U. S. ENVIRONMENTAL PROTECTION AGENCY
                            Washington, D.C. 20460

                                   OFFICE OF
                              CHEMICAL SAFETY AND
                             POLLUTION PREVENTION

                                                                PC Code: 128850
                                                             DP Barcode: 387412
                                                             Date: May 30, 2012

MEMORANDUM

SUBJECT:	Refined Drinking Water Assessment for Glufosinate-ammonium Use on Rice

TO:		Kathryn Montague, Product Manager
      Dan Kenny, Branch Chief
      Herbicide Branch
		Registration Division (7505P)

		Barbara Madden, Team Leader
		Risk Integration Minor Use, and Emergency Response Branch
		Registration Division (7505P)

      William Donovan, Risk Assessor
      Michael Metzger, Branch Chief
      Risk Assessment Branch V
      Health Effects Division (7509P)

FROM:	Chuck Peck, Environmental Engineer
		Environmental Risk Branch IV
		Environmental Fate and Effects Division (7507P)

REVIEWED	Jim Carleton, Ph.D., Senior Scientist
BY:		Environmental Risk Branch IV
		Environmental Fate and Effects Division (7507P)

 APPROVED	Meredith Laws, Acting Branch Chief
BY:		Environmental Risk Branch IV
		Environmental Fate and Effects Division (7507P)

Executive Summary
      Glufosinate-ammonium (CAS 77182-82-2), an herbicide, is an active ingredient in (1) Liberty[(R)] 280 SL (EPA Reg. No. 264-829), used for broadcast burndown applications prior to planting or pre-emergence of rice and for use on glufosinate-tolerant rice, and (2) Liberty[(R)] (EPA Reg. No. 264-660), used for weed control on Liberty(R) tolerant rice.  A previous refined Tier II drinking water assessment (drinking water memo dated October 20, 2010; DP Barcode 381992) evaluated applications of glufosinate-ammonium to rice and reported acute and chronic estimated drinking water concentrations (EDWCs) of 831 and 615 ug/L, respectively.  These EDWCs exceeded the chronic drinking water level of concern (75 ug/L, revised in 2012 based on updated consumption data in the Dietary Exposure Evaluation Model (DEEM) to 97 ug/L).

      Since the publishing of these EDWCs, Bayer Crop Science (BCS) has made revisions to the labels and OPP has reevaluated the residues of concern and updated modeling parameters in an effort to refine the potential exposure estimates. In May 2012, BCS submitted proposed label revisions on the Liberty[(R)] and Liberty[(R)]280 SL labels for burndown and postemergent applications to rice and for rice-seed propagation.  Analysis of environmental fate data indicated incorrect input values had been used in the previous drinking water assessment and, as such, these values have been updated (DP Barcode 387412, 09/06/2011).  In January 2012, OPP's Health Effects Division's (HED) Residues of Concern Knowledgebase Subcommittee (ROCKS) met to re-evaluate the inclusion of three glufosinate-ammonium degradates [3-methylphosphinico-propionic acid (MPP, HOE 061517), 2-methylphosphinico-acetic acid (MPA, HOE 064619), and 2-acetamido-4-methylphosphinico-butanoic acid (NAG, HOE 085355/HOE 099730)] into the drinking water assessment, as had been done in previous assessments.  Based on the recommendation of the ROCKS, only MPP is retained as a degradate of concern.  Additionally, based on their different modes of action, drinking water estimates for MPP and glufosinate-ammonium are derived separately (D397644, 03/29/2012).

      The revised Liberty[(R)] 280 SL label allows for a maximum of 79 fl oz product/acre/season (1.44 lbs ai/acre/season) to be applied for rice seed propagation to dry fields with a 7-day post-application holding period for flood waters.  This application may be split into two applications of approximately 40 fl oz product/acre/season (0.72 lbs ai/acre/season), applied with a 10 to 14-day retreatment interval (RTI), where the first application is made to a dry field and the second application to a flooded field.  If this is done, then longer holding periods for the flood water are required: 55-days for 4-inch paddy depths and 30-days for 8-inch paddy depths.

      For applications on glufosinate-tolerant rice plants, a maximum of 48 fluid ounces (fl oz) product/acre/season (0.88 lbs ai/acre/season) is allowed, and a 7-day post-application flood water holding period is required.  If the maximum single application rate of 0.88 lbs ai/acre is used, then the application must be made to dry fields.  If the applications is split into two applications of 24 fl oz product (0.44 lbs ai/acre), with the second application made from 10-14 days after the first application, then the applications may be made to either dry or flooded fields.  If two flooded-field applications of 0.44 lbs ai/acre are made, then the paddy depth must be at least 8 inches.  If the applications are to dry fields, then the paddy depth must be at least 4 inches.  A 7-day post-application holding period of paddy water is required for both dry and flooded-field applications.  Finally, for burndown applications the Liberty[(R)] 280 SL label allows a maximum single application rate of 36 fl oz product/acre (0.66 lbs ai/A), with a 7-day post-application holding period for flood waters.

      The application rates for the Liberty[(R)] label are similar to those prescribed on the Liberty[(R)] 280 SL label.  For rice seed propagation, the Liberty[(R)] label allows for a maximum of 112 fl oz product/acre/season (1.46 lbs ai/acre/season) to be applied to dry fields with a 7-day post-application holding period.  This application may be split into two applications of 56 fl oz product/acre/season (0.73 lbs ai/acre/season), applied with a 10 to 14-day RTI, where the first application is made to a dry field and the second application applied to a flooded field, but longer holding periods for flood water are incurred: 55-days for 4-inch paddy depths and 30-days for 8-inch paddy depths.  For applications on glufosinate-tolerant rice, a maximum of 68 fl oz product/acre/season (0.89 lbs ai/acre/season) can be made, requiring a 7-day post-application flood water holding period.  If the maximum single application rate of 0.89 lbs ai/acre is used, the application must be made to dry fields.  If the applications is split into two applications of 34 fl oz product (0.44 lbs ai/acre), with the second application made from 10-14 days after the first application, the applications may be made to dry or flooded fields.  If two flooded-field applications of 0.44 lbs ai/acre are made, the paddy depth must be at least 8 inches in depth; if dry applications are made, a minimum 4-inch paddy depth is required.  A 7-day post-application holding period of paddy water is required for dry and flooded-field applications.  The Liberty[(R)] label does not allow for burndown applications for rice.

      EFED modeled applications of glufosinate-ammonium to dry and flooded fields as specified on the two labels.  The scenarios modeled are provided in Table 1. Given regional practices in rice production, 4-inch and 8-inch paddies were modeled along with the holding periods specified on the labels.

	EFED estimated acute EDWCs for glufosinate-ammonium and MPP using the refined Tier I Rice Model and Pesticide Flooded Application Model (PFAM) without the index reservoir.  Several methods were employed to estimate chronic EDWCs.   In the first method, the acute concentrations from PFAM without the index reservoir were allowed to degrade over a 365-day period, using the aerobic aquatic degradation half-lives, and the average concentration over the one-year period was calculated.  Using this method, chronic values are approximately 76% and 3% lower than the acute values for glufosinate-ammonium and MPP, respectively.  In the second method, EFED evaluated monitoring data for other pesticides (molinate, thiobencarb, and fipronil) used predominantly for rice growing to assess trends in chronic and acute concentrations. These data indicated that long-term average concentrations (chronic) could be 50 to 90% lower than the peak concentrations (acute).  EFED applied these factors to the PFAM without-the-index-reservoir acute concentrations to estimate chronic concentrations.  Lastly, EFED examined the use of PFAM, coupled with the index reservoir, to estimate acute and chronic concentrations in a receiving waterbody.  Reductions in peak to annual average concentrations using PFAM with the index reservoir were in the range of 73-85% and 4-47% for glufosinate-ammonium and MPP, respectively.  It should be noted that the use of PFAM in estimating drinking water concentrations in surface waters from rice paddies is still under evaluation. However, as the effluent concentrations derived in PFAM have been through internal and external review and evaluation, EFED recommends the use of the acute EDWCs derived from PFAM without-the-index-reservoir, and chronic EDWCs derived using aerobic aquatic degradation kinetics, as conservative estimates for glufosinate-ammonium and MPP in drinking water.  The highest estimates for the use scenarios are provided in Table 1.  A full range of EDWCs for different uses of glufosinate-ammonium on rice and different areas of the country derived using PFAM without-the-index-reservoir is provided in Table 11.  It should be noted that the highest modeled EDWCs, as reported in Table 1, occurred in California.  While rice is grown in California, the Liberty[(R)] 280 SL and Liberty[(R)] products are currently not registered in California and, therefore, are not used for weed control on rice in California.

      For glufosinate-ammonium and MPP, flooded applications generated the highest PFAM concentrations, followed by the dry application scenarios.  This is different than the trend of results from the refined Tier I analysis, where the highest single application rate (1.46 lbs ai/A) generated the highest acute EDWCs, with all other applications generating similar acute EDWCs.  The glufosinate-ammonium PFAM concentrations in the flooded applications were similar to those obtained with the refined Tier I model, while the dry applications were significantly lower (approximately 82%).  For MPP, the PFAM concentrations for dry applications were significantly lower (approximately 99%), while reductions in the flooded applications were comparable to the dry glufosinate-ammonium applications (approximately 82%).  It appears that for glufosinate-ammonium and MPP during flooded applications, the majority of the compound remains in the water column after application, resulting in higher concentrations than occur following dry applications.  A comparison of these PFAM results to maximum water column concentrations reported in an aquatic field dissipation (AFD) study for glufosinate ammonium, adjusted to a 4-inch paddy depth, indicates that the PFAM concentrations were similar to those in the AFD study for glufosinate-ammonium (83 μg/L versus 93 μg/L for a dry application and 375 μg/L versus 288 μg/L for a flooded application of 0.44 lbs ai/A, twice).  For MPP, the concentrations were similar for the flooded application (40 μg/L versus 42 μg/L), but much lower for the dry applications (2.8 μg/L versus 102 μg/).

