(IV), U(IV) and Pu(IV) and for Am(III) and Pu(III) based on their experimental results. A PROPMIC parameter was proposed for Np(V) based on the results of Ams et al. (2013) using Chromohalobacter sp in 2 M and 4 M NaClO4 solutions. PROPMIC parameters were also developed on an element-by-element basis, assuming the highest PROPMIC values developed for the different oxidation states of each element. For example, the oxidation-state-specific PROPMIC parameters proposed for Pu(III) were less than the proposed Pu(IV) parameters, so the Pu(IV) parameters were recommended for both plutonium oxidation states.
Reed et al. (2013) recommended revision of the CAPMIC parameters to correspond with a biomass of 10[9] cell/ml, multiplied by the moles of actinide per cell reported by Francis (1998) and measured values from Reed et al. (2013). This approach differs from the approach taken for the CCA, which used the actinide concentration at which microbial growth was no longer observed. The revised approach resulted in significantly lower CAPMIC parameter values than were developed for the CCA PA (Table 7-3). Reed et al. (2013, Section 5.2) based the maximum 10[9] cells on the number of haloarchea in optimal growth media rather than all microbes (bacteria and haloarchea) and the absence of growth. This represents a change in the conceptual model, which is acknowledged by Reed et al. (2013) because this change is explicitly addressed as a changed assumption (Assumption 1, Reed et al. 2013, Section 5.2). Because the WIPP PA model only allows for a single set of microbial colloid parameters, Roselle et al. (2013) selected the higher PROPMIC and CAPMIC parameter values Reed et al. (2013) developed. The number selected was from the bacterium as this had a higher sorption than the archaeon.
EPA noted (Economy 2016) that Reed et al. (2013) used results from microbial adsorption tests conducted for two hours, which is much shorter than the 11- to 21-day experiments used for previous WIPP microbial adsorption tests (Papenguth 1996; DOE 1996, Appendix SOTERM-2014). EPA requested evidence that the two-hour period was sufficient to achieve steady state conditions. Patterson (2016b) responded that the tests were designed only to evaluate biosorption, and not intracellular accumulation or biomineralization (surface-catalyzed precipitation) on the cell surface. This represents a significant departure from the original microbial colloid conceptual model. In the tests, the sorption kinetics were determined to typically take less than 90 minutes to reach steady state, so DOE expected the two-hour period for the experiments to be adequate. Patterson (2016b) stated that longer-term data were obtained over periods of days to weeks, with no significant change in the absence of precipitation. However, the Nd(III) data provided in the response (Patterson 20a6b, Figure 1) show an apparent upward trend between 3 and 20 hours. In the response, this slight increase is noted but the small changes are said to be within the uncertainty of the data. The information provided does not demonstrate steady state, because it is possible that experiments conducted over days or weeks could have shown increases that exceed the data uncertainties.

EPA (Economy 2016) noted that the Th(IV) solutions used in the biosorption tests were passed through 20 nm filters both before the tests were conducted and at the end of the tests (Reed et al. 2013). However, intrinsic colloids less than 10 nm have been reported for thorium (Reed et al. 2013) so these solutions may have contained intrinsic colloids. The presence of intrinsic colloids in the test solutions would result in dissolved concentrations that were less than the total concentrations. PROPMIC is calculated from the ratio of sorbed and dissolved concentrations:

PROPMIC= [Actinide or Analog Sorbed][Dissolved Actinide or Analog]
                                                                           (36)

Overestimating the dissolved concentration would result in an underestimated value for PROPMIC. In response, DOE (Patterson 2016b) addressed two possible cases: one in the absence of colloids and one in the presence of colloids in the test solutions. Because significant concentrations of colloids are likely to be present in the solutions, only the second case appears to be applicable. DOE stated in their response that the effects of colloids depends on whether colloids are sorbed: if colloids are not sorbed, then biosorption is underestimated, and if colloids are preferentially sorbed, biosorption is overestimated. DOE stated that experimental data have shown that colloids are preferentially sorbed and seem to facilitate biomineralization on the cell surface, but DOE did not provide any supporting data in its response. In addition, DOE stated that Th(IV) colloids are "always present at WIPP-relevant pH and we were not able to isolate true dissolved species sorption given our experimental limitations." Although DOE explained the use of 20-nm filtration to remove microbes at the end of the tests, DOE did not explain why additional 2-nm filtration was not performed to remove intrinsic colloids both before and after the sorption tests were conducted.
Uncertainties have been introduced into the calculation of the revised PROPMIC parameters (Table 7-3) because of the short experiment durations and the possible effects of intrinsic colloids on the biosorption results. Although the CRA-2014 PA PROPMIC parameters increased for U(IV), U(VI), Pu(III) and Pu(IV), these parameters decreased for Th(IV), Np(IV), Np(V) and Am(III). Because of a change to the conceptual model, the CAPMIC concentrations decreased for all elements (Table 7-3). For the +IV actinides in Salado and Castile brines, CAPMIC (2.3 x 10[-6] M) would not limit the microbial colloids for the baseline solubility plus mean uncertainty. However, for Pu(III) and Am(III), CAPMIC is low compared to the baseline dissolved concentration and would likely affect the actinide concentrations associated with microbial colloids (Section 8.0).
DOE has changed the conceptual model assumptions for calculating CAPMIC, microbial cultures present, and bioaccumulation/biomineralization. In addition, EPA disagrees with many of the experimental parameters chosen and as a result believe the CRA-2014 microbial colloid parameter updates are not adequately justified for implementation. EPA expects this model to continue to be updated for the CRA-2019. Until then, EPA will accept the higher CAPMIC values originally developed for the CCA PA. DOE has stated that they continue to study actinide biosorption, intracellular accumulation and biomineralization (Patterson 2016b). Because these studies are incomplete and the effects of intracellular accumulation and biomineralization may not have been assessed in the more recent tests, EPA will also continue to accept the higher PROPMIC parameter (either from the CCA PA or Reed et al. 2013) until the issues cited in this review are resolved.
Colloids Summary and Conclusions

DOE (2014b) provided the first update to the colloids parameters since the CCA PA. During review of the colloid parameters, it was noted:
 Humic colloid parameters remained unchanged
 Parameter values reflecting mineral fragment colloids (CONCMIN) remain unchanged, which is consistent with the available data
 Whether mineral fragment colloids might form during reaction of MgO in WIPP brines and transport actinides remains unresolved and formation of these mineral fragment colloids should be experimentally investigated
 The revised intrinsic colloid parameter (CONCINT) for Th(IV) should reflect upper bounding values for samples collected within the pcH range of 8.5  -  10.5
 The Pu(III) and Am(III) intrinsic colloid parameters do not appear to be documented by the references cited by Reed et al. (2013) and the Pu(III) intrinsic colloid parameter does not appear to be an upper, bounding value
 DOE changed the upper bounds for the microbially associated actinides (CAPMIC) for the CRA-2014 PA based on a revised conceptual model assumption regarding the upper limit on microbially associated actinides. Previous PAs assumed that the maximum concentration is the concentration at which no growth was observed, but DOE changed this upper limit to the concentration calculated from an assumed maximum cell concentration multiplied by the moles of actinides per cell. These departures from the original conceptual model suggest the revised microbial colloidal conceptual model and revised parameters have not been adequately justified. Consequently, it would be appropriate for DOE to continue to evaluate the model and parameters for the CRA-2019 and continue using the original CAPMIC values until this evaluation is complete
 Because of the short reaction times in tests reported by DOE, the current microbial proportionality constants (PROPMIC) likely reflect only biosorption and not bioaccumulation or biomineralization. This represents a conceptual model change because previous WIPP PAs included biosorption, bioaccumulation and biomineralization. DOE has stated that further research into the biosorption, bioaccumulation and biomineralization processes is planned, and these investigations should include longer reaction times to substantiate steady state conditions and that the effects of bioaccumulation and biomineralization have been addressed
 Uncertainty has been introduced into the biosorption data developed by DOE because of the possible inclusion of intrinsic colloids in the experimental solutions which may or may not have sorbed; EPA expects DOE to use 2 nm filtration to remove intrinsic colloids from their post-test solutions in future experiments to establish the dissolved concentration for calculating PROPMIC parameter values.