Table 1.EDWCs for glufosinate-ammonium use on rice derived using PFAM (maximum values appear in bold)
Use rate, Number of apps, retreatment interval, water holding period, application timing
                                  Acute EDWC
                                    (ug/L)
                        Annual Average EDWC[1] (ug/L)

                                     4"
                                     8"
                                     4"
                                     8"
                             Glufosinate-ammonium
1.46 lbs ai/acre, 1, NA, 7 days, dry
                                      173
                                      86
                                      42
                                      21
0.73 lbs ai/acre, 2, 10, 30, dry and flooded[2]
                                      NA
                                      320
                                      NA
                                      78
0.73 lbs ai/acre, 2, 10, 55, dry and flooded[2]
                                      390
                                      NA
                                      95
                                      NA
0.89 lbs ai/acre, 1, NA, 7 days, dry
                                      106
                                      53
                                      26
                                      13
0.66 lbs ai/acre, 1, NA, 7 days, dry
                                      74
                                      38
                                      18
                                       9
0.44 lbs ai/acre, 2, 10 days, 7 days, dry
                                      88
                                      45
                                      22
                                      11
0.44 lbs ai/acre, 2, 10 days,7 days, flooded
                                      NA
                                      375
                                      NA
                                      92
                                      MPP
1.46 lbs ai/acre, 1, NA, 7 days, dry
                                      5.7
                                      2.9
                                      5.5
                                      2.8
0.73 lbs ai/acre, 2, 10, 30, dry and flooded[2]
                                      NA
                                      88
                                      NA
                                      85
0.73 lbs ai/acre, 2, 10, 55, dry and flooded[2]
                                      183
                                      NA
                                      177
                                      NA
0.89 lbs ai/acre, 1, NA, 7 days, dry
                                      3.5
                                      1.8
                                      3.4
                                      1.7
0.66 lbs ai/acre, 1, NA, 7 days, dry
                                      2.2
                                      1.1
                                      2.1
                                      1.1
0.44 lbs ai/acre, 2, 10 days, 7 days, dry
                                      3.0
                                      1.5
                                      2.9
                                      1.5
0.44 lbs ai/acre, 2, 10 days,7 days, flooded
                                      NA
                                      45
                                      NA
                                      44
 1. Annual average EDWC calculated by taking acute EDWC and allowing it to degrade for 365 days, using the aerobic aquatic degradation half-life, and then taking the average value over the 365-day period.
2. For this scenario the first application is made to a dry field, while the second application is made to a flooded field.
NA  -  not allowed.
Background and Purpose of this Drinking Water Assessment
      Glufosinate-ammonium (CAS 77182-82-2) is an active ingredient in Liberty[(R)] 280 SL (EPA Reg. No. 264-829) and Liberty[(R)] (EPA Reg. No. 264-660).  Among their many uses, both products can be used for control of weeds in rice.  Glufosinate-ammonium is a nonselective herbicide that is applied post-emergent to weeds and pre-emergent to rice that is not genetically modified to be tolerant to glufosinate-ammonium, or post-emergent to rice that is genetically modified to be tolerant to glufosinate-ammonium.  A previous refined Tier II drinking water assessment (DP Barcode 381992, October 20, 2010) evaluated applications of glufosinate-ammonium to rice and reported acute and chronic estimated surface water concentrations  of total toxic residues of 831 and 615 ug/L, respectively.  These EDWCs exceeded the chronic drinking water level of concern (75 ug/L, revised in 2012 based on updated consumption data in the Dietary Exposure Evaluation Model (DEEM) to 97 ug/L).  In response to this drinking water assessment, Bayer Crop Science (BCS) submitted label revisions to the Liberty[(R)]280 SL and Liberty[(R)] labels.  BCS also provided information to update the organic-carbon normalized distribution coefficients (KOCs) used in the previous assessment.  OPP also reconsidered the inclusion of three glufosinate-ammonium degradates [3-methylphosphinico-propionic acid (MPP, HOE 061517), 2-methylphosphinico-acetic acid (MPA, HOE 064619), and 2-acetamido-4-methylphosphinico-butanoic acid (NAG, HOE 085355/HOE 099730)] in its assessment.  Based on a re-evaluation by HED's ROCKS, only glufosinate-ammonium and its degradate MPP are considered residues of concern and, because of their different modes of action, should be assessed separately (D397644, 03/29/2012).  As a result of these revisions to the label, fate parameters, and residues of concern, a revised drinking water assessment was conducted.
 Use Characterization
      The revised Liberty[(R)] 280 SL label allows for a maximum of 79 fl oz product/acre/season (1.44 lbs ai/acre/season) to be applied for rice seed propagation to dry fields with a 7-day post-application holding period for flood waters.  This application may be split into two applications of approximately 40 fl oz product/acre/season (0.72 lbs ai/acre/season), applied with a 10 to 14-day retreatment interval (RTI), where the first application is made to a dry field and the second application to a flooded field.  If this is done, then longer holding periods for the flood water are required: 55-days for 4-inch paddy depths and 30-days for 8-inch paddy depths.

      For applications on glufosinate-tolerant rice plants, a maximum of 48 fluid ounces (fl oz) product/acre/season (0.88 lbs ai/acre/season) is allowed, and a 7-day post-application flood water holding period is required.  If the maximum single application rate of 0.88 lbs ai/acre is used, then the application must be made to dry fields.  If the applications is split into two applications of 24 fl oz product (0.44 lbs ai/acre), with the second application made from 10-14 days after the first application, then the applications may be made to either dry or flooded fields.  If two flooded-field applications of 0.44 lbs ai/acre are made, then the paddy depth must be at least 8 inches.  If the applications are to dry fields, then the paddy depth must be at least 4 inches.  A 7-day post-application holding period of paddy water is required for both dry and flooded-field applications.  Finally, for burndown applications the Liberty[(R)] 280 SL label allows a maximum single application rate of 36 fl oz product/acre (0.66 lbs ai/A), with a 7-day post-application holding period for flood waters.

      The application rates for the Liberty[(R)] label are similar to those prescribed on the Liberty[(R)] 280 SL label.  For rice seed propagation, the Liberty[(R)] label allows for a maximum of 112 fl oz product/acre/season (1.46 lbs ai/acre/season) to be applied to dry fields with a 7-day post-application holding period.  This application may be split into two applications of 56 fl oz product/acre/season (0.73 lbs ai/acre/season), applied with a 10 to 14-day RTI, where the first application is made to a dry field and the second application applied to a flooded field, but longer holding periods for flood water are incurred: 55-days for 4-inch paddy depths and 30-days for 8-inch paddy depths.  For applications on glufosinate-tolerant rice, a maximum of 68 fl oz product/acre/season (0.89 lbs ai/acre/season) can be made, requiring a 7-day post-application flood water holding period.  If the maximum single application rate of 0.89 lbs ai/acre is used, the application must be made to dry fields.  If the applications is split into two applications of 34 fl oz product (0.44 lbs ai/acre), with the second application made from 10-14 days after the first application, the applications may be made to dry or flooded fields.  If two flooded-field applications of 0.44 lbs ai/acre are made, the paddy depth must be at least 8 inches in depth; if dry applications are made, a minimum 4-inch paddy depth is required.  A 7-day post-application holding period of paddy water is required for dry and flooded-field applications.  The Liberty[(R)] label does not allow for burndown applications for rice.

      Table 2 summarizes the uses on the proposed labels.

Table 2. Labeled uses for glufosinate-ammonium on rice
                                      Use
                    Maximum Application Rate (lbs ai/acre)
                            Number of Applications
                         Minimum Retreatment Interval
                         Minimum paddy depth (inches)
                             Flooded or dry field
                      Minimum Water Holding Period (days)
                              Liberty[(R)] 280 SL
Rice Seed
                                     1.44
                                       1
                                      NA
                                       4
                                      Dry
                                       7

                                     0.73
                                       2
                                      10
                                       4
                                1[st] app dry,
                               2[nd] app flooded
                                      55

                                     0.73
                                       2
                                      10
                                       8
                                1[st] app dry,
                               2[nd] app flooded
                                      30
Glufosinate-tolerant Rice
                                     0.88
                                       1
                                      NA
                                       4
                                      Dry
                                       7

                                     0.44
                                       2
                                      10
                                       4
                                      Dry
                                       7

                                     0.44
                                       2
                                      10
                                       8
                                    Flooded
                                       7
Burndown
                                     0.66
                                       1
                                      NA
                                       4
                                      Dry
                                       7
                                  Liberty[(R)]
Rice Seed
                                     1.46
                                       1
                                      NA
                                       4
                                      Dry
                                       7

                                     0.73
                                       2
                                      10
                                       4
                                1[st] app dry,
                               2[nd] app flooded
                                      55

                                     0.73
                                       2
                                      10
                                       8
                                1[st] app dry,
                               2[nd] app flooded
                                      30
Glufosinate-tolerant Rice
                                     0.89
                                       1
                                      NA
                                       4
                                      Dry
                                       7

                                     0.44
                                       2
                                      10
                                       4
                                      Dry
                                       7
Rice Seed
                                     0.44
                                       2
                                      10
                                       8
                                    Flooded
                                       7

       Glufosinate-ammonium use in conjunction with rice production is limited, with less than 500 lbs used on rice between 2007 and 2010 (USEPA, 2011).  Data on the amount of use of glufosinate-ammonium on tolerant rice are not readily available.  It should be noted, though, that usage of glufosinate-ammonium on rice can be inferred based on the planting of rice that is tolerant to glufosinate-ammonium (e.g., Liberty Link Rice).  According to the available public information, only two liberty link rice lines, LL62[(R)] and LL06[(R)], have been released for planting.  Because of concerns in the European Union and other foreign markets resulting from contamination of conventional rice with genetically engineered rice, detected in shipments in 2006, growing genetically engineered rice has not become economically popular (Lemaux, date unknown).  As such, one could infer that if farmers are not growing glufosinate-tolerant rice, then use of glufosinate-ammonium on glufosinate-tolerant rice plants, or use for propagation of glufosinate-tolerant rice seed is also unlikely.  Data on the amount of use of glufosinate-ammonium as a burndown treatment are not available.

Environmental Fate
      A complete summary of the environmental fate of glufosinate-ammonium may be found in Table 3.  Glufosinate-ammonium is stable to hydrolysis and aqueous photolysis, and degrades slowly in anaerobic aquatic environments (half-life of 415 days) (MRIDs 40345656, 41323115, 46258601).  Primary routes of degradation for glufosinate-ammonium are soil photolysis (half-life = 17 days), aerobic soil metabolism (half-lives ranging from 8.5 to 23 days) and aerobic aquatic metabolism (half-lives ranging from 1 to 87 days) (MRIDs 41920102, 41323119, 40345659A, 40345660, and 45204402/01).  Glufosinate-ammonium is mobile in sand (organic carbon-normalized soil partition coefficient, Koc, = 16.5 L/kgoc) and moderately mobile in silt loam soils (Kocs of 268 and 605 L/kgoc) according to the FAO classification system (FAO, 2000; MRID 40345662/48394101).  Koc values are somewhat less variable than Kd values for glufosinate-ammonium, as the coefficient of variation (CV) across the three soils for the organic carbon partition coefficient (99.7%) is less than that for the soil-water partition coefficient (116%).  Therefore, the Koc model is more appropriate for glufosinate-ammonium. In several terrestrial field dissipation (TFD) studies, glufosinate-ammonium dissipated from the upper six inches of soil with half-lives of 1-2 weeks (MRIDs 43110402, 43766915, 43766916, 47542601).  This range of dissipation rates is generally consistent with those expected from the submitted fate and transport studies.  In terrestrial field dissipation studies, glufosinate-ammonium did not leach further than six inches in loam or clay soils or further than 24 inches in a sand soil.  Major degradates (e.g., those that made up >=10% applied parent equivalents) of glufosinate-ammonium include 3-methylphosphinico-propionic acid (MPP, HOE 061517), 2-methylphosphinico-acetic acid (MPA, HOE 064619), and 2-acetamido-4-methylphosphinico-butanoic acid (NAG, HOE 085355).  Based on findings by HED's ROCKS, only the parent and MPP are considered in this assessment (D397644, 03/29/2012).

      An aquatic field dissipation (AFD) study (MRID 45204403) was conducted to simulate applications to rice.  Two applications of 0.45 lb ai/A of glufosinate-ammonium were made to bare ground in Louisiana in late August followed by flooding and holding of the water for 60 days.  Additionally, two applications of 0.45 lb ai/A of glufosinate-ammonium were made to a flooded, non-cropped field in California in late August.  At both sites, the first application was made at a time corresponding to the 2- to 4-leaf stage of rice (typically 14-28 days after planting) and the second application was made 14 days later, corresponding to the 2- to 3-tiller stage.  At both test sites, applications were made using broadcast ground equipment.  In Louisiana, the plot was flooded within 24 hours of the second application.  In California, the plot was flooded prior to the first application and both applications were made to the water.  At both test sites, the plots were maintained under static water conditions after flooding and the water depth was maintained by addition of water as necessary to a depth of 12 inches.  It should be noted that this depth is greater than depths reported in the crop profiles for California (4-8 inches; Hill, 1997) and Louisiana (4 inches; Ship, 2002).  Measured concentrations in these studies are expected to be lower than what would occur in water bodies with smaller depths.  Plots were drained 60 days after the second application in Louisiana and 90 days after the second application in California.  At the Louisiana site, the maximum concentration of glufosinate-ammonium in the flood water was 31 ug/L (Day 4).  In California, the maximum concentration of glufosinate-ammonium in the flood water was 96 ug/L (Day 0).  If these concentrations were adjusted for a 4-inch paddy depth, the maximum glufosinate-ammonium concentrations in the water would be 93 ug/L (LA) and 288 ug/L (CA).

      Based on comments from the registrant (MRID 48394101), the Kocs previously used by EFED were based on organic matter and not organic carbon content.  The values used in this assessment and presented in Tables 3 and 4 have been corrected by taking the previous values and multiplying by 1.72, an assumed, standard correction factor for organic matter to organic carbon conversion, as used in OPPTS Guideline 835.1230.

Table 3. Physical/chemical properties and environmental fate data for glufosinate-ammonium.
                                   Property
                                     Value
                                    Source
                                   Comments
Chemical Name
           Ammonium-(2RS)-2-amino-4-(methylphosphinato)butyric acid
                                       -
                                       -
Molecular Weight (g/mol)
                                     198.2
                                       -
                                       -
Solubility in Water (mg/L, 20°C)
                                  1.37x10[6]
                                 MRID 00263025
                                       -
Vapor Pressure (torr, 25°C)
                                <7.5x10[-9]
                                 MRID 44032901
                                       -
Henry's Law Constant
(atm-m[3]/mole)
                                1.43 x 10[-12]
                                       -
       Estimated using vapor pressure, molecular weight and solubility.
Hydrolysis Half-life (25°C) (days)
                 No significant degradation at pH 5, 7, and 9
                                 MRID 40345656
                                       -
Aqueous Photolysis Half-life (@ pH 7 (days)
                          No significant degradation
                                 MRID 41323115
                                       -
Soil Photolysis Half-life (days)
                                      17
                                 MRID 41920102
                                       -
Aerobic Soil Metabolism Half-life (days)
    8.5 (sandy loam)
    8.7 (silt loam)
    8.8 (loamy sand)
                                 MRID 41323119
                               1.62 ppm applied

    20.6 (sandy loam)
    21.4 (silt loam)
    23.0 (sandy loam)
                                MRID 40345659A
                               7.23 ppm applied
Aerobic Aquatic Metabolism Half-life (days)
    38 (sand)
    57 (silt loam)
                                     MRID
                                   40345660
                                8.3 ppm applied

    11 (loam)
    87 (sand)
                                     MRID
                                  45204402/01
                                1 ppm  applied

    1 (sand)

                                0.1 ppm applied
Anaerobic Aquatic Metabolism Half-life (days)
    415
                                 MRID 46258601
                                       -
Organic Carbon-Normalized Soil Partition Coefficient (mL/goc) (Koc)
    16.5 (sand)
    268 (silt loam)
    605 (silt loam)
                                MRID 40345662/
                                  48394101[1]
                                      --
Terrestrial field dissipation half-lives (days)
    8 (cropped plot)
                                 MRID 43110402
                                       -

    16.5 (cropped plot)
    15.1 (bare ground)
                                 MRID 43766915

    10.3 (cropped plot)
    10.6 (bare ground)
                                 MRID 43766916

    14 (grapes)
                                 MRID 47542601

Aquatic field dissipation half-lives (days)
                       <7 (soil, 1st application, LA)
                        12 (soil, 2nd application, LA)
                    3 (water, 1st and 2nd applications, CA)
                                 MRID 45204403
                                       -
  1.    Based on comments from the registrant (MRID 483941-01), the Kocs previously used by EFED were based on organic matter and not organic carbon content.  The values reported in this table have been corrected by taking the old values and multiplying by 1.72.

      Available environmental fate data for MPP, estimated from registrant submitted studies and EPISuite, are provided in Table 4.  Based on aerobic soil metabolism studies for glufosinate-ammonium (MRID 41323119) and MPP (MRID 41323118), MPP degrades in soil at approximately the same rate as glufosinate-ammonium, with half-lives ranging from 6.5 to 23 days.  Degradation in aerobic aquatic environments for MPP was estimated from a study submitted for glufosinate-ammonium (MRID 45204402/01), indicating that MPP is persistent in water with a half-life ranging from 441 to 2,054 days.  It should be noted that there is significant uncertainty surrounding the degradation of MPP in water, as the study was not actually conducted by applying MPP and the half-life developed for the sand system applied with 1 ppm glufosinate-ammonium was based on four data points.  MPP has roughly the same mobility as glufosinate-ammonium, with a Koc < 17 L/kgoc in sand and Kocs of 145 and 272 L/kgoc in silt loam soils (MRID 40345662/48394101).  Like for glufosinate-ammonium, Koc values for MPP are somewhat less variable than Kd values, as the CV across the three soils for the organic carbon partition coefficient (88.2%) is less than that for the soil-water partition coefficient (102%).  Therefore the Koc model is more appropriate for MPP.  In a TFD study (MRID 43110402) conducted in a Florida citrus orchard, residues of MPP were detected at depths up to 36 inches.  Two other TFD studies (MRIDs 43766915 and 43766916), conducted on transgenic corn and soybeans, indicated MPP was less mobile, restricted to the top 6 inches of the soil.  A fourth TFD study (MRID 47542601), where glufosinate-ammonium was applied to grapes, indicated MPP was not detected below 18 inches.  Estimated dissipation half-lives for MPP ranged from 33 to 169 days in the TFD studies.  Again, it should be noted that there is significant uncertainty surrounding these half-lives, as the studies were not conducted using applied MPP, but with applied glufosinate-ammonium, some of which degraded to MPP.  In the AFD study (MRID 45204403) conducted at the Louisiana site, the maximum concentration of MPP in flood water was 34 ug/L (Day 14, 110% of maximum glufosinate-ammonium concentration).  In the California AFD study, the maximum concentration of MPP in flood water was 14 ug/L (Day 4, 15% of maximum glufosinate-ammonium concentration).  If these concentrations were adjusted for a 4-inch paddy depth, the maximum MPP concentrations in the water would be 102 ug/L (LA) and 42 ug/L (CA).

Table 4. Physical/chemical properties and environmental fate data for MPP.
                                   Property
                                     Value
                                    Source
                                   Comments
Chemical Name
                      3-methylphosphinico-propionic acid
                                       -
                                       -
Molecular Weight (g/mol)
                                    152.09
                                       -
                                       -
Solubility in Water (mg/L, 20°C)
                                   1x10[6]
                                   EPISuite
                                       -
Vapor Pressure (torr, 25°C)
                                 1.27x10[-][5]
                                   EPISuite
                                       -
Henry's Law Constant
(atm-m[3]/mole)
                                2.54 x 10[-12]
                                       -
       Estimated using vapor pressure, molecular weight and solubility.
Aerobic Soil Metabolism Half-life (days)
    6.5 (sandy loam)
                                 MRID 41323118
                            0.5 ppm MPP[1] applied

    18.6 (sandy loam)

                            1.0 ppm MPP[1] applied

    19.8 (sandy loam)
    23.2 (silt loam)
    21.1 (sandy loam)
                               MRID 41323119[4]
                            1.62 ppm GA[1] applied
Aerobic Aquatic Metabolism Half-life (days)
    2,054 (sand)
                                     MRID
                                45204402/01[4]
                             1 ppm  GA[1] applied

    441 (sand)

                             0.1 ppm GA[1] applied
Organic Carbon-Normalized Soil Partition Coefficient (mL/goc) (Koc)
    <17 (sand)
    145 (silt loam)
    272 (silt loam)
                                MRID 40345662/
                                  48394101[2]
                                      --
Terrestrial field dissipation half-lives (days)
     68 (cropped plot)
                               MRID 43110402[4]
                                      --

    169 (cropped plot)
     95 (bare ground)
                               MRID 43766915[4]

    33 (cropped plot)
    67 (bare ground)
                               MRID 43766916[4]

    34 (grapes)
                               MRID 47542601[4]

Aquatic field dissipation half-lives (days)
                       16 (soil, 2nd application, LA)[3]
                   9.4 (water, 1st and 2nd applications, CA)
                               MRID 45204403[4]
                                       -
   1.    MPP  -  3-methylphosphinico-propionic acid. GA  -  glufosinate-ammonium.
   2.    Based on comments from the registrant (MRID 483941-01), the Kocs previously used by EFED were based on organic matter and not organic carbon content.  The values reported in this table have been corrected by taking the old values and multiplying by 1.72.
   3.    Insufficient data to estimate soil half-life after 1[st] application.
   4.    Half-lives based on studies conducted using glufosinate ammonium.
 Relevant Rice Growing Practices
      Greater than 85% of the rice production in the United States occurs in Arkansas, California, Mississippi, Louisiana, and Texas.  Rice has also been grown in Florida, Missouri, Oklahoma, and Tennessee.  Survey data from the National Agricultural Statistics Service (NASS) on rice acres planted in the United States for the year 2010 are provided in Table 5.