Total Mobilized Actinide Calculations for PA

The total mobilized actinides in WIPP brines is defined for PA as the sum of the dissolved actinide concentrations and the colloidal actinide concentrations. The information required to calculate the dissolved and colloidal actinide concentrations for each PA realization includes:
 Whether the scenario uses the Salado or Castile end-member brine composition (DOE 2014b, Appendix SOTERM-2014, Table SOTERM-22)
 The baseline dissolved concentrations of +III, +IV, +V, and +VI actinides in the brine (Table 6-17)
 Sampled uncertainty values for the +III and +IV actinide solubilities (Section 6.6.6)
 Whether actinides with multiple possible oxidation states are present in their relatively reduced [U(IV), Np(IV) and Pu(III)] or relatively oxidized [U(VI), Np(V) and Pu(IV)] oxidation states
 The constant concentrations of intrinsic and mineral-fragment colloids (Table 7-1)
 The proportionality constants and maximum concentrations of humic and microbial colloid actinides (Tables 7-2 and 7-3); of these parameters, only the proportionality constant for the +III actinide humic colloids in ERDA-6 brine is sampled for PA; all other proportionality constants and the maximum concentrations are fixed values
 The total inventory of the actinide
CRA-2014 PA Total Mobilized Actinide Results

DOE (2014b, Appendix SOTERM-2014, Section SOTERM-5.0) provided a discussion of the PA calculations of the actinide source term and example calculations of actinide solubility and colloidal actinides. Table SOTERM-23 (DOE 2014b, Appendix SOTERM-2014) summarized dissolved and colloidal actinide concentrations calculated using median parameter values for the CCA PAVT, PABC04, PABC09 and CRA-2014 PA.
Baseline dissolved actinide concentrations calculated using EQ3/6, the mean uncertainty parameters and the mean colloidal actinide parameters for the CRA-2014 PA are summarized in Table 8-1. Because the constant concentrations of mineral fragment colloids and intrinsic colloids are relatively small, contributions to the total mobile actinide concentrationss are low from these types of colloids for all actinide oxidation states for calculations using the mean CRA-2014 PA parameter values (Table 8-1). In GWB brine, total mobilized Am(III) is dominated by dissolved species, with the next largest contribution from humic colloids. For total mobilized Am(III) concentrations in ERDA-6 brine, higher concentrations are associated with humic colloids because of a higher proportionality constant for Am(III) humic colloids in Castile brine, with the next largest contribution from dissolved species. For both GWB and ERDA-6 brine, Am(III) concentrations associated with microbial colloids are low because they are limited by the maximum concentration. For Pu(III) in both GWB and ERDA-6 brine, the largest contributions to total mobilized concentrations are from microbial colloids, with significant additional contributions from dissolved species and humic colloids.
For +IV actinides in both GWB and ERDA-6 brine, humic colloids are the most significant contributors to the total mobilized concentrations (Table 8-1). Microbial colloids are the second-most important contributors, with dissolved species constituting only 11% of the total mobilized concentrations. Total mobilized Np(V) is dominated by microbial colloids and total mobilized U(VI) is dominated by dissolved species.
The overall mean CCDF for DBR was lower for the CRA-2014 PA than for the PABC09 (DOE 2014b Appendix PA-2014, Figure PA-78). This decrease was likely caused by the lower sampled range for the probability of a drilling intrusion encountering pressurized brine (GLOBAL:PBRINE) for the CRA-2014 PA. The lower mean DBR CCDF for the CRA-2014 PA may also be related to lower total mobilized actinide concentrations in the CRA-2014 PA. Evidence supporting the effects of total mobilized actinide concentrations on DBR includes the observation that the sampled parameter SOLMOD3:SOLVAR (the solubility uncertainty for the +III oxidation state) accounted for a significant fraction of the uncertainty in DBRs (Kirchner 2013). In addition, the median total mobilized Pu(III) and Am(III) concentrations in Castile brine in the CRA-2014 PA were only 18% and 5% of their respective values in the PABC09 (DOE 2014b, Appendix SOTERM, Table SOTERM-23). There is no evidence that the +IV actinides significantly affected DBRs, because the median total mobilized +IV concentrations increased between the PABC09 and CRA-2014 PA while the overall mean CCDF for DBR decreased.
The total mobilized Am(III) concentrations in the CRA-2014 PA for both GWB and ERDA-6 brine were lower than the PABC09 concentration because: 1) the mean solubilities were reduced because of a more negative mean SOLMOD3:SOLVAR value; 2) the humic colloid concentration was lower because it is proportional to the mean solubility; and 3) the microbial colloidal Am(III) concentration was reduced by the smaller actinide solubility, a smaller proportionality constant and smaller maximum concentration. The largest difference for Am(III) was observed for the microbial colloid concentration, which decreased by more than two orders of magnitude from the PABC09 to the CRA-2014 PA. The overall decreases in the total mobilized Am(III) concentrations for both GWB and ERDA-6 brine were nearly an order of magnitude between the PABC09 and CRA-2014 PA (Tables 8-1 and 8-2).
The changes in total mobilized Pu(III) concentrations from the PABC09 to the CRA-2014 PA differed for the two WIPP brines. The total mobilized Pu(III) concentration in GWB brine for the CRA-2014 PA decreased relative to the PABC09 by only about 10% because lower dissolved Pu(III) and humic colloid Pu(III) concentrations were partially offset by a higher microbial colloid Pu(III) concentration. On the other hand, the total mobilized Pu(III) concentration in ERDA-6 brine for the CRA-2014 PA was less than half of the PABC09 concentration because of a lower mean Pu(III) solubility and humic colloid concentration.
CRA14_SEN4 PA Total Mobilized Actinide Results

The total mobilized actinides for the CRA14_SEN4 PA are summarized in Table 8-3. The total mobilized actinide concentrations for the CRA14_SEN4 PA differed from the concentrations for the CRA-2014 PA because of the revised solubility uncertainty distributions used for the +III and +IV actinides (Xiong and Domski 2016a). The revised solubility uncertainty distributions increased the mean +III actinide solubilities and decreased the mean +IV actinide solubilities used in the CRA14_SEN4 PA. Because the humic colloid and microbial colloid concentrations are proportional to the dissolved actinide solubilities, the humic colloid, microbial colloid and total colloidal concentrations also changed relative to the CRA-2014 PA.
Sensitivity of Total Mobilized Actinide Concentrations to Parameter Changes
The changes to microbial colloid proportionality constants and maximum microbial colloid concentrations for Am(III) and Pu(III) for the CRA-2014 PA and CRA14_SEN4 significantly affected total mobilized Am(III) and Pu(III) concentrations relative to the PABC09. For the PABC09, using the CCA PA microbial colloid parameters resulted in total mobilized Am(III) concentrations composed of 75% (GWB) and 63% (ERDA-6) microbial colloids. In contrast, the Am(III) concentrations associated with microbial colloids for the CRA-2014 PA were 4% of the total mobilized concentration in both GWB and ERDA-6 brine, whereas for the CRA14_SEN4 PA, the Am(III) concentrations associated with microbial colloids were 0.3% of the total mobilized concentration. Microbial colloids accounted for 20% (GWB) and 12% (ERDA-6) of the total mobilized Pu(III) in the PABC09, 58% (GWB) and 44% (ERDA-6) of the total mobilized Pu(III) in the CRA-2014 PA and 20% (GWB) and 19% (ERDA-6) of the total mobilized Pu(III) in the CRA14_SEN4 PA.
The total mobilized actinide concentrations for Castile brines in the PABC09, CRA-2014 PA and CRA14_SEN4 PA are compared in Table 8-4. For Am(III) and Pu(III), which are the actinide oxidation states most likely to affect releases, the highest total mobilized concentrations were observed in the CRA14_SEN4 PA. The total mobilized Am(III) concentration in the CRA14_SEN4 would have been even higher (1.09 x 10[-5] M) if the Am(III) maximum microbial colloid concentration had not been changed from its previously unlimited value to 3.1 x 10[-8] M (Table 7-3). The total mobilized Pu(III) concentration for the CRA14_SEN4 PA increased by nearly an order of magnitude over the concentrations predicted for the PABC09 or CRA-2014 PA. The total mobilized Pu(III) concentration would have been even higher (1.75 x 10[-5] M), but was limited by the downward revision of the maximum microbial colloid concentration from 6.8 x 10[-5] M to 2.3 x 10[-6] M (Table 7-3).
Because the baseline solubility of Pu(III) is higher than the Pu(IV) baseline solubility in WIPP PA and humic and microbial colloid concentrations are proportional to aqueous solubilities, it would be reasonable to expect that mean and median total mobilized Pu(III) concentrations will be greater than mean and median total mobilized Pu(IV) concentrations. For the PABC09 and CRA14_SEN4 PA, total mobilized Pu(III) concentrations were significantly higher than total mobilized Pu(IV) concentrations (Table 8-4). However, for the CRA-2014 PA both mean and median total mobilized Pu(IV) concentrations exceeded the mean and median total mobilized Pu(III) concentrations (Table 8-4; DOE 2014b Appendix SOTERM-2014, Table SOTERM-23). For the CRA-2014 PA, the mean Pu(III) dissolved concentration was only slightly higher than the mean Pu(IV) dissolved concentration because of the negative mean Pu(III) solubility uncertainty parameter and the positive mean Pu(IV) solubility uncertainty parameter. The total mobilized Pu(IV) concentration was also increased relative to Pu(III) by the higher mean humic colloid concentration for Pu(IV) (Table 8-1).