      Table 5. Summary of acres planted with rice in 2010[1]
                                     State
                           Acreage Planted
(x 1000)
                                    Percent
                               Production
(cwt)
                                    Percent
                                   Arkansas
                                     1,791
                                      49%
                                  115,675,000
                                      48%
                                  California
                                      558
                                      15%
                                  44,326,000
                                      18%
                                  Louisiana
                                      540
                                      15%
                                  32,625,000
                                      13%
                                 Mississippi
                                      305
                                     8.4%
                                  20,756,000
                                     8.5%
                                   Missouri
                                      253
                                     7.0%
                                  16,254,000
                                     6.7%
                                    Texas
                                      189
                                     5.2%
                                  13,468,000
                                     5.5%
                                United States
                                     3,636

                                  243,014,000

      1  Based on survey data from the from the Quick Stats Database of the National Agricultural Statistics Service available at http://quickstats.nass.usda.gov/results/6432EC45-FA6F-3DE6-A48F-98F1917AAEA8.

      In general, rice fields are prepared (disking and harrowing) prior to seeding in order to destroy winter vegetation and reduce the chance of seedling drift.  The fields are then leveled in order to maintain a flood and reduce runoff.  After land preparation, rice seeds are planted via water-seeding or dry-seeding on well prepared seed beds.  If dry-seeded, the fields are then flushed with irrigation water to obtain a uniform seed germination and seedling emergence.  In between planting and establishment of permanent flood (3 to 4 weeks after the seeding), fields may be flushed several times to maintain moisture in the soil. Once the permanent flood is established, it is maintained until 2 to 3 weeks before harvesting.  Flooded water is released and fields are drained 2 to 3 weeks before harvesting to facilitate harvesting operations that use machines.

      In California (Hill et al., 1998), rice is grown primarily in continuously flooded, flow-through systems, where the rice is seeded by airplane into the water.  However, the practice of dry seeding of rice in California has increased due to weed resistance issues (Hill et al., 2006).  Weed control begins within days of planting and continues until the canopy closes over. Flooded paddies are normally drained 2-3 weeks prior to harvest.  Information from the University of California Rice Research and Information Program (http://www.plantsciences.ucdavis.edu/uccerice/rice_production/planting_water_mgmt.htm) states that paddy water depths typically range from four to greater than eight inches.

      In the southeastern United States, rice can be planted via water-seeding, drill-seeded, or broadcast dry; however, most of the planting is via water-seeding to control red rice.  In dry seed planting, 4-6 weeks may elapse between planting and permanent flood establishment, although flooding fields immediately after seeding can improve weed control. A uniform flood depth of less than four inches (1-2 inches is preferred) is usually maintained prior to rice emergence, and then the flood depth is increased to four inches (Shipp, 2002).

      Typical planting and harvesting periods were collected from the USDA (2010) and are provided in Table 6 below.

      Table 6. Typical planting and harvesting dates*
                                     State
                                   Planting
                                  Harvesting
                                   Arkansas
                              April 14  -  May 1
                               Sep 9  -  Oct 10
                                  California
                                  May 1 - 25
                               Sep 15  -  Nov 1
                                   Louisiana
                              March 28  -  May 1
                               Aug 4  -  Sep 15
                                     Texas
                             March 23  -  April 26
                                Aug 7  -  Sep 4
                                  Mississippi
                              April 18  -  May 16
                                Sep 5  -  Oct 6
       * Data from USDA (2010)
Modeling
      The refined Tier I Rice Model and PFAM were used to estimate pesticide concentrations in rice paddy effluent.  These estimated concentrations result in conservative estimates of drinking water concentrations and reflect possible concentrations that may occur when a drinking water intake is near the location where paddy water is released, and mixing with uncontaminated water is minimal.  While the Tier I Rice Model output has been shown to generate concentrations that are similar to measured concentrations in paddy waters, and is conservative when compared with surface water concentrations downstream of paddy discharges, monitoring data are not available to support estimation of chronic exposures.  It is likely that long-term mean concentrations that do not account for dilution will not be representative of chronic drinking water concentrations, because release of paddy water into surface water will not be continuous but instead will occur during specific times of the year, such that water used as a source of drinking water is not continuously influenced by rice paddy effluent (USEPA, 2007).
       This assessment attempts to estimate glufosinate-ammonium and MPP chronic drinking water concentrations by exploring several techniques to account for residue dissipation.  The first method applies the aerobic aquatic metabolism decay rates of glufosinate-ammonium and MPP to the acute concentration, to account for degradation over a 365-day period, and then averages the concentrations over this period to estimate an annual average concentration (Sections 5.1 and 5.2).  The second method uses monitoring data for various other pesticides that are applied primarily to rice, in order to estimate typical percent reductions in going from acute to chronic concentrations (Section 5.3).  Lastly, an analysis of Safe Drinking Water Information System (SDWIS) data confirms that reservoirs and lakes in rice growing areas are commonly used as community drinking water supplies (Appendix A).  Thus the third technique investigates dilution as well as degradation of paddy effluent (i.e. PFAM results) that discharges into an index reservoir (the standard water body EFED uses for surface water source drinking water assessments) to characterize chronic concentrations (Section 5.4).
 Refined Tier I Rice Model
      The EFED Tier 1 Rice Model (v1.0, May 8, 2007) was used to estimate aquatic exposures for applications directly to water.  The model estimates exposure in a water body holding a 10 cm water depth.   When a pesticide is applied to a water body, the model assumes that it will instantaneously partition between a water phase and a sediment phase based on the partition coefficient of the chemical.  The formula of the Tier I Rice Model v1.0 is as follows:

      and, if appropriate:
                                 Kd = 0.01Koc
	where
            Cw	water concentration (ug/L)
            mai'	mass applied per unit area (kg/ha)
            Kd	water-sediment partitioning coefficient (L/kg)
            Koc	organic carbon partitioning coefficient (L/kg)

      The Tier I Rice Model was calibrated to generate estimates that are similar to or greater than dissolved concentrations measured within rice paddies and in discharged paddy water.  The calibration involved determination of the sediment interaction depth by calibrating the model to exceed residues measured in paddy water in the vast majority of dissipation studies.  The model does not account for pesticide degradation, volatilization, dilution, or other dissipation processes.  The model was not evaluated or calibrated for concentrations measured in sediment.  Guidance for using the Tier I Rice Model may be found on the U.S. Environmental Protection Agency (EPA) Water Models web-page (USEPA, 2007).

      In the previous drinking water assessment (DP Barcode 381992, October 20, 2010), provisional refinements were made to EFED's Tier I Rice Model to incorporate aerobic soil and aerobic aquatic degradation into the equations discussed above.  Additionally, instead of considering one application, the equation was modified to allow for multiple applications and degradation over time.  The following equation was used to estimate the amount of mass in the paddy as a function of time:

      where
      	M(t)	mass in paddy per area of paddy at time t (kg/ha)
      	Mo	initial mass per area of paddy applied to paddy (kg/ha)
      	t	time (days)
            k	aerobic soil or aquatic metabolism rate constant (days[-1]), depending on application timing

If the pesticide is applied prior to flooding, the aerobic soil rate constant would be used in the mass equation.  Once flooding occurred, the aerobic aquatic metabolism rate constant is used.

      The concentrations in the paddy water were then estimated using the following equation:

      where
      	EEC(t)		concentration in paddy water at time t (ug/L)
      	CF2		conversion factor (1x10[9] ug/kg / 1x104 m[2]/ha / 1x10[3] L/m[3] = 100)
      dw		water column depth (0.1016 and 0.2032 m in this assessment)
            dsed	sediment depth (0.01 m)
      	θ		sediment porosity (0.509 unitless)
      	ρ		sediment bulk density (1300 kg/m[3])
      	Kd		water-sediment distribution coefficient (L/kg)
      	CF1		conversion factor (1000 L/m[3])

      In this assessment instead of the 0.00105 in the denominator (Tier I Rice model), a value of 0.0010669 was used to represent a four inch depth and a value of 0.0020829 was used for the eight inch depth (Tier I refined Rice Model).  As the prior assessment was conducted considering the total toxic residue, aerobic soil and aerobic aquatic half-lives were estimated for the combined glufosinate-ammonium, MPP, MPA, and NAG residues observed in fate studies.  In this assessment, glufosinate-ammonium and MPP were the only modeled residues of concern and they were modeled separately.  For glufosinate-ammonium, a Koc of 297 L/kgoc was used in the modeling (average of the three reported values).  The 90[th] percentile upper confidence bound on the mean of six aerobic soil metabolism and five aquatic half-life rates for residues of glufosinate-ammonium (19 days (k = 0.0356 days[-1]) and 63 days (k = 0.0111 days[-1]), respectively) were used in modeling.

      For MPP, a Koc of 145 L/kgoc was used in the modeling (average of the three reported values).  The 90[th] percentile upper confidence bound on the mean of five aerobic soil metabolism and two aquatic half-life rates for residues of MPP (22 days (k = 0.0315 days[-1]) and 3,730 days (k = 0.0002 days[-1]), respectively) were used in modeling.  For MPP, application rates were adjusted by a factor of 0.77 to account for differences in molecular weight (152/198 = 0.77).  For Tier I modeling, the glufosinate-ammonium was assumed to instantaneously convert to MPP.  This has the effect of generating conservative estimates for MPP, as the model does not account for the gradual formation and decline of MPP, but rather assumes it is present at day 1.

      For all dry applications, flooding occurred 24 hours after the last application.  Release of the permanent flood waters from the fields was assumed to begin 7 days after the last application.  For flooded-field applications, release of the permanent flood waters from the fields was assumed to begin 7 days after the last application.  Results from the refined Tier I Rice Model are shown in Table 7.  For all applications, the acute concentration of glufosinate-ammonium is slightly higher than those for MPP.  Chronic values are approximately 76% and 3% lower than the acute values for glufosinate-ammonium and MPP, respectively.

Table 7. EDWCs using the refined Tier I rice model.
Use rate, Number of apps, retreatment interval, water holding period[1], application timing
                              Acute EDWC in ug/L
                         Annual Average EDWC in ug/L

                                     4"
                                     8"
                                     4"
                                     8"
                             Glufosinate-ammonium
1.46 lbs ai/acre, 1, NA, 7 days, dry[2,3]
                                     1,006
                                      592
                                      246
                                      145
0.73 lbs ai/acre, 2, 10 days, 30 days, 1[st] app dry and 2[nd] app flooded
                                      NA
                                      431
                                      NA
                                      105
0.73 lbs ai/acre, 2, 10 days, 55 days, 1[st] app dry and 2[nd] app flooded
                                      555
                                      NA
                                      136
                                      NA
0.89 lbs ai/acre, 1, NA, 7 days, dry[2,3]
                                      613
                                      361
                                      150
                                      88
0.66 lbs ai/acre, 1, NA, 7 days, dry[2][,3]
                                      455
                                      268
                                      111
                                      66
0.44 lbs ai/acre, 2, 10 days, 7 days, dry[2,3]
                                      516
                                      303
                                      126
                                      74
0.44 lbs ai/acre, 2, 10 days,7 days, flooded[4]
                                      NA
                                      350
                                      NA
                                      94
                                    MPP[5]
1.46 lbs ai/acre, 1, NA, 7 days, dry[2,3]
                                      969
                                      536
                                      937
                                      518
0.73 lbs ai/acre, 2, 10 days, 30 days, 1[st] app dry and 2[nd] app flooded
                                      NA
                                      510
                                      NA
                                      493
0.73 lbs ai/acre, 2, 10 days, 55 days, 1[st] app dry and 2[nd] app flooded
                                      918
                                      NA
                                      888
                                      NA
0.89 lbs ai/acre, 1, NA, 7 days, dry[2,3]
                                      591
                                      327
                                      571
                                      316
0.66 lbs ai/acre, 1, NA, 7 days, dry[2][,3]
                                      438
                                      242
                                      424
                                      234
0.44 lbs ai/acre, 2, 10 days, 7 days, dry[2,3]
                                      505
                                      279
                                      488
                                      270
0.44 lbs ai/acre, 2, 10 days,7 days, flooded[4]
                                      NA
                                      333
                                      NA
                                      322
   1 The water holding period is the period water is held from the date of the last application.
   2 For dry applications, the field was flooded 24-hours after the last application.
   3 Release modeled 7 days after flooding.
   4 Release modeled 7 days after last application.
   5 Assumes 77% conversion of glufosinate-ammonium to MPP, based on an adjustment for molecular weight (152/198).
 Pesticides in Flooded Applications Model (PFAM)
      PFAM version 0.70 was developed as a flooded agriculture model for use in pesticide regulatory work.  This model has been developed under USEPA Guidance (USEPA, 2009c).  External independent evaluations have been positive (Luo, 2011).  As this model is under evaluation, it is still considered provisional.  PFAM is conceptualized in Figure 1 and includes both hydrological processes and chemical processes.  The water body depth may change due to precipitation, refill, drainage, evaporation, and weir-height changes.  The model consists of two regions -- a water column and a benthic region.  Each individual region is completely mixed and at equilibrium with all phases within the individual region, and equilibrium within each region follows a linear isotherm (USEPA, 2004).  The two regions are coupled by a first-order mass-transfer process.  Equations for chemical processes (i.e., hydrolysis, bacterial metabolism, photolysis, and sorption) within each region are similar to USEPA, 2004.   The model accounts for plant growth, metabolism, hydrolysis, photolysis, and metabolism in water, sediment, and soil (when no water is present), sorption, and volatilization.  Water, sediment, and pesticide may flow out of the field.  Changes in water body conditions (temperature, water levels, wind speed, etc.) and the resulting changes in degradation rates occur on a daily time step.    Pesticide application and flooding sequences are mapped onto the time series in 1-year cycles for the length of the simulation. For instance, a typical exposure assessment assumes the pesticide is used every year for 30 years (the length of most EFED meteorological files).  Output is delivered as released mass of pesticide as well as a daily time series of concentrations.

Figure 1. Conceptual model of hydrological and chemical processes in PFAM.

      PFAM requires the entry of specific dates of applications, emergence, and flooding events.  Using regional-specific data provided in Table 8, PFAM-required dates were developed for the various modeling scenarios as shown in Table 9.  The midpoint of the planting dates in Table 6 was used in the modeling to estimate emergence dates.  Emergence generally occurs 5-20 days after planting (University of Arkansas, 2006), so, for this assessment, emergence dates were assumed to occur 10 days after planting.  The midpoint of the harvesting dates in Table 6 was used in the modeling to estimate the Days from Zero Height to Full Height input on PFAM's Crop tab.  It was assumed that paddies are drained at least 21 days prior to harvesting (Hill, et al., 1997).  The Days from Zero Height to Full Height were estimated as the difference between the release date (harvest date minus 21 days) and the emergence date.  For single, dry applications, pesticides were applied one day prior to flooding.  For burndown applications, pesticides were applied one day prior to planting followed by flooding one day later.   For dry, tolerant rice applications, the first application occurred 10 days after planting (assumed emergence period), with the second application occurring 10 days later.  Flooding occurred one day after the second application.  For the wet, tolerant rice applications (2 x 0.44 lbs ai/A), flooding occurred one day prior to planting, with applications beginning 10 days after planting.  For multiple applications for rice seed propagation (2 x 0.73 lbs ai/A), flooding occurred one day after planting, with the first application occurring 5 days before planting (dry) and the second application occurring 5 days after planting (wet).  For all scenarios, to simulate the release of water, flood water heights and weir heights were set to their respective values (see Table 10) for Event 1, but were set to 0 meters on the release day specified as Event 2.  For dry applications, release of the permanent flood waters from the fields was assumed to begin 7 days after flooding.  For flooded applications, release of the permanent flood waters from the fields was assumed to begin seven days after the last application.  A four-inch water column depth (0.1016 m) and an eight-inch water column depth (0.2032 m) were modeled at all sites to characterize the influence of water depth on the EDWCs.

      The remaining required input parameters used in PFAM were derived from chemical and fate properties and information provided in the crop profiles. These inputs are provided in Table 10.  Inputs for the Crop tab, except for the zero height reference which is the emergence date, and the Physical tab were left at the pre-determined, default levels (see Appendix C).  Inputs on the Output tab include the area of application, the area of the surrounding watershed, and the curve number for the surrounding watershed.  EFED recently updated the percent cropped area (PCA) estimates for use in drinking water assessments (USEPA, 2012).  Using 8-digit hydrologic units (HUC 8) and National Land Cover Data, EFED has developed a PCA of 20% for rice.  Using this PCA value times the area of the index reservoir watershed (1,728,000 m[2]), the area of application (345,600 m[2]) was derived.  The area of the surrounding watershed (1,382,400 m[2]) was the difference in the index reservoir watershed and the area of application.  The curve number of the surrounding watershed (83) was obtained from the PRZM Users Manual, Table 5.10, assuming the surrounding watershed land was contoured pasture with fair hydrologic conditions and a soil group of D, typical of rice growing areas.

      As PFAM allows for modeling of the parent and up to two degradates, glufosinate-ammonium and MPP were modeled as a parent and degradate during the same PFAM runs.  In PFAM, degradates are handled exactly like the parent in regard to their transformations. The production of degradates is based on first-order degradation of the parent compound and can be modeled as the result of aqueous, soil, or benthic sediment biodegradation, photolysis or hydrolysis. The stoichiometry of the degradate production, supplied by the pesticide study submissions, is also used as data input to the model.  MPP parameters were entered on the Degradate 1 tab.

Table 8. Crop-specific dates used for PFAM modeling (Crop tab)
                                     State
                                  Planting[1]
                               Emergence Date[2]
                                Release Date[3]
                                 Harvesting[1]
                                      AR
                                   April 22
                                     May 2
                                  September 3
                                 September 24
                                      CA
                                    May 13
                                    May 23
                                 September 17
                                   October 8
                                      LA
                                   April 14
                                   April 24
                                   August 4
                                   August 25
                                      TX
                                    April 9
                                   April 19
                                    July 31
                                   August 21
                                      MS
                                     May 2
                                    May 12
                                   August 30
                                 September 20
[1] Midpoint of dates provided in Table 6.
	2 Assumed to occur 10 days after planting.
	[3] Assumed to occur 21 days before harvesting.

Table 9. Site-specific input dates used for PFAM modeling (Applications and Floods tabs), based in agronomic practices reported in crop profiles
Use rate, Number of apps, retreatment interval, water holding period, application method (wet or dry)
                                     State
                                     App 1
                                     App 2
                                     Flood
                                   (Event 1)
                                    Release
                                   (Event 2)
                                    # days
1.46 lbs ai/acre (1.64 kg ai/ha), 1, NA, 7 days, dry
and
0.89 lbs ai/acre (1.0 kg ai/ha), 1, NA, 7 days, dry
AR
                                                                          2-May
                                      NA
                                                                          3-May
                                                                         10-May
                                       7

CA
                                                                         23-May
                                      NA
                                                                         24-May
                                                                         31-May
                                       7

LA
                                                                         24-Apr
                                      NA
                                                                         25-Apr
                                                                          2-May
                                       7

TX
                                                                         19-Apr
                                      NA
                                                                         20-Apr
                                                                         27-Apr
                                       7

MS
                                                                         12-May
                                      NA
                                                                         13-May
                                                                         20-May
                                       7
0.73 lbs ai/acre (0.818 kg ai/ha), 2, 10 days, 7 days, 1[st] app dry and 2[nd] app flooded[1]
AR
                                                                         17-Apr
                                    27-Apr
                                                                         23-Apr
                                                                         21-Jun
                                      59

CA
                                                                          8-May
                                    18-May
                                                                         14-May
                                                                         12-Jul
                                      59

LA
                                                                          9-Apr
                                    19-Apr
                                                                         15-Apr
                                                                         13-Jun
                                      59

TX
                                                                          4-Apr
                                    14-Apr
                                                                         10-Apr
                                                                          8-Jun
                                      59

MS
                                                                         27-Apr
                                     7-May
                                                                          3-May
                                                                          1-Jul
                                      59
0.73 lbs ai/acre (0.818 kg ai/ha), 2, 10 days, 7 days, 1[st] app dry and 2[nd] app flooded[2]
AR
                                                                         17-Apr
                                    27-Apr
                                                                         23-Apr
                                                                         27-May
                                      34

CA
                                                                          8-May
                                    18-May
                                                                         14-May
                                                                         17-Jun
                                      34

LA
                                                                          9-Apr
                                    19-Apr
                                                                         15-Apr
                                                                         19-May
                                      34

TX
                                                                          4-Apr
                                    14-Apr
                                                                         10-Apr
                                                                         14-May
                                      34