Table 8-1.	CRA-2014 PA Mean Dissolved and Colloidal Actinides
                       Radionuclide and Oxidation State
                                     Brine
                     Baseline Dissolved Concentration (M)
                     Mean SOLVAR (Brush and Domski 2013c)
                              Mean Solubility (M)
                         Mineral Fragment Colloids (M)
                            Intrinsic Colloids (M)
                              Humic Colloids (M)
                            Microbial Colloids (M)
                              Total Colloidal (M)
                              Total Mobilized (M)
                                    Am(III)
                                      GWB
                                 2.59  10[-6]
                                    -0.678
                              5.44  10[-7] (77%)
                                 2.60  10[-8]
                                 4.00  10[-9]
                                 1.03  10[-7]
                               3.10  10[-][8 d]
                             1.64  10[-][7] (23%)
                                7.08  10[-][7]
                                    Pu(III)
                                      GWB
                                 2.59  10[-6]
                                    -0.678
                              5.44  10[-7] (33%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 1.03  10[-7]
                                 9.57  10[-7]
                             1.11  10[-][6] (67%)
                                 1.65  10[-6]
                                    Th(IV)
                                      GWB
                                 6.05  10[-8]
                                     0.673
                              2.85  10[-7] (11%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                1.80  10[-][6]
                                5.01  10[-][7]
                             2.34  10[-][6] (89%)
                                2.63  10[-][6]
                                     U(IV)
                                      GWB
                                 6.05  10[-8]
                                     0.673
                              2.85  10[-7] (11%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                1.80  10[-][6]
                                 5.01  10[-7]
                              2.34  10[-6] (89%)
                                 2.63  10[-6]
                                    Np(IV)
                                      GWB
                                 6.05  10[-8]
                                     0.673
                              2.85  10[-7] (11%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                1.80  10[-][6]
                                 5.01  10[-7]
                              2.34  10[-6] (89%)
                                 2.63  10[-6]
                                    Pu(IV)
                                      GWB
                                 6.05  10[-8]
                                     0.673
                              2.85  10[-7] (11%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                1.80  10[-][6]
                                 5.01  10[-7]
                              2.34  10[-6] (89%)
                                 2.63  10[-6]
                                     Np(V)
                                      GWB
                                 2.77  10[-7]
                                     --[a]
                              2.77  10[-7] (35%)
                                 2.60  10[-8]
                                     --[b]
                                 2.52  10[-10]
                                4.88  10[-][7]
                             5.14  10[-][7] (65%)
                                7.91  10[-][7]
                                     U(VI)
                                      GWB
                                 1.00  10[-3]
                                     --[a]
                              1.00  10[-3] (99%)
                                 2.60  10[-8]
                                 3.00  10[-8]
                                1.10  10[-5 c]
                                2.30  10[-6] d
                               1.34  10[-5] (1%)
                                 1.01  10[-3]
                                    Am(III)
                                    ERDA-6
                                 1.48  10[-6]
                                    -0.678
                              3.11  10[-7] (44%)
                                 2.60  10[-8]
                                 4.00  10[-9]
                                3.42  10[-][7]
                               3.10  10[-][8 d]
                             4.03  10[-][7] (56%)
                                7.13  10[-][7]
                                    Pu(III)
                                    ERDA-6
                                 1.48  10[-6]
                                    -0.678
                              3.11  10[-7] (25%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 3.42  10[-7]
                                 5.47  10[-7]
                              9.34  10[-7] (75%)
                                 1.25  10[-6]
                                    Th(IV)
                                    ERDA-6
                                 7.02  10[-8]
                                     0.673
                              3.31  10[-7] (11%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                2.08  10[-][6]
                                5.82  10[-][7]
                             2.71  10[-][6] (89%)
                                3.04  10[-][6]
                                     U(IV)
                                    ERDA-6
                                 7.02  10[-8]
                                     0.673
                              3.31  10[-7] (11%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 2.08  10[-6]
                                 5.82  10[-7]
                              2.71  10[-6] (89%)
                                 3.04  10[-6]
                                    Np(IV)
                                    ERDA-6
                                 7.02  10[-8]
                                     0.673
                              3.31  10[-7] (11%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 2.08  10[-6]
                                 5.82  10[-7]
                              2.71  10[-6] (89%)
                                 3.04  10[-6]
                                    Pu(IV)
                                    ERDA-6
                                 7.02  10[-8]
                                     0.673
                              3.31  10[-7] (11%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 2.08  10[-6]
                                 5.82  10[-7]
                              2.71  10[-6] (89%)
                                 3.04  10[-6]
                                     Np(V)
                                    ERDA-6
                                 8.76  10[-7]
                                     --[a]
                              8.76  10[-7] (36%)
                                 2.60  10[-8]
                                     --[b]
                                 6.48  10[-9]
                                 1.54  10[-5]
                             1.57  10[-][6] (64%)
                                2.45  10[-][6]
                                     U(VI)
                                    ERDA-6
                                 1.00  10[-3]
                                     --[a]
                              1.00  10[-3] (99%)
                                 2.60  10[-8]
                                 3.00  10[-8]
                                1.10  10[-5 c]
                                2.30  10[-6 d]
                               1.34  10[-5] (1%)
                                 1.01  10[-3]
a  -  Np(V) and U(VI) solubility uncertainty parameters are not sampled for PA
b  -  Np(V) intrinsic colloids are not included in the PA calculations
c  -  CAPHUM value
d  -  CAPMIC value
Table 8-2.	PABC09 Mean Dissolved and Colloidal Actinides
                       Radionuclide and Oxidation State
                                     Brine
                     Baseline Dissolved Concentration (M)
                       Mean SOLVAR (Xiong et al. 2009a)
                              Mean Solubility (M)
                         Mineral Fragment Colloids (M)
                            Intrinsic Colloids (M)
                              Humic Colloids (M)
                            Microbial Colloids (M)
                              Total Colloidal (M)
                              Total Mobilized (M)
                                    Am(III)
                                      GWB
                                 1.66  10[-6]
                                    -0.142
                              1.20  10[-6] (21%)
                                 2.60  10[-8]
                                     0.00
                                 2.27  10[-7]
                                 4.31  10[-6]
                              4.56  10[-6] (79%)
                                 5.76  10[-6]
                                    Pu(III)
                                      GWB
                                 1.66  10[-6]
                                    -0.142
                              1.20  10[-6] (66%)
                                 2.60  10[-8]
                                     0.00
                                 2.27  10[-7]
                                 3.59  10[-7]
                              6.13  10[-7] (34%)
                                 1.81  10[-6]
                                    Th(IV)
                                      GWB
                                 5.63  10[-8]
                                    -0.346
                               2.54  10[-8] (9%)
                                 2.60  10[-8]
                                     0.00
                                 1.60  10[-7]
                                 7.87  10[-8]
                              2.65  10[-7] (91%)
                                 2.90  10[-7]
                                     U(IV)
                                      GWB
                                 5.63  10[-8]
                                    -0.346
                              2.54  10[-8] (12%)
                                 2.60  10[-8]
                                     0.00
                                 1.60  10[-7]
                                 5.33  10[-11]
                              1.86  10[-7] (88%)
                                 2.11  10[-7]
                                    Np(IV)
                                      GWB
                                 5.63  10[-8]
                                    -0.346
                               2.54  10[-8] (5%)
                                 2.60  10[-8]
                                     0.00
                                 1.60  10[-7]
                                 3.05  10[-7]
                              4.90  10[-7] (95%)
                                 5.16  10[-7]
                                    Pu(IV)
                                      GWB
                                 5.63  10[-8]
                                    -0.346
                              2.54  10[-8] (12%)
                                 2.60  10[-8]
                                 1.00  10[-9]
                                 1.60  10[-7]
                                 7.61  10[-9]
                              1.95  10[-7] (88%)
                                 2.20  10[-7]
                                     Np(V)
                                      GWB
                                 3.90  10[-7]
                                     --[a]
                               3.90  10[-7] (8%)
                                 2.60  10[-8]
                                     0.00
                                 3.55  10[-10]
                                 4.68  10[-6]
                              4.71  10[-6] (92%)
                                 5.10  10[-6]
                                     U(VI)
                                      GWB
                                 1.00  10[-3]
                                     --[a]
                              1.00  10[-3] (99%)
                                 2.60  10[-8]
                                     0.00
                                 1.10  10[-5]
                                 2.10  10[-6]
                               1.31  10[-5] (1%)
                                 1.01  10[-3]
                                    Am(III)
                                    ERDA-6
                                 1.51  10[-6]
                                    -0.142
                              1.09  10[-6] (17%)
                                 2.60  10[-8]
                                     0.00
                                 1.20  10[-6]
                                 3.92  10[-6]
                              5.14  10[-6] (83%)
                                 6.23  10[-6]
                                    Pu(III)
                                    ERDA-6
                                 1.51  10[-6]
                                    -0.142
                              1.09  10[-6] (41%)
                                 2.60  10[-8]
                                     0.00
                                 1.20  10[-6]
                                 3.27  10[-7]
                              1.55  10[-6] (59%)
                                 2.64  10[-6]
                                    Th(IV)
                                    ERDA-6
                                 6.98  10[-8]
                                    -0.346
                               3.15  10[-8] (9%)
                                 2.60  10[-8]
                                     0.00
                                 1.98  10[-7]
                                 9.75  10[-8]
                              3.22  10[-7] (91%)
                                 3.53  10[-7]
                                     U(IV)
                                    ERDA-6
                                 6.98  10[-8]
                                    -0.346
                              3.15  10[-8] (12%)
                                 2.60  10[-8]
                                     0.00
                                 1.98  10[-7]
                                 6.61  10[-11]
                              2.24  10[-7] (88%)
                                 2.56  10[-7]
                                    Np(IV)
                                    ERDA-6
                                 6.98  10[-8]
                                    -0.346
                               3.15  10[-8] (5%)
                                 2.60  10[-8]
                                     0.00
                                 1.98  10[-7]
                                 3.78  10[-7]
                              6.02  10[-7] (95%)
                                 6.33  10[-7]
                                    Pu(IV)
                                    ERDA-6
                                 6.98  10[-8]
                                    -0.346
                              3.15  10[-8] (12%)
                                 2.60  10[-8]
                                 1.00  10[-9]
                                 1.98  10[-7]
                                 9.44  10[-9]
                              2.35  10[-7] (88%)
                                 2.66  10[-7]
                                     Np(V)
                                    ERDA-6
                                 8.75  10[-7]
                                     --[a]
                               8.75  10[-7] (8%)
                                 2.60  10[-8]
                                     0.00
                                  6.48 10[-9]
                                 1.05  10[-5]
                              1.05  10[-5] (92%)
                                 1.14  10[-5]
                                     U(VI)
                                    ERDA-6
                                 1.00  10[-3]
                                     --[a]
                              1.00  10[-3] (99%)
                                 2.60  10[-8]
                                     0.00
                                 1.10  10[-5]
                                 2.10  10[-6]
                               1.31  10[-5] (1%)
                                 1.01  10[-3]
a  -  	Np(V) and uranium (VI) solubility uncertainty parameters are not sampled for PA
Table 8-3.	CRA14_SEN4 PA Mean Dissolved and Colloidal Actinides
                       Radionuclide and Oxidation State
                                     Brine
                     Baseline Dissolved Concentration (M)
                     Mean SOLVAR (Xiong and Domski 2016a)
                              Mean Solubility (M)
                         Mineral Fragment Colloids (M)
                            Intrinsic Colloids (M)
                              Humic Colloids (M)
                            Microbial Colloids (M)
                              Total Colloidal (M)
                              Total Mobilized (M)
                                    Am(III)
                                      GWB
                                 2.59  10[-6]
                                     0.484
                             7.89  10[-][6] (83%)
                                 2.60  10[-8]
                                 4.00  10[-9]
                                1.50  10[-][6]
                                3.10  10[-8 d]
                             1.56  10[-][6] (23%)
                                9.45  10[-][6]
                                    Pu(III)
                                      GWB
                                 2.59  10[-6]
                                     0.484
                              7.89  10[-6] (34%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 1.50  10[-6]
                                2.30  10[-6 d]
                              3.83  10[-6] (33%)
                                1.17  10[-][5]
                                    Th(IV)
                                      GWB
                                 6.05  10[-8]
                                    -0.011
                             5.90  10[-][8] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 3.72  10[-7]
                                 1.04  10[-7]
                             5.21  10[-][7] (90%)
                                5.80  10[-][7]
                                     U(IV)
                                      GWB
                                 6.05  10[-8]
                                    -0.011
                              5.90  10[-8] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 3.72  10[-7]
                                 1.04  10[-7]
                              5.21  10[-7] (90%)
                                 5.80  10[-7]
                                    Np(IV)
                                      GWB
                                 6.05  10[-8]
                                    -0.011
                              5.90  10[-8] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 3.72  10[-7]
                                 1.04  10[-7]
                              5.21  10[-7] (90%)
                                 5.80  10[-7]
                                    Pu(IV)
                                      GWB
                                 6.05  10[-8]
                                    -0.011
                              5.90  10[-8] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 3.72  10[-7]
                                 1.04  10[-7]
                              5.21  10[-7] (90%)
                                 5.80  10[-7]
                                     Np(V)
                                      GWB
                                 2.77  10[-7]
                                     --[a]
                              2.77  10[-7] (35%)
                                 2.60  10[-8]
                                     --[b]
                                 2.52  10[-10]
                                 4.88  10[-7]
                              5.14  10[-7] (65%)
                                 7.91  10[-7]
                                     U(VI)
                                      GWB
                                 1.00  10[-3]
                                     --[a]
                              1.00  10[-3] (99%)
                                 2.60  10[-8]
                                 3.00  10[-8]
                                1.10  10[-5 c]
                                2.30  10[-6 d]
                               1.34  10[-5] (1%)
                                 1.01  10[-3]
                                    Am(III)
                                    ERDA-6
                                 1.48  10[-6]
                                     0.484
                             4.51  10[-][6] (26%)
                                 2.60  10[-8]
                                 4.00  10[-9]
                                 4.96  10[-6]
                                3.10  10[-8 d]
                             5.02  10[-][6] (53%)
                                9.53  10[-][6]
                                    Pu(III)
                                    ERDA-6
                                 1.48  10[-6]
                                     0.484
                              4.51  10[-6] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 4.96  10[-6]
                                2.30  10[-6 d]
                             7.29  10[-][6] (62%)
                                1.18  10[-][5]
                                    Th(IV)
                                    ERDA-6
                                 7.02  10[-8]
                                    -0.011
                             6.84  10[-][8] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 4.31  10[-7]
                                 1.20  10[-7]
                             5.98  10[-][7] (90%)
                                6.66  10[-][7]
                                     U(IV)
                                    ERDA-6
                                 7.02  10[-8]
                                    -0.011
                              6.84  10[-8] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 4.31  10[-7]
                                 1.20  10[-7]
                              5.98  10[-7] (90%)
                                 6.66  10[-7]
                                    Np(IV)
                                    ERDA-6
                                 7.02  10[-8]
                                    -0.011
                              6.84  10[-8] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 4.31  10[-7]
                                 1.20  10[-7]
                              5.98  10[-7] (90%)
                                 6.66  10[-7]
                                    Pu(IV)
                                    ERDA-6
                                 7.02  10[-8]
                                    -0.011
                              6.84  10[-8] (10%)
                                 2.60  10[-8]
                                 2.00  10[-8]
                                 4.31  10[-7]
                                 1.20  10[-7]
                              5.98  10[-7] (90%)
                                 6.66  10[-7]
                                     Np(V)
                                    ERDA-6
                                 8.76  10[-7]
                                     --[a]
                              8.76  10[-7] (36%)
                                 2.60  10[-8]
                                     --[b]
                                  6.48 10[-9]
                                 1.54  10[-5]
                              1.57  10[-6] (64%)
                                 2.45  10[-6]
                                     U(VI)
                                    ERDA-6
                                 1.00  10[-3]
                                     --[a]
                              1.00  10[-3] (99%)
                                 2.60  10[-8]
                                 3.00  10[-8]
                                1.10  10[-5 c]
                                2.30  10[-6 d]
                               1.34  10[-5] (1%)
                                 1.01  10[-3]
a  -  Np(V) and U(VI) solubility uncertainty parameters are not sampled for PA
b  -  Np(V) intrinsic colloids are not included in the PA calculations
c  -  CAPHUM value
d  -  CAPMIC value
Table 8-4.	Mean Total Mobilized Concentrations in Castile Brine for the PABC09, CRA-2014 PA and CRA14_SEN4 PA