MS
                                                                         27-Apr
                                     7-May
                                                                          3-May
                                                                          6-Jun
                                      34
0.66 lbs ai/acre (0.732 kg ai/ha), 1, NA, 7 days, dry
AR
                                                                         21-Apr
                                      NA
                                                                         23-Apr
                                                                         30-Apr
                                       7

CA
                                                                         12-May
                                      NA
                                                                         14-May
                                                                         21-May
                                       7

LA
                                                                         13-Apr
                                      NA
                                                                         15-Apr
                                                                         22-Apr
                                       7

TX
                                                                          8-Apr
                                      NA
                                                                         10-Apr
                                                                         17-Apr
                                       7

MS
                                                                          1-May
                                      NA
                                                                          3-May
                                                                         10-May
                                       7
0.44 lbs ai/acre (0.49 kg ai/ha), 2, 10 days, 7 days, dry
AR
                                                                          2-May
                                                                         12-May
                                                                         13-May
                                                                         20-May
                                       7

CA
                                                                         23-May
                                                                          2-Jun
                                                                          3-Jun
                                                                         10-Jun
                                       7

LA
                                                                         24-Apr
                                                                          4-May
                                                                          5-May
                                                                         12-May
                                       7

TX
                                                                         19-Apr
                                                                         29-Apr
                                                                         30-Apr
                                                                          7-May
                                       7

MS
                                                                         12-May
                                                                         22-May
                                                                         23-May
                                                                         30-May
                                       7
0.44 lbs ai/acre (0.49 kg ai/ha), 2, 10 days, 7 days, flooded[3]
AR
                                                                          2-May
                                                                         12-May
                                                                         21-Apr
                                                                         19-May
                                      28

CA
                                                                         23-May
                                                                          2-Jun
                                                                         12-May
                                                                          9-Jun
                                      28

LA
                                                                         24-Apr
                                                                          4-May
                                                                         13-Apr
                                                                         11-May
                                      28

TX
                                                                         19-Apr
                                                                         29-Apr
                                                                          8-Apr
                                                                          6-May
                                      28

MS
                                                                         12-May
                                                                         22-May
                                                                          1-May
                                                                         29-May
                                      28
   1. Applicable for a 4-inch paddy depth.  # days  reflects 55-day holding period after 2[nd] application.
   2. Applicable for an 8-inch paddy depth.  # days reflects 30-day holding period after 2[nd] application
   3. Release modeled 7 days after 2[nd] application.

Table 10.  PFAM inputs specific to glufosinate-ammonium/MPP modeling runs
                                Input Parameter
                                     Value
                                    Source
                                    Comment
                                 Chemical Tab
Water Column Half-life (days) at 20°C
                                      63
                          MRIDs 40345660, 45204402/01
Represents the 90[th] percentile upper confidence bound on the mean (39) of 5 aerobic aquatic half-life values.[1]
Benthic Compartment Half-Life (days) at 20°C
                                     1246
                                 MRID 46258601
3 times the anaerobic aquatic half-life.[1]
Unflooded Soil Half-life (days) at 20°C
                                      19
                           MRIDs 41323118, 40345659A
Represents the 90[th] percentile upper confidence bound on the mean (15) of 6 aerobic soil half-life values.[1]
Aqueous Near Surface
Half-life (days) at 40° Latitude
                                    1x10[8]
                                 MRID 41323115
No evidence of aqueous photolysis.
Hydrolysis Half-life (days)
                                    1x10[8]
                                 MRID 40345656
No evidence of degradation.
Organic Carbon Partition Coefficient (mL/goc) (Koc)
                                      297
                                 MRID 40345662
Average of three KOC values.
Molecular Weight (g/mol)
                                      198
                                      ---
                                      ---
Vapor Pressure (torr)
                                  7.5x10[-9]
                                 MRID 44032901
                                      ---
Solubility (mg/L)
                                  1.37x10[6]
                                 MRID 00263025

Heat of Henry (J/mol)
                                     49884
                                      ---
Estimated using HENRYWIN program in EPISuite (see Appendix B).
Henry Reference Temperature ([o]C)
                                      20

                               Applications Tab
Number of Applications
                                  See Table 9

Application dates
                                  See Table 9

                                 Location Tab
Meteorological files
                                  AR (w13963)
                                  LA (w13970)
                                  CA (w23232)
                                  MS (w03940)
                                  TX (w13958)
                                      ---
Meteorological data available in PE5 installation. Stations correspond to
Little Rock, AR (w13963), Baton Rouge, LA (w13970), Sacramento, CA (w23232), Jackson, MS (w03940), and Austin, TX (w13958)
                                  Floods Tab
Number of Flood Events
                                       2
                                      ---
First event is flooding, second event is release of flood waters.
Date for Event 1
                                  See Table 9

Fill Level (m), Event 1
                                   0.1016[2]
                                   0.2032[3]
                                      ---
Crop profiles
Weir Level (m), Event 1
                                   0.1270[2]
                                   0.2286[3]
                                      ---
Assumed 1 inch clearance between top of weir and water.
Min. Level (m), Event 1
                                   0.1016[2]
                                   0.2032[3]
                                      ---
Keeps paddy full
Number of Days, Event 2
                                  See Table 9

Fill Level (m), Event 2
                                       0
                                      ---
Simulate release of water
Weir Level (m), Event 2
                                       0
                                      ---
Simulate release of water
Min. Level (m), Event 2
                                       0
                                      ---
Simulate release of water
Turn Over (1/day), Events 1 and 2
                                       0
                                      ---
Turnover is used for those systems that use a continuous flow through the system and are maintained at a set depth. Set to 0.
                                   Crop Tab
Zero Height Reference
                                   AR: 05/02
                                   CA: 05/23
                                   LA: 04/24
                                   TX: 04/19
                                   MS: 05/12
                                      ---
Emergence date, assumed to occur 10 days after planting. See Table 8 for dates.
Days from Zero Height to Full Height
                                    AR: 124
                                    CA: 118
                                    LA: 102
                                    TX: 103
                                    MS: 111
                                      ---
Number of days from emergence to maturation. Maturation expected to occur 21 days prior to harvesting. See Table 8 for dates.
Days from Zero Height to Removal
                                    AR: 145
                                    CA: 139
                                    LA: 123
                                    TX: 124
                                    MS: 132
                                      ---
Number of days from emergence to harvest. See Table 8 for dates.
Maximum Fractional Areal Coverage
                                      1.0
                                      ---
Conservative assumption
                                 Physical Tab
Reference depth (m)
                                   0.1270[2]
                                   0.2286[3]

Set to same depth as weir height, per PFAM guidance.
                                  Output Tab
Area of Application (m[2])[4]
                                    345,600
                                      ---
20% of the watershed drainage acreage (1,728,000 m[2]), based on draft HUC-8 PCA for rice
Area of Surrounding Watershed (m[2]) 4
                                   1,382,400
                                      ---
Difference in watershed drainage acreage (1,728,000 m[2]) and area of application
Curve Number of Surrounding Watershed[4]
                                      83
                                      ---
Curve number for a contoured pasture with Fair hydrologic conditions and a Soil Group of D
                                Degradate 1 Tab
Moles of Degradate Produced per mole of Parent
   Water
   Benthic
   Unflooded soil
   Photolysis
   Hydrolysis

                                       1
                                       0
                                       1
                                       0
                                       0
                                      ---
Estimates based on stochiometry of glufosinate ammonium degradation to MPP and whether MPP was formed in the particular studies. MPP was a degradate in the aerobic soil and aerobic aquatic studies, but not in the anaerobic aquatic, aqueous photolysis, and hydrolysis studies for glufosinate-ammonium (DP 397388, 1/4/2012).
Water Column Half-life (days) at 20°C
                                     3730
                          MRIDs 40345660, 45204402/01
Represents the 90[th] percentile upper confidence bound on the mean (1247) of 2 aerobic aquatic half-life values.[1]
Benthic Compartment Half-Life (days) at 20°C
                                    1x10[8]
                                      ---
Assumed stable.
Unflooded Soil Half-life (days) at 20°C
                                      22
                           MRIDs 41323118, 40345659A
Represents the 90[th] percentile upper confidence bound on the mean (18) of 5 aerobic soil half-life values.[1]
Aqueous Near Surface
Half-life (days) at 40° Latitude
                                    1x10[8]
                                      ---
Assumed stable.
Hydrolysis Half-life (days)
                                    1x10[8]
                                      ---
Assumed stable.
Organic Carbon Partition Coefficient (mL/goc) (Koc)
                                      145
                                 MRID 40345662
Average of three KOC values for MPP.  A value of <17 L/kg was assumed to be 17 in the calculation.
Molecular Weight (g/mol)
                                    152.09
                                      ---
                                   EPISuite
Vapor Pressure (torr)
                                  3.7x10[-5]
                                      ---
                                   EPISuite
Solubility (mg/L)
                                    1x10[6]
                                      ---
                                   EPISuite
Heat of Henry (J/mol)
                                     49884
                                      ---
Estimated using HENRYWIN program in EPISuite (see Appendix B).
Henry Reference Temperature ([o]C)
                                      20

1  EFED input parameter guidance is located at: : http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm
[2] Heights used for a 4-inch paddy.
[3] Heights used for an 8-inch paddy.
[4]Used when employing the index reservoir function in PFAM.

      The 90[th] percentile peak (acute) and annual average (chronic) concentrations for the Arkansas, California, Louisiana, Mississippi, and Texas PFAM runs without the use of the index reservoir are provided in Table 11.  The 90[th] percentile concentration was selected to reflect the 1-in-10 year peak concentration, similar to the method used in EFED's drinking water modeling using PRZM/EXAMS.  The effluent concentrations are obtained from the pp1 files generated by PFAM (see Appendix D).  Maximum concentrations for the effluent are estimated for each year using concentration values designated as "overflow" or "manual release".  "Dry field runoff" values are excluded from the analysis, as these values are indicative of times when a rice paddy is not present.  These estimated concentrations result in conservative estimates of acute drinking water concentrations and reflect possible concentrations that may occur when the drinking water intake is near the location where effluent is released, and mixing with uncontaminated water is minimal.  The annual average EDWC was estimated by taking the upper 90[th] percentile concentration and allowing it to degrade in water for 365 days using the aerobic aquatic metabolism half-life of 63 and 3,730 days for glufosinate-ammonium and MPP, respectively, and estimating the average daily concentration during the 365 days. The values for a particular application, regardless of the site, are fairly close, as the main differences between the sites are dates of precipitation which results in overflow removing water from the paddy. As a result, the site with the lowest precipitation, the California scenario, provides the highest glufosinate-ammonium concentrations.  For all sites and applications, chronic values are approximately 76% and 3% lower than the acute values for glufosinate-ammonium and MPP, respectively.  The concentrations in the 8-inch paddies are also roughly 50% lower than those in the 4-inch paddies, except for the scenarios with multiple applications for rice seed propagation, where concentrations in the 8-inch paddies were 90% of the concentrations in 4-inch paddies.

Table 11. EDWCs using the PFAM without-the-index-reservoir.
Use rate, number of apps, retreatment interval, water holding period, application timing
                                     Site
                             Glufosinate-ammonium
                                      MPP

                                  Acute EDWC
                                    (ug/L)
                          Annual Average EDWC (ug/L)
                                  Acute EDWC
                                    (ug/L)
                          Annual Average EDWC (ug/L)

                                     4"
                                     8"
                                     4"
                                     8"
                                     4"
                                     8"
                                     4"
                                     8"
1.46 lbs ai/acre, 1, NA, 7 days, dry[1]
                                      AR
                                      CA
                                      LA
                                      MS
                                      TX
                                      165
                                      173
                                      163
                                      161
                                      163
                                      84
                                      86
                                      84
                                      83
                                      84
                                      40
                                      42
                                      40
                                      39
                                      40
                                      21
                                      21
                                      20
                                      20
                                      21
                                      4.5
                                      5.0
                                      5.1
                                      5.7
                                      5.2
                                      2.3
                                      2.5
                                      2.7
                                      2.9
                                      2.7
                                      4.3
                                      4.8
                                      4.9
                                      5.5
                                      5.0
                                      2.3
                                      2.4
                                      2.6
                                      2.8
                                      2.6
0.73 lbs ai/acre, 2, 10 days, 30/55 days, dry and flooded[2]
                                      AR
                                      CA
                                      LA
                                      MS
                                      TX
                                      344
                                      390
                                      318
                                      313
                                      344
                                      294
                                      320
                                      288
                                      282
                                      296
                                      84
                                      95
                                      78
                                      77
                                      84
                                      72
                                      78
                                      70
                                      69
                                      72
                                      159
                                      173
                                      158
                                      183
                                      181
                                      73
                                      81
                                      83
                                      88
                                      85
                                      154
                                      167
                                      153
                                      177
                                      175
                                      71
                                      78
                                      80
                                      85
                                      82
0.89 lbs ai/acre, 1, NA, 7 days, dry[1]
                                      AR
                                      CA
                                      LA
                                      MS
                                      TX
                                      101
                                      106
                                      100
                                      99
                                      100
                                      51
                                      53
                                      51
                                      51
                                      51
                                      25
                                      26
                                      24
                                      24
                                      24
                                      13
                                      13
                                      13
                                      12
                                      13
                                      2.7
                                      3.1
                                      3.1
                                      3.5
                                      3.1
                                      1.4
                                      1.5
                                      1.6
                                      1.8
                                      1.7
                                      2.6
                                      3.0
                                      3.0
                                      3.4
                                      3.0
                                      1.4
                                      1.5
                                      1.6
                                      1.7
                                      1.6
0.66 lbs ai/acre, 1, NA, 7 days, dry
                                      AR
                                      CA
                                      LA
                                      MS
                                      TX
                                      71
                                      74
                                      71
                                      71
                                      72
                                      37
                                      38
                                      37
                                      36
                                      37
                                      17
                                      18
                                      17
                                      17
                                      18
                                       9
                                       9
                                       9
                                       9
                                       9
                                      1.8
                                      2.0
                                      2.1
                                      2.2
                                      2.1
                                      0.9
                                      1.0
                                      1.1
                                      1.1
                                      1.1
                                      1.7
                                      1.9
                                      2.0
                                      2.1
                                      2.0
                                      0.9
                                      1.0
                                      1.0
                                      1.1
                                      1.0
0.44 lbs ai/acre, 2, 10 days, 7 days, dry[1]
                                      AR
                                      CA
                                      LA
                                      MS
                                      TX
                                      83
                                      88
                                      83
                                      81
                                      84
                                      43
                                      45
                                      42
                                      42
                                      43
                                      20
                                      22
                                      20
                                      20
                                      21
                                      11
                                      11
                                      10
                                      10
                                      10
                                      2.7
                                      2.9
                                      2.8
                                      3.0
                                      2.9
                                      1.4
                                      1.5
                                      1.4
                                      1.5
                                      1.5
                                      2.6
                                      2.8
                                      2.7
                                      2.9
                                      2.8
                                      1.3
                                      1.4
                                      1.4
                                      1.5
                                      1.4
0.44 lbs ai/acre, 2, 10 days, 7 days, flooded[3]
                                      AR
                                      CA
                                      LA
                                      MS
                                      TX
                                      NA
                                      365
                                      375
                                      356
                                      360
                                      365
                                      NA
                                      89
                                      92
                                      87
                                      88
                                      89
                                      NA
                                      39
                                      40
                                      39
                                      45
                                      42
                                      NA
                                      38
                                      39
                                      38
                                      44
                                      41
   1 Release modeled 7 days after flooding.  Time period allows for dry applied product to partition to water phase from sediment phase.  The field was flooded 24-hours after the last application.
   2 1[st] application is made to dry soil, while the 2[nd] application is made while the rice paddy is flooded.  The 30-day holding period is applicable to an 8-inch paddy depth, while a 55-day holding period is applicable to a 4-inch paddy depth.
   3 Release modeled 7 days after last application.  As flooding for wet applications occurs prior to planting, holding times are greater than 7 days.

      For glufosinate-ammonium and MPP, flooded applications generated higher PFAM concentrations than dry application scenarios.  This is different than the trend of results from the refined Tier I analysis, where the highest single application rate (1.46 lbs ai/A) generated the highest acute EDWCs, with all other applications generating similar acute EDWCs.  This difference is due primarily to how each model treats mass transfer between the water column and the sediment.  PFAM treats mass transfer as a diffusion-driven, time-limited process, while the refined Tier 1 model assumes instantaneous equilibrium of chemical between sediment and water phases.  The glufosinate-ammonium PFAM concentrations in the flooded applications were similar to those obtained with the refined Tier I model, while the dry applications were significantly lower (approximately 82%).  For MPP, the PFAM concentrations for dry applications were significantly lower (approximately 99%), while reductions in the flooded applications were comparable to the dry glufosinate-ammonium applications (approximately 82%).  It appears that for glufosinate-ammonium and MPP during flooded applications, the majority of the compound remains in the water column after application, resulting in higher concentrations than occur following dry applications.

      While the PFAM runs do not entirely match the conditions under which the glufosinate-ammonium AFD was conducted, a comparison of the results from the two datasets for a 4-inch paddy depth indicates striking similarities.  For glufosinate-ammonium, the PFAM concentrations were very similar to those from the AFD study (83 μg/L versus 93 μg/L for a Louisiana effluent (dry application) and 375 μg/L versus 288 μg/L for a California effluent (flooded application)).  For MPP, the concentrations were similar for the flooded application (40 μg/L versus 42 μg/L for a California effluent (flooded application)), but much lower for the dry applications (2.8 μg/L versus 102 μg/L for a Louisiana effluent (dry application)).
Monitoring Data
      Glufosinate-ammonium and MPP

      Glufosinate-ammonium is included as one of the analytes monitored in U.S. surface and groundwater under the USGS's National Water Quality Assessment (NAWQA) program (http://water.usgs.gov/nawqa).  In surface water, glufosinate-ammonium was detected as recoverable, filtered glufosinate (0.7 micron glass fiber filter) 7 times out of 177 samples collected in Washington and Bolivar counties (rice-growing counties) in Mississippi, with a concentration range of 0.02 to 3.23 ug/L.  In 68 samples collected in Merced, San Joaquin, and Stanislaus counties in California, glufosinate-ammonium was not detected (detection limit of 0.1 ug/L).  Surface water and sediment monitoring data were not available for glufosinate-ammonium from the California Department of Pesticide Regulation's (CDPR) database.  It should be emphasized that these monitoring values are not directly comparable to modeling results.  As the monitored data is for glufosinate and not MPP, and as most monitored values are not targeted to rice growing regions or to the particular time of the year that paddy waters are discharged, modeled estimates are expected to be higher than non-targeted monitoring results.

	Other Pesticides Used Exclusively on Rice

	A review of the molinate Reregistration Eligibility Decision (RED) was conducted to determine if monitoring data near drinking water intakes could be used to assess the level of annual concentrations relative to peak concentrations in surface water which might be found in the vicinity of drinking water treatment plant intakes.  Molinate is an herbicide that was exclusively used on rice, so concentrations in surface water could be directly attributed to rice use.  Table 12 presents the summary data obtained from Table 3 of Appendix E of the RED for drinking water intakes for the cities of Sacramento, CA, West Sacramento, CA, St. Francisville, LA, and Melville, LA.  Surface water intakes for cities in Arkansas and Mississippi were not available in rice producing areas of the States.  Sacramento gets its drinking water from the intake that is below the confluence of the American and Sacramento Rivers.  Approximately 71.4 % of their water is obtained from the Sacramento River.  For West Sacramento, California, all of their water is obtained from the Sacramento River.  To adjust for the source of water, the maximum and annual concentrations for the City of Sacramento were divided by the average percent of Sacramento River that Sacramento receives (71.4 % from 1991-2000), and multiplied by 100.  Based on this data, time-weighted annual mean concentrations reported in the RED are approximately 80-88% lower than the maximum concentrations.  However, molinate is more volatile than glufosinate-ammonium, so other active ingredients were also examined.

	Thiobencarb is another herbicide that has been used exclusively on rice.  Laboratory studies indicate thiobencarb is stable in aquatic environments.  Thiobencarb is moderately mobile to immobile and semivolatile.  In aquatic field dissipation studies, thiobencarb dissipated relatively quickly in the water column, with DT50s less than 9 days.  Given this information, the fate of thiobencarb appears similar to that of glufosinate-ammonium and its degradates.  EFED examined the CDPR surface water database for thiobencarb data, particularly for Colusa Basin Drain, which is used to provide irrigation water and drain water from fields in Colusa County.  Colusa County is one of the top rice-producing counties in California, with many rice paddies potentially discharging to the Colusa Basin Drain upstream from Colusa Basin Drain # 5.  Data were available for 1995 through 2002 for Colusa Basin Drain # 5.  Additional data for 1994 were obtained from the thiobencarb RED.  Data were collected between the months of March and July each year.  Sampling intervals ranged from two to five days, except for the time between the first and second sampling periods, which was between 22 and 35 days.  An analysis of the data (Table 13) indicates a reduction of 48-93% in time-weighted average seasonal concentrations when compared to peak concentrations. An average reduction value of 74% was obtained from the annual values.  It should be noted that the lowest reduction, 48%, occurred during the shortest sampling period.