                       Radionuclide and Oxidation State
                                    PABC09
                                      (M)
                                 CRA-2014 PA
                                      (M)
                                CRA14_SEN4 PA
                                      (M)
                                    Am(III)
                                 6.23  10[-6]
                                 7.13  10[-7]
                                 9.53  10[-6]
                                    Pu(III)
                                 2.64  10[-6]
                                 1.25  10[-6]
                                 1.18  10[-5]
                                    Th(IV)
                                 3.53  10[-7]
                                 3.04  10[-6]
                                 6.66  10[-7]
                                     U(IV)
                                 2.56  10[-7]
                                 3.04  10[-6]
                                 6.66  10[-7]
                                    Np(IV)
                                 6.33  10[-7]
                                 3.04  10[-6]
                                 6.66  10[-7]
                                    Pu(IV)
                                 2.66  10[-7]
                                 3.04  10[-6]
                                 6.66  10[-7]
                                     Np(V)
                                 1.14  10[-5]
                                 2.45  10[-6]
                                 2.45  10[-6]
                                     U(VI)
                                 1.01  10[-3]
                                 1.01  10[-3]
                                 1.01  10[-3]

Consideration of the effects of parameter changes on total mobilized actinide concentrations for the PABC09, CRA-2014 PA and CRA14_SEN4 PA has demonstrated that:
 Mineral fragment and intrinsic colloids contribute relatively little to total mobilized actinide concentrations using the mean parameters from these PAs
 For both GWB and ERDA-6 brine, mean Am(III) total mobilized concentrations are significantly decreased relative to PABC09 concentrations by the smaller PROPMIC and CAPMIC parameters used in the CRA-2014 PA and CRA14_SEN4 PA
 For both GWB and ERDA-6 brine, mean Am(III) and Pu(III) total mobilized concentrations for the CRA-2014 PA are significantly decreased relative to PABC09 and CRA14_SEN4 PA concentrations by the negative mean SOLMOD3:SOLVAR parameter
 Because of the negative SOLMOD3:SOLVAR parameter and positive SOLMOD4:SOLVAR parameter used in the CRA-2014 PA and different PROPHUM parameters for Pu(III) and Pu(IV), mean and median total mobilized Pu(IV) concentrations are higher than the mean and median total mobilized Pu(III) concentrations, even though the baseline Pu(III) solubilities in GWB and ERDA-6 were significantly greater than the baseline Pu(IV) solubilities
These results demonstrate the importance of the PROPMIC and CAPMIC parameters developed by DOE for the CRA-2014 PA (Section 7.4) and the importance of consistently applying a defensible set of criteria for selection solubility data for calculating the SOLMOD3:SOLVAR and SOLMOD4:SOLVAR parameters (Sections 6.6.6 and 6.7).

Actinides Included in Performance Assessment Calculations

Total mobilized actinide concentrations are important only for calculating DBRs and releases by transport through the Culebra to the WIPP boundary. The primary release mechanism at high probability -- cuttings and cavings -- is independent of the extent of total actinide mobilization in brine. Spallings, which is not currently predicted to be an important release mechanism, are also not affected by actinides mobilized in brine.
Although actinide solubilities and colloidal actinide concentrations are calculated for thorium, uranium, neptunium, plutonium, americium and curium, not all elements and isotopes contribute significantly to DBR or releases through the Culebra and Salado. DOE has noted that inclusion of all radionuclides in the WIPP inventory in the PA calculations would be both computationally complex and prohibitively time consuming. Consequently, the number of radionuclides directly included in PA was reduced using an algorithm developed for the CCA (DOE 2014b, Appendix PA-2014, Section PA-4.3.2 and Figure PA-17).  Isotopes identified for calculations of CRA-2014 PA DBR releases included 23 radioisotopes of plutonium, americium, uranium, thorium, neptunium, curium, cesium and strontium (Kicker and Zeitler 2013). These isotopes represent 99.96% of the potential releases, in EPA units, of the CRA-2014 PA inventory at the time of repository closure (Kicker and Zeitler 2013).
Ten plutonium, americium, uranium and thorium isotopes were identified for calculations of releases by transport through the Culebra and Salado (Kicker and Zeitler 2013). DOE reduced the number of isotopes identified for the transport release calculations based on total inventories, removing radionuclides with inventories that total less than one EPA unit. DOE further omitted [137]Cs and [90]Sr because their short half lives cause their concentrations to drop below one EPA unit within 150 years. [241]Pu was added to the list of modeled isotopes because it is a parent isotope for [241]Am and has a significant inventory at the start of the calculation. [229]Th and [233]U were included because they have potential releases close to one EPA unit and were included in previous PA calculations. The isotopes [14]C, [243]Cm, [59]Ni and [63]Ni were excluded because their contributions to the overall potential release are negligible. The remaining ten isotopes were [241]Am, [23][8]Pu, [239]Pu, [240]Pu, [241]Pu, [242]Pu, [233]U, [234]U, [229]Th and [230]Th. This list of isotopes accounts for 97.83%, in EPA units, of the potential releases at the time of repository closure in the CRA-2014 PA inventory (Kicker and Zeitler 2013).
The inventories of the ten isotopes identified for calculating releases by transport through the Culebra and Salado were combined into a smaller number of "lumped" inventory parameters based on the reasonable assumption that radionuclides of the same element will transport at the same rate. Therefore, the inventories of the uranium isotopes; [239]Pu, [240]Pu and [242]Pu; and thorium isotopes were combined into the lumped inventory parameters U234L, PU239L and TH230L, respectively. [238]Pu was not included in the PU239L parameter because of its short half life. As a final step, the inventory of [241]Pu was combined with the [241]Am inventory to create the inventory for AM241L in the transport calculations (Kicker and Zeitler 2013). Depending on the half-lives of the isotopes, combining the inventories based on curies or on moles will result in differing combined activities. The lumped parameters were combined using an approach that maximized their combined activities (Kicker and Zeitler 2013).

Culebra Dolomite Distribution Coefficients

Radionuclides may reach the Culebra member of the Rustler Formation via brine flow through a borehole that intersects the repository (DOE 2014b, Appendix PA-2014, Section 6.8.3). Radionuclides introduced into the Culebra may be transported through natural groundwater flow to the accessible environment (DOE 2004, Section <IP_ADDRESS>). Predictions of radionuclide transport and release through the Culebra are affected by sorption onto minerals along this potential pathway. Accordingly, DOE developed distribution coefficients (Kds) to express a linear relationship between sorbed and aqueous concentrations of the radionuclides (DOE 2004, Section <IP_ADDRESS>.1).
Distribution Coefficients Used in the CCA PA and PAVT

The distribution coefficients used in the CCA PA and PAVT were developed using data from experiments carried out at Los Alamos National Laboratory (LANL) and SNL:
 Sorption studies carried out with `dolomite-rich Culebra rock' by Triay and coworkers
 Sorption studies performed using pure dolomite from Norway and synthetic NaCl solutions by Brady and coworkers
 Transport studies using intact core samples of Culebra rock by Lucero and coworkers
Brush (1996) compiled and evaluated sorption data from these sources , and the selected Kd values were used in the CCA PA and CCA PAVT (Table 10-1). Brush and Storz (1996) subsequently updated these values because of slight errors in the calculated Kd values. The differences between the Kd values Brush (1996) and by Brush and Storz (1996) reported resulted from corrections of errors in the mass of dolomite used to calculate Kd values in the sorption studies by Brady and coworkers and corrections of errors in the density of brine Triay and coworkers used to calculate Kd values in the experiments (Brush and Storz 1996). The revised values from Brush and Storz (1996) were not available in time for inclusion in the CCA PA or PAVT. However, Brush and Storz (1996) stated that the relatively small changes in the Kd values were unlikely to significantly impact PA results.
Brush (1996) and Brush and Storz (1996) provided Kd values for actinide elements for both deep (Castile and Salado) brines and Culebra brines. For conservatism, only the ranges of Kds for the brine having the smaller mean value were used in PA. A uniform distribution was used for all Kd ranges in the CCA PA (Brush 1996; Brush and Storz 1996). Sorption experiments were conducted in the presence of organic ligands, but DOE did not revise the ranges of Kds to include the potential effects of organic ligands because they assumed that metals present in the waste (such as iron and nickel) would compete with actinides for the organic ligands and would limit the effects of ligands on actinide sorption (Brush 1996, Brush and Storz 1996).
Table 10-1.	Comparison of Matrix Kd Values for the CCA PA and PAVT With Matrix Kd Values for the CRA-2004 PA, PABC04, CRA-2009 PA and PABC09
                              Kd Range [m[3]/kg]
                         CCA PA and PAVT (Brush 1996)
                            CRA-2004 PA, PABC04 and
                                  CRA-2009 PA
                            (Brush and Storz 1996)
                          High Concentration Organics
                           (Brush and Storz 1996)[a]
                            PABC09 and CRA-2014 PA
                               (Clayton 2009)[b]
Am(III)
                                  0.02 - 0.5
                                  0.02 - 0.4
                               0.00505 - 0.00740
                                 0.005 - 0.4
Pu(III)
                                  0.02 - 0.5
                                  0.02 - 0.4
                                      --
                                  0.005 - 0.4
Th(IV)
                                   0.9 - 20
                                   0.7 - 10
                              0.000467 - 0.00469
                                  0.0005 - 10
U(IV)
                                   0.9 - 20
                                   0.7 - 10
                                      --
                                  0.0005 - 10
Np(IV)
                                   0.9 - 20
                                   0.7 - 10
                                      --
                                  0.0005 - 10
Pu(IV)
                                   0.9 - 20
                                   0.7 - 10
                                      --
                                  0.0005 - 10
Np(V)
                                  0.001 - 0.2
                                  0.001 - 0.2
                                0.00 - 0.00249
                                 0.00003 - 0.2
U(VI)
                                0.00003 - 0.03
                                0.00003 - 0.02
                                0.00 - 0.0101
                                0.00003 - 0.02
a  -  	SPC brine (Salado) with 0.0489 M acetate, 0.003417 M citrate, 1.1  10[-5] M EDTA, and 0.00288 M lactate; H-17 brine (Culebra) and ERDA-6 brine (Castile) with 0.0195 M acetate, 0.00340 M citrate, 1.46  10[-5] M EDTA, and 0.00076 M lactate
b  -  	Log-uniform distribution
EPA (1998c) evaluated the ranges and distributions of Kd values used in the CCA PA. EPA accepted the ranges used in the CCA, but disagreed with the uniform distribution for these ranges. EPA instead specified a log-uniform distribution for the Kd ranges for use in the PAVT.
Distribution Coefficients Used in the CRA-2004 PA, PABC04 and CRA-2009 PA

Hansen and Leigh (2003) provided a reconciliation of parameter values between the CCA (PAVT) and the CRA-2004 PA. Hansen and Leigh (2003) documented that there were no changes in the ranges of Kd values for U(VI), U(IV), Pu(III), Pu(IV), Th(IV), and Am(III) between the CCA PA and the PAVT. Hansen and Leigh (2003) noted the two errors in the procedures used to calculate the matrix Kds that were identified by Brush and Storz (1996), and reiterated the conclusion of Brush and Storz (1996) that these errors would have minimal effects on the CCA PA and the PAVT results.
The CRA-2004 PA, PABC04 and CRA-2009 PA used the revised Kd ranges from Brush and Storz (1996) (Table 10-1). During the review of the CRA-2004 PA and PABC04, EPA (2006c) evaluated the Kd ranges and log-uniform distribution of the data. The key references EPA (2006c) used in their evaluation of the matrix Kd values were Brush (1996), Brush and Storz (1996), Wall (2001) and Hansen and Leigh (2003).
In discussing the parameter baseline, including Kds, Wall (2001) appeared to use the EPA (1998d) conclusions to address key technical issues, such as the use of brines with lower pH than expected in the repository and the effect of organic ligands. EPA (1998d) agreed that the effect of organic ligands on actinide sorption could be ruled out because sufficient concentrations of non-actinide cations would be available to complex with these ligands. However, aqueous speciation data and modeling calculations available from the CRA-2004 (EPA 2006c, Section 8.0) indicated that solubilities of the +III and +V actinides could be affected by organic ligands. Consequently, EPA (2006c) re-examined the Kd values measured in brines with organic ligands (Brush and Storz 1996).
The reported values for low and intermediate organic ligand concentrations were judged to be the most applicable to WIPP repository conditions. The ranges of Kd values reported for Am(III) and U(VI) in the presence of organic ligands were found to be consistent with the ranges of Kd values used in the CRA-2004 PA and PABC04. However, values on the order of approximately 4 ml/g were observed for Th(IV) in the presence of organic ligands, whereas the range of values used in the CRA-2004 PA and PABC04 was 700 to 10,000 ml/g. This lower Kd for Th(IV) appeared to be inconsistent with the results of geochemical modeling because Th(IV) does not appear to be significantly complexed by organic ligands (EPA 2006c, Section 8.5). EPA (2006c) concluded that this inconsistency may have been the result of higher CO2 fugacities and lower pH in the experiments than are expected in the WIPP repository.
The data regarding the potential effects of organic ligands on actinide sorption were previously considered by EPA (1998b). At that time, EPA concluded that the results of the speciation calculations, Kd values reported in the literature and expected increased adsorption under alkaline conditions indicated that the Kd ranges exclusive of the organic ligands were sufficiently representative of actinide solid/liquid partitioning for modeling actinide transport in the Culebra.
Comparison of the Kd ranges used in the CCA PA and PAVT with the ranges used in the CRA-2004 PA, PABC04 and CRA-2009 PA, indicated that the differences were small (Table 10-1). Furthermore, the revised Kd ranges are more conservative because the lower Kd values should result in less sorption. EPA (2006c) determined that the revised Kd ranges used in the CRA-2004 PA and PABC04 were acceptable, because no new experimental sorption data were available, the changes to the Kd ranges were minor and conservative and these changes had been previously reviewed and found acceptable by EPA (1998b).
Distribution Coefficients used in the PABC09 and CRA-2014 PA