	Fipronil is an insecticide used on rice.  Fipronil is stable to hydrolysis under mildly acidic to neutral pH conditions, but hydrolyzes under alkaline conditions. Field persistence of fipronil is low to moderate in water and soil (USEPA, 2001). Fipronil monitoring data were available for 2000 and 2001 for parishes in Louisiana in the Mermentau River Basin (Appendix C, USEPA 2001).  In response to reports of low crawfish production in 1999 possibly due to the use of fipronil, a study of fipronil toxicity was conducted in the Mermentau River Basin. Eighteen stations throughout the rice belt were sampled weekly from March through August in 2000. In 2001, a follow up study was undertaken on eleven of the same stations. Weekly water column samples were collected beginning in March and continued until no detects were observed at any of the stations or August, whichever came first. Analysis of the data (Table 14) shows a reduction of 65-92% in time-weighted average seasonal concentrations compared to peak concentrations.  An average reduction value of 84% was obtained from the annual values.  It should be noted that, while the samples were collected from March to August, permanent flood field water is normally held until two weeks prior to harvest (mid July through September, depending upon when the rice was planted), at which time it is released. It is believed that this practice contributes the greatest loads of fipronil to the Mermentau River Basin (USEPA, 2001), so maximum values, as well as chronic averages, could be higher.

      Based on the percent reduction from acute to chronic values generated from this monitoring data, and using the acute estimates generated using PFAM, chronic EDWCs were estimated using percent reductions of 50% (Table 15).  Glufosinate-ammonium and MPP results followed the same trends as discussed earlier in Section 5.2.  Glufosinate-ammonium chronic values were 2x higher than the PFAM values reported in Table 11, while MPP chronic values were 2x lower than the PFAM values.

Table 12. Comparison of molinate acute and chronic concentrations at surface water intakes

Location
(Source of Data[2])

              Frequency of Detection (Range of detection limits)
                                   Molinate
                       Parent + degradates of concern[1]

                       Maximum Concentration[3]  (ug/L)

                         Time-weighted Annual Means[3]
                                    (ug/L)
                                % Reduction[6]

                           Maximum Concentration[3]
                                    (ug/L)

                         Time-weighted Annual Means[3]
                                    (ug/L)
                                % Reduction[6]
Sacramento, California
(Raw water from the Sacramento River  after holding periods (1991-2000 data))[4]

                                    65/117
                                  (0.1 ug/L)
                                     1.52
                                     0.29
                                      81%
                                     2.37
                                     0.45
                                      81%
West Sacramento, California
(Raw water from the Sacramento River  after holding periods (1991-2000 data) adjusted for the percent flow data at City of  Sacramento)[5]

                                    65/117
                                  (0.1 ug/L)
                                     2.13
                                     0.41
                                      81%
                                     3.32
                                     0.64
                                      81%
Arkansas and Mississippi  (no surface water intakes in rice-production areas)
                                Not Applicable
                                       0
                                       0
                                      NA
                                       0
                                       0
                                      NA
New Orleans Area (Raw water from 1996-1999 USGS data in the Mississippi River at St. Francisville, LA)

                                     13/58
                                 (0.004 ug/L)
                                     0.117
                                     0.014
                                      88%
                                     0.18
                                     0.022
                                      88%
St. Mary Parish, Louisiana Intakes at Atchafalaya River  (Raw water from 1996-1999 USGS data at Melville, LA)

                                     15/57
                                 (0.004 ug/L)

                                     0.105
                                     0.018
                                      83%
                                     0.17
                                     0.028
                                      84%
   1.    Maximum and annual mean concentrations for molinate adjusted by a factor of 1.56 to account for hydroxy molinate (2-,3-,4-), molinate sulfoxide and acid, and 4-keto molinate, considered degradates of concern which were identified in registrant submitted studies and open literature.
   2.    Indicates source that authors of the molinate RED used to develop the estimates.
   3.    Maximum concentration is the upper 95th percentile of the annual maximum concentration values. Time-weighted annual means are the upper 95[th] percentile of the annual means. In estimating annual means, non-detect values were set equal to the limit of detection.
   4.    Sacramento gets their drinking water from the intake that is below the confluence of the American and Sacramento Rivers.  Approximately 71.4 % of their water is obtained from the Sacramento River.
   5.    For West Sacramento, California, all of their water is obtained from the Sacramento River.  To adjust for the source of water, the maximum and annual concentrations for the City of Sacramento were divided by the average percent of Sacramento River that Sacramento receives (71.4 % from 1991-2000), and multiplied by 100.
   6.    The % Reduction is calculated by dividing the difference in the maximum concentration and the time-weighted average mean by the maximum concentration and multiplying by 100.

Table 13. Comparison of peak and time-weighted average seasonal concentrations for thiobencarb at Colusa Basin Drain #5
                                     Year
                                     % NDs
                            Sampling Period (days)
                          Peak concentration (ug/L)
                      TWA Seasonal Concentration (ug/L)
                                  % Reduction
                                     1994
                                      0%
                                      63
                                     37.4
                                     2.75
                                      93%
                                     1995
                                      38%
                                      97
                                     3.67
                                     0.93
                                      75%
                                     1996
                                      30%
                                      87
                                     16.55
                                     2.74
                                      83%
                                     1997
                                      19%
                                      87
                                     12.30
                                     2.23
                                      82%
                                     1998
                                      29%
                                      100
                                     11.00
                                     2.49
                                      77%
                                     1999
                                      7%
                                      72
                                     11.80
                                     3.38
                                      71%
                                     2000
                                      0%
                                      44
                                     10.70
                                     5.61
                                      48%
                                     2001
                                      26%
                                      100
                                     5.90
                                     1.82
                                      69%
                                     2002
                                      35%
                                      100
                                     7.58
                                     2.41
                                      68%

Table 14. Comparison of peak and time-weighted average seasonal concentrations for fipronil in Louisiana[1]
                                    Parish
                                    Site ID
                                     2000
                                     2001

                               Max Conc (ug/L)
                           TWA Seasonal Conc (ug/L)
                                  % Reduction
                               Max Conc (ug/L)
                           TWA Seasonal Conc (ug/L)
                                  % Reduction
Vermilion
IX-01
                                     0.37
                                     0.029
                                     92.0%
                                      --
                                      --
                                      --
Acadia
IX-02
                                     6.78
                                     1.100
                                     83.3%
                                      --
                                      --
                                      --
Acadia
IX-03
                                     4.09
                                     0.391
                                     90.5%
                                      --
                                      --
                                      --
Vermilion
IX-04
                                     1.74
                                     0.261
                                     85.0%
                                     0.52
                                     0.052
                                     89.9%
Acadia
IX-05
                                     4.08
                                     0.722
                                     82.3%
                                     2.19
                                     0.316
                                     85.6%
Acadia
IX-06
                                     2.81
                                     0.593
                                     78.9%
                                     1.06
                                     0.266
                                     74.9%
Acadia
IX-07
                                     8.41
                                     0.910
                                     89.2%
                                      --
                                      --
                                      --
Acadia
IX-08
                                     3.47
                                     0.398
                                     88.5%
                                     1.44
                                     0.500
                                     65.3%
Jefferson Davis
IX-09
                                     4.16
                                     0.409
                                     90.2%
                                      --
                                      --
                                      --
Jefferson Davis
IX-10
                                     4.88
                                     0.393
                                     91.9%
                                      --
                                      --
                                      --
Jefferson Davis
IX-11
                                     3.12
                                     0.436
                                     86.0%
                                     2.38
                                     0.475
                                     80.0%
Cameron
IX-15
                                     0.79
                                     0.128
                                     83.7%
                                     0.41
                                     0.065
                                     84.0%
Acadia
IXWM-03
                                     4.76
                                     0.682
                                     85.7%
                                      --
                                      --
                                      --
Acadia
IXWM-04
                                     4.98
                                     1.030
                                     79.3%
                                     2.83
                                     0.448
                                     84.2%
Jefferson Davis
IXWM-05
                                     6.24
                                     0.741
                                     88.1%
                                     5.17
                                     0.485
                                     90.6%
Acadia
IXWM-06
                                     2.63
                                     0.258
                                     90.2%
                                     0.98
                                     0.316
                                     67.8%
Jefferson Davis
IXWM-07
                                     1.53
                                     0.185
                                     87.9%
                                     1.54
                                     0.246
                                     84.0%
Acadia
IXWM-08
                                     4.09
                                     0.699
                                     82.9%
                                     2.66
                                     0.347
                                     86.9%
   1.    Stations were sampled weekly from March through August in 2000. In 2001, weekly water column samples were collected beginning in March and continued until no detects were observed at any of the stations or August, whichever came first.

Table 15. Chronic EDWCs assuming 50% reduction based on monitoring data for other rice pesticides
Use rate, number of apps, retreatment interval, water holding period, application timing
                                     Site
                             Glufosinate-ammonium
                                      MPP

                                  Acute EDWC
                                    (ug/L)
                          Annual Average EDWC (ug/L)
                                  Acute EDWC
                                    (ug/L)
                          Annual Average EDWC (ug/L)

                                     4"
                                     8"
                                     4"
                                     8"
                                     4"
                                     8"
                                     4"
                                     8"
1.46 lbs ai/acre, 1, NA, 7 days, dry[1]
                                      AR
                                      CA
                                      LA
                                      MS
                                      TX
                                      165
                                      173
                                      163
                                      161
                                      163
                                      84
                                      86
                                      84
                                      83
                                      84
                                      83
                                      87
                                      82
                                      81
                                      82
                                      42
                                      43
                                      42
                                      42
                                      42
                                      4.5
                                      5.0
                                      5.1
                                      5.7
                                      5.2
                                      2.3
                                      2.5
                                      2.7
                                      2.9
                                      2.7
                                      2.3
                                      2.5
                                      2.6
                                      2.9
                                      2.6
                                      1.2
                                      1.3
                                      1.4
                                      1.5
                                      1.4
0.73 lbs ai/acre, 2, 10 days, 30/55 days, dry and flooded[2]
                                      AR
                                      CA
                                      LA
                                      MS
                                      TX
                                      344
                                      390
                                      318
                                      313
                                      344
                                      294
                                      320
                                      288
                                      282
                                      296
                                      172
                                      195
                                      159
                                      157
                                      172
                                      147
                                      160
                                      144
                                      141
                                      148
                                      159
                                      173
                                      158
                                      183
                                      181
                                      73
                                      81
                                      83
                                      88
                                      85
                                      80