The CRA-2009 PA used the same ranges of Kd values and distributions as the CRA-2004 PA and PABC04 (DOE 2009, Appendix PA-2009, Section PA-<IP_ADDRESS>). However, higher ligand concentrations were predicted for the PABC09 than for previous PAs (Table 6-1). EPA (2010b) found that the predicted acetate and citrate concentrations for the PABC09 were similar to concentrations in the high-ligand-concentration experiments Brush and Storz (1996) reported, and the predicted EDTA concentration exceeded the concentrations reported for the high-ligand-concentration experiments.
The range of Kd values measured for Np(V) in the presence of high concentrations of organic ligands is compared to the range of Kd values Brush and Storz (1996) established in Table 10-1. Although the measured Kd values for Np(V) in the presence of high concentrations of organic ligands falls at the lower end of the Kd range used in PA, the effects on PA of lower Kd values for Np(V) are likely insignificant because of the relatively small amount of neptunium in the WIPP inventory. The range of Kd values measured for U(VI) with high concentrations of organic ligands is reasonably similar to the range selected for PA by Brush and Storz (1996). Consequently, EPA (2010b) concluded that the effects of organic ligands on the transport of Np(V) or U(VI) through the Culebra are not likely to be significant for PA.
For both Am(III) and Th(IV), however, EPA (2010b) observed that the Kd ranges measured in experiments with high concentrations of organic ligands were significantly lower than the Kd ranges used in the CCA PAVT, PABC04 and CRA-2009 PA calculations. Because of the importance of Am(III), Pu(III) and Pu(IV) to total releases from the repository, smaller Kd values for the +III and +IV oxidation state have the potential to significantly affect PA results. For that reason, EPA requested additional information from DOE regarding the possible effects of increased ligand concentrations and lower Kd values on +III and +IV actinide releases by transport through the Culebra (Comment 3-C-25, Cotsworth 2009c).
DOE re-evaluated the range of Kd values used to assess retardation of actinides during transport through the Culebra (Moody 2010b). There was no evidence that the upper bound should be changed. Consequently, the upper bounds for the Kd ranges were maintained, but the lower bounds were reduced to account for the possibility of higher organic ligand concentrations.

Clayton (2009) provided details regarding the changes in the Kd ranges. The lower limit for the U(VI) Kd range was not modified, because this value was already effectively equal to zero. Although neptunium is not currently included in the Culebra transport calculations, DOE updated its Kd range to maintain consistency. Because the Kd ranges sampled for PA are assumed to have a log-uniform distribution, assumption of a lower Kd limit of zero is not valid. Consequently, the lower limit of the U(VI) Kd was also assumed for the lower limit for the Np(V) Kd (Table 10-1). The Kd ranges used in the PABC09 are summarized in Table 10-1. The revised lower-bound values for the Kds used in the PABC09 are consistent with the lower limits of the ranges observed in the experiments with high concentrations of organic ligands. Consequently, EPA (2010b) found that the Kd ranges were appropriately conservative for use in the PABC09.

The Kd ranges sampled for the PABC09 were also used for the CRA-2014 PA (DOE 2014b, Appendix PA, Table PA-1). During review of the CRA-2014, EPA requested that DOE assess the possible effects on the Kd ranges of changed organic ligand concentrations since the PABC09 (Edwards 2014, Comment 1-23-1) and DOE responded with an assessment of the organic ligand inventory changes (Franco 2015b). The acetate, citrate and EDTA concentrations calculated for the CRA-2014 PA were consistent with the concentrations used for the PABC09 (Table 6-1). Oxalate concentrations for all brine volumes were higher in the CRA-2014 PA than in the PABC09. However, because aqueous speciation calculations indicate that oxalate is likely to precipitate as the Ca-oxalate solid whewellite and is unlikely to affect actinide speciation in WIPP brines (Brush and Domski 2013b), oxalate is not expected to significantly affect sorption in the Culebra. Additionally, the lower bounds of the sampled Kd ranges are quite low (Table 10-1), which would maximize Culebra transport.

Organic kitty litter was mistakenly used as an absorbent for nitrate salts for Waste Stream LA-MIN02-V.001 (NMED 2014) and 349 drums of this waste were placed in Panels 6 and 7 (Wallace 2014) which lead to the radionuclide release. EPA requested that DOE assess the potential effects of the organic kitty litter added to the LANL waste on the Culebra Kds (Edwards 2014, Comment 1-23-1). In their response to this comment (Franco 2015b), DOE deferred the Culebra Kd portion to their response to Comment 1-C-1 (Franco 2015c). DOE noted that the possible 41,000 lbs of kitty litter in the LANL waste cellulosic kitty litter makes up less than 0.4% of the currently expected total amount of cellulose and 0.1% of the total CPR based on a scaled full repository volume (Franco 2015c). DOE identified microbial degradation of the kitty litter and changes to the repository chemical conditions through CO2 production as the process likely to affect Culebra Kds. DOE stated that the excess MgO emplaced in the repository would adequately control CO2 generation by microbial degradation of cellulosic kitty litter and prevent any effects on Culebra Kds, which is reasonable. DOE also cited an earlier assessment of the effects of increased CPR on repository performance (Dunagan et al. 2005). However, this study was performed before parameters such as microbial degradation rates and probabilities were changed (Tables 3-9 and 3-10) and other PA changes have been made such as an updated water balance. Consequently, it is not clear that this older PA sensitivity analysis still applies. DOE did not directly address the potential effects of organics or surfactants that might be leached from the kitty litter, but only a small amount of waste included the kitty litter sorbent, which includes four waste streams placed in 507 drums (Shrader 2015). The small volume of waste likely precludes any significant effects on Culebra Kds.

No additional Culebra sorption data have been developed since the PABC09. Because the concentrations of acetate, citrate and EDTA in WIPP repository brines are consistent with the high-organic-ligand concentrations used in the Brush and Storz (1996) experiments, the Kd ranges sampled for the CRA-2014 PA continue to be appropriate.

Culebra Dolomite Distribution Coefficients Conclusions

 Radionuclides may reach the Culebra, the most transmissive member of the Rustler Formation. This could happen through brine flow up a borehole and will be affected by sorption onto the minerals along this pathway.
 Sorption data were collected from experiments using Culebra dolomite. The Kd values remain the same as those used in the PABC09 and reflect updated data on the effects of higher concentrations of organic ligands on actinide sorption.
 The Kd values likely provide a conservative assessment of actinide transport and continue to remain appropriate.

Effects of Heterogeneous Waste Loading

All solubility calculations carried out for WIPP PA assume that waste would be homogeneously loaded in the repository. At the time of the CCA, no information was available regarding the potential patterns of waste loading that might occur, and this assumption was appropriate. However, EPA noted during their review of the CRA-2004 PA that Panel 1 waste loading information indicated that the assumption of homogeneity was incorrect. Because shipments of waste placed in Panel 1 included an intensive campaign of waste shipments from the Rocky Flats site, 54% of the waste in Panel 1 was from that site (Leigh 2003).
EPA (2006c) evaluated the potential effects of waste heterogeneity on the results of PA. This evaluation included the potential effects on actinide solubility calculations and the MgO EF calculations. As described in Section 4.7.1, DOE has accounted for the potential effects of heterogeneous waste loading on the MgO EF by implementing an approved emplacement plan that ensures the required minimum EF is maintained. Kouba (2013) provided an analysis showing that the EF value in each panel exceeds the required minimum value of 1.2.
The Environmental Evaluation Group (EEG) questioned whether the assumption of homogeneous chemical conditions in the repository is appropriate (Oversby 2000). As an alternative to the approach used by DOE, EEG recommended calculating releases that would occur if a small number of waste drums containing relatively high concentrations of actinides and organic ligands are intersected by a borehole. This waste could be leached by drilling fluid, and if this borehole then reached an underlying brine pocket, EEG maintained that an unacceptable release of actinides could result. EEG recommended that DOE assess repository performance by calculating the range of actinide concentrations that could be released under this scenario, and multiplying by the probability of the scenario. This approach was considered on several occasions (EPA 1998a, TEA 2000, TEA 2001). EPA observed that WIPP PAs are based on a probabilistic approach, rather than the calculation of solubilities associated with a worst-case scenario as suggested by EEG, and this probabilistic approach had been peer-reviewed and found acceptable. EPA also found that the effects of waste heterogeneity on PA were adequately addressed by the uncertainties assigned to the actinide solubilities (EPA 1998a, TEA 2000, TEA 2001).
During review of the CRA-2004 PA, EPA requested additional information from DOE regarding the potential effects of heterogeneous waste loading on the assumption of homogeneous chemical conditions throughout the entire repository (Cotsworth 2004a, Comment G-12). DOE responded with an assessment of the potential effects of higher amounts of plutonium in Panel 1 than would be expected, based on homogeneous emplacement throughout a 10-panel repository (Detwiler 2004c). DOE stated that the only likely effect of this higher-than-expected loading of plutonium in Panel 1 would be increased radiolysis that could cause plutonium to speciate as plutonium(V) or plutonium(VI), instead of the reduced oxidation states predicted for the repository (see Section 5.0). However, because of microbial consumption of CPR and the amounts of metallic iron in the repository, including the large amounts of stainless steel in the pipe overpacks in Panel 1, DOE stated that reducing conditions and lower plutonium oxidation states would still be maintained in Panel 1.
Heterogeneous waste loading is unlikely to affect actinide solubilities if a relatively high proportion of the actinide inventory is present in a single panel, because of the assumption of chemical equilibrium with respect to the solubilities of actinide solid phases. However, the placement of waste with a high proportion of the organic ligand inventory in a panel could affect actinide solubilities. DOE has accounted for this possibility in the actinide solubility calculations by assuming the entire organic ligand inventory is dissolved in the amount of brine released from the repository (Brush and Domski 2013a). This approach ensures that the effects of heterogeneous waste placement are adequately accounted for in the actinide solubility calculations, provided that the inventory of organic ligands is bounded.
Use of shielded containers for disposal of RH waste will increase the lead inventory in panels filled after EPA's approval was received (Edwards 2011, 2013). Evaluation of the lead inventory increase associated with placement of candidate RH waste streams indicates that the repository-average lead inventory could increase to 17% of the mass of iron-based waste and packaging materials (Section 3.2). Because the excess lead will only be placed in panels filled after the approval of RH shielded containers, the lead inventory in those panels may be higher than 17%. Consequently, it would be appropriate to include bounding calculations for the CRA-2019 to evaluate possible increased lead inventory based on candidate waste forms that could be placed in shielded containers (Section 3.2). The principal effects of lead on PA are likely to be caused by gas generation through anoxic corrosion. Because the increased amount of lead and increased lead surface area will depend on the amounts of RH waste emplaced in shielded containers, the magnitude of the effects of increased lead on gas generation and repository performance cannot be quantitatively addressed at this time. However, it is reasonable to assume that the sampled uncertainty of the anoxic corrosion gas generation rates (Section 3.1.2) adequately bounds the effects of anoxic corrosion of the possible increase in the lead inventory. Consequently, the effects of heterogeneous placement of lead in the repository is unlikely to have major effects on PA.
The panel closures are expected to have extremely low permeabilities (Camphouse et al. 2012) and will effectively limit the contact of brine between panels. The effects of heterogeneous CPR inventory and iron inventory placement leading to variable gas generation between panels have been considered and found to be unlikely to significantly affect PA (TEA 2004). In addition, the heterogeneous distribution of organic ligands is conservatively bounded by assuming the entire inventory is dissolved in the mobilized brine volume. Consequently, the available information indicates that the effects of heterogeneous waste loading have been adequately addressed in PA.
In summary:

 The effects of heterogenous waste loading are unlikely to significantly affect actinide solubilities because of the assumption of chemical equilibrium between actinide solid phases.
 DOE accounts for organic ligands by dissolving the entire inventory in the minimum amount of brine needed for a release from the repository.
 Even though lead inventories may increase, uncertainty in gas generation rates likely bounds the effects of this increase. Consequently, lead is unlikely to have major effects on PA based on current information. By performing a bounding calculation for CRA-2019, DOE can deomonstrate this.
 Although panel closures will likely have extremely low permeabilities effectively limiting brine contact between panels, heterogeneous CPR and iron inventory, as well as gas generation, are unlikely to significantly affect PA and have been adequately been addressed.

Effects of Chemical Processes on Repository Water Balance

Chemical processes that may affect the quantity of water in the repository include microbial degradation of CPR, MgO hydration to form brucite or hydromagnesite, dehydration of hydromagnesite to form magnesite and anoxic corrosion of iron-based metal or other metals. For the CCA PAVT, PABC04 and PABC09, brine consumption from anoxic corrosion of iron-based metal was the only chemical process included in the repository water balance. SC&A (2008b) performed a qualitative evaluation of the possible effects of various chemical processes on brine generation and consumption, and concluded that the potential importance of CPR degradation, MgO hydration and carbonation and FeS precipitation on the water balance in the repository warranted additional evaluation.
EPA observed during their review of the CRA-2009 (DOE 2009) that the potential effects of processes other than anoxic corrosion on the repository water balance had not been addressed (Cotsworth 2009a, Comment 1-C-5). Consequently, EPA requested additional information from DOE regarding whether production of water by CPR degradation and consumption of water by brucite hydration and hydromagnesite persistence might significantly affect predicted repository performance. DOE acknowledged the potential for water consumption or production with reactions involving magnesium-based minerals and microbial degradation of CPR (Moody 2009a). DOE concluded that although it would be appropriate to include all water balance considerations in future PA calculations, the uncertainties associated with water balance changes caused by brucite hydration, hydromagnesite persistence in the repository and production of water by CPR degradation precluded adding these processes to the repository water balance calculations at that time. EPA (2010b) agreed that this conclusion was reasonable, but recommended DOE continue evaluating the effects of chemical processes on the repository water balance for the CRA-2014.
For the CRA-2014 PA, DOE (2014b) modified the repository water balance to include multiple chemical processes. In addition to the flow of water in and out of the repository, the chemical processes in the revised water balance included major brine production and consumption reactions (DOE 2014b, Appendix PA, Section PA-4.2.5). The reactions included in the water balance are: brine consumption by anoxic corrosion of iron-based metals, brine production by sulfidation of iron-based metals, brine production by microbial degradation of CPR materials, brine consumption by MgO hydration and brine production by hydromagnesite conversion to magnesite. Camphouse (2013b) summarized the reactions included in the water balance and Clayton (2013) provided justification for the implementation and stoichiometric coefficients for the water balance.
Anoxic Corrosion and Sulfidation of Iron-Based Metals

Anoxic corrosion consumes water, which has been accounted for in PAs preceeding the CRA-2014. Brine consumption through anoxic corrosion occurs via the reaction:

Fe(s) + 2 H2O(l) --> Fe(OH)2(s) + H2(g)
                                                                           (37)

Reaction (37) results in the consumption of two moles of water for each mole of iron consumed (Clayton 2013). The rates of this anoxic corrosion reaction in humid and inundated conditions are based on sampled parameters (Section 3.1.2).
The CRA-2014 PA assumes that H2S created by CPR degradation [reaction (11)] preferentially reacts with iron hydroxide versus metallic iron (CRA-2014 Appendix MASS, Section MASS-6.3). These sulfidation reactions are shown in reactions 6 and:

Fe(s) + H2S(g) --> FeS(s) + H2(g)
                                                                           (38)

The assumption that H2S preferentially reacts with Fe(OH)2(s) [reaction (6)] increases brine production and decreases gas production compared to the assumption that all or some of the H2S reacts with metallic iron [reaction (38)]. EPA requested supporting data for this water-balance assumption from DOE and an evaluation of the potential magnitude of its effects on the water balance (Edwards 2015b, Comment 2-C-3). In response, DOE (Bryson 2015a) cited the results of WIPP project-specific iron sulfidation experiments Telander and Westerman performed (1993, 1997). These test results showed that FeS(s) and not Fe(OH)2(s) formed during reaction of H2S with steel in the presence of Brine A, which provides reasonable justification for assuming H2S will preferentially react with any Fe(OH)2(s) present in the repository.
DOE also assumed for the CRA-2014 PA water balance that all H2S produced by CPR degradation will instantanously and completely react with the iron in steel to produce FeS(s) (Clayton 2013). EPA requested information justifying this assumption (Edwards 2015a, Comment 4-C-5; Economy 2016), noting that it appears to contradict evidence of steel passivation by H2S Telander and Westerman (1997) reported (Section 3.1.2). DOE agreed to remove reactions (6) and (38) from calculations of the water balance and gas production in PA by setting the stoichiometric coefficients to zero for Appendix PA equations PA.89 and PA.90 (DOE 2014b). Removing reaction (6) from PA calculations will increase the amount of gas in the repository and decrease the amount of brine. The effects of the change to the stoichiometric coefficients for the steel sulfidation reactions are addressed in Section 13.0 as part of the assessment of the CRA14_SEN4 PA.

Lead Corrosion

EPA requested additional information related to effects of the increased lead inventory from the use of shielded RH waste containers on gas generation and the water balance (Edwards 2015c, Comment 3-C-7). DOE's response to this comment (Bryson 2015b) is discussed in detail as part of the gas generation evaluation (Section 3.2). As previously noted for gas generation, the potential mass of lead compared to iron using the candidate waste streams identified by Crawford and Taggart (2007) indicates that lead corrosion may have a small effect on water balance. Because WIPP-specific lead corrosion rate data are available (Roselle 2013), EPA believes additional work is necessary to consider the effects of lead corrosion on water balance in the CRA-2019 PA calculations if bounding calculations using candidate waste forms indicate significant masses of lead will be present in the repository.

Microbial Degradation of CPR

The CPR microbial degradation reactions [reactions (10) and (11), Section 3.3.1) and rate parameters used to calculate gas generation for the CRA-2014 PA (Table 3-9) were used to calculate the production of water by CPR degradation (Camphouse 2013b). Because the moles of water produced differs for reaction (10) (denitrification) and reaction 11 (sulfate reduction), the average stoichiometry model (Wang and Brush 1996) is used to calculate the production of gas and water by CPR degradation:

[C6H10O5]/6 + microbes --> y H2(g) + z H2O + unknowns
                                                                           (39)

Clayton (2013) describes the calculation of y and z in equation (39) using the maximum amount of CPR that can be degraded within 10,000 years in a particular vector and the potential amounts of CPR that can be degraded by denitrification [reaction (10)] and sulfate reduction [reaction (11)]. This approach adequately incorporates the effects of CPR degradation on water balance.
Backfill Reactions

The MgO backfill reactions included in the water balance were: 1) MgO hydration to form brucite [reaction (14)], which consumes water; 2) the carbonation of brucite to form hydromagnesite [reaction (17)], which does not consume or produce water; and 3) conversion of hydromagnesite to magnesite [reaction (20)], which produces water (Camphouse 2013b). Clayton (2013) also included reaction of MgO directly to magnesite when brucite is not available:

MgO(s) + CO2(g) --> MgCO3(s)
                                                                           (40)

MgO Hydration Rate

The parameters used to represent the rate of MgO hydration to brucite (reaction 14, Section 4.1.1) were WAS_AREA:BRUCITEC in ERDA-6 brine, WAS_AREA:BRUCITES in GWB brine and WAS_AREA:BRUCITEH under humid conditions. Clayton (2013) developed these parameters using MgO hydration rates and associated uncertainties for each brine type developed by Nowak and Clayton (2007). EPA requested that DOE provide a copy of the Excel file used to calculate the MgO hydration rates (Edwards 2015b, Comment 2-C-7) and DOE provided this file (2-C-7 data spreadsheet.xlsx, Franco 2015a).
EPA noted that the MgO hydration rate data were based on experiments conducted with Premier MgO, whereas MgO in the repository has been provided by three vendors, with a relatively small amount from National Magnesia Chemicals and larger amounts from Premier Chemicals and Martin Marietta Magnesia Specialties (Edwards 2015b, Comment 2-C-6). Wall (2005) performed preliminary tests with the Martin Marietta MgO and concluded that it reacted to form brucite faster than Premier MgO, which indicates that using hydration rates obtained only with Premier MgO would underestimate the MgO hydration rate and overestimate the amount of brine in the repository. EPA requested that DOE provide the inundated and humid hydration rates for MgO from the three vendors, the potential effects of the various hydration rates on repository performance and the amounts of National Magnesia Chemicals, Premier MgO and Martin Marietta MgO that will be present in the WIPP repository at the time of closure, and assumptions regarding the future source(s) of MgO.
In their response, DOE reported that the percentages of the different MgO sources emplaced in WIPP were 0.4% National Magnesia Chemicals, 30.1% Premier Chemicals and 69.5% Martin Marietta (Franco 2015c). Because of the relatively small amount of National Magnesia Chemicals MgO, DOE has not characterized its hydration rate and the effects of its hydration rate on the repository water balance are likely to be minimal. However, Martin Marietta MgO represents a large fraction of the MgO currently emplaced in the repository and the proportion of this material is likely to become more significant over time because Martin Marietta is the current MgO supplier. DOE acknowledged that the hydration rate of Martin Marietta MgO is higher than Premier MgO (Franco 2015c). Deng et al. (2009) reported some results of Martin Marietta MgO hydration experiments, but Franco (2015c) stated that these experiments had not been completed. Franco (2015c) asserted that using the slower hydration rates for Premier MgO was a conservative assumption, but did not provide evidence to substantiate this claim. Franco (2015c) also stated that releases did not appear to be sensitive to the MgO hydration rates based on sensitivity analyses of the sampled PA parameters (Kirchner 2013).
During an EPA/DOE technical exchange meeting in February 2016, EPA requested that DOE provide additional information on the status of the Martin Marietta MgO hydration experiments. DOE later stated that the Martin Marietta hydration rate data had "not been processed into the form that is usable for PA" (Shrader 2016). However, DOE committed to combining the Martin Marietta MgO hydration rate data with the Premier MgO hydration rate data for use in future PAs.

MgO and Brucite Carbonation Rates

The reaction rates for the MgO and brucite carbonation reactions are defined by the sampled CPR degradation rate because the available CO2 is produced by CPR degradation. The Gas Generation conceptual model includes the assumption that essentially all CO2 is rapidly consumed by reaction with the MgO backfill (SCA 2008b). DOE assumed for the CRA-2014 PA that CO2 preferentially reacts with brucite instead of unreacted MgO via the carbonation (see reaction 17).
For the stoichiometric coefficients used in PA, this reaction was normalized by Clayton (2013) to the moles of CO2:

1.25 Mg(OH)2(s) + CO2 --> 0.25 Mg5(CO3)4(OH)2::4H2O(s)
                                                                           (41)

EPA observed that assuming CO2 preferentially reacts with brucite instead of with MgO:

1.25 MgO(s) + CO2 + 1.25 H2O --> 0.25 Mg5(CO3)4(OH)2::4H2O(s)
                                                                           (42)

increases brine in the repository because more brine is consumed by reaction (42) than by reaction (41). EPA requested that DOE provide supporting data for this assumption and an evaluation of the potential magnitude of the effects of this assumption on the water balance (Edwards 2015b, Comment 2-C-3). In response, DOE cited WIPP-specific experiments carried out to evaluate MgO hydration and carbonation (Xiong and Snider 2003, Xiong and Lord 2008). The cited experiments appear to substantiate the assumption that brucite to hydromagnesite is the reasonably expected pathway for carbonation. The DOE response noted the differences in the results of the CRA14-BV and CRA14-0 PA calculations, which only differed in the implementation of updated iron corrosion rates that reduced the rate of brine consumption and implementation of the revised water balance calculations. DOE stated that the overall impact of the changes in iron corrosion rates and water balance calculations were small.
Clayton (2013) stated that "In the event that CO2 generation is occurring, but brucite is not available in BRAGFLO simulations, MgO will be converted directly to magnesite:" EPA has repeatedly reviewed the potential for magnesite formation in the WIPP repository directly from MgO and determined that magnesite formation directly from MgO will not occur under WIPP conditions and the formation of hydromagnesite should be assumed instead of magnesite (EPA 1998d, SCA 2008, EPA 2010b). EPA stated that correctly including the formation of hydromagnesite [reaction (42)] instead of magnesite [reaction (40)] would decrease the amount of water in the repository and asked that DOE assess the effects on the water balance of incorrectly assuming the formation of magnesite instead of hydromagnesite (Edwards 2015c, Comment 3-C-6). In response, DOE stated that this assumption remained in the BRAGFLO code to be compatible with previous versions and to reduce potential numerical instabilities (Bryson 2015b). DOE also stated that the direct conversion of MgO to magnesite occurred only in the first 20 years when CO2 has been generated but no brine is yet available. DOE calculated that the potential decrease in water is a maximum of 0.081 m[3], which is insignificant to the overall water balance.
Hydromagnesite Conversion Rate

DOE (Clayton 2013) formulated the conversion reaction from hydromagnesite to magnesite for inclusion in the BRAGFLO calculations as:

Mg5(CO3)4(OH)2::4H2O(s) --> 4 MgCO3(s) + Mg(OH)2(s) + 4 H2O
                                                                           (43)

Clayton (2013) calculated the range for the hydromagnesite conversion rate used in the CRA-2014 PA from reaction times of 100 years to 10,000 years based on a general statement by EPA (1998d) that hydromagnesite would undergo slow conversion to magnesite over hundreds to thousands of years (Table 12-1). This represents a selective reading of EPA (1998d), because EPA (1998d) also noted that "deposits of hydromagnesite are found in some evaporite basins dated as late Quaternary in age (<23.7 million years) (Stamatakis, 1995), indicating that the hydromagnesite has persisted in a metastable state for a long period with only partial conversion to magnesite and other magnesium carbonates." DOE also ignored a more recent EPA review regarding hydromagnesite metastability (SCA 2008a). SCA (2008a) reviewed the available experimental and natural analogue data and found that "a slow rate of hydromagnesite to magnesite conversion in WIPP could result in the persistence of hydromagnesite throughout much, if not all, of the 10,000-year regulatory period." SCA (2008a) concluded that hydromagnesite conversion is best represented by a range of zero conversion (only hydromagnesite remains after 10,000 years) to complete conversion (only magnesite remains after 10,000 years), with a uniform distribution across this range.
Table 12-1.	Hydromagnesite Conversion Rates (WAS_AREA:HYMAGCON)
                                 Minimum Rate
                             (mol kg[-1] sec[-1])
                                 Maximum Rate
                             (mol kg[-1] sec[-1])
                                 Reaction Time
                                    (years)
                                    Source
                                6.8 x 10[-12]
                                6.8 x 10[-10]
                                100  -  10,000
                                Clayton (2013)
                                       0
                              < 6.8 x 10[-10]
                              > 100 - infinite
                                 Peake (2016a)
                                5.7 x 10[-11]
                                 3.7 x 10[-9]
                                 18  -  1,180
                                Shrader (2016)
                                       0
                                3.4 x 10[-10]
                                200 - infinite
                                This evaluation

EPA noted that the minimum reaction time (maximum reaction rate) for the hydromagnesite conversion reaction was uncertain and requested that DOE provide additional justification for the selection of the hydromagnesite reaction rate limits used in the CRA-2014 PA (Edwards 2015b, Comment 2-C-4; Peake 2016a, Addendum to Comment 2-C-4(a)). EPA recommended a minimum reaction rate of 0 mol kg[-1] sec[-1] (Table 12-1) to include the possibility of no hydromagnesite conversion to magnesite during the 10,000-year repository performance period (Peake 2016a). EPA also requested that DOE re-evaluate the maximum reaction rate (consistent with a minimum reaction time of greater than 100 years) that is consistent with the available data (Peake 2016a).