LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 1 4.0 Bank Filtration 4.1 Introduction Bank filtration is a surface water pretreatment process that uses the bed and bank of a river ( or lake) and the adjacent aquifer as a natural filter. In optimal locations and under optimal conditions, bank filtration is suitable for accomplishing sufficient Cryptosporidium removal to partially meet the requirements of the Long Term 2 Enhanced Surface Water Treatment Rule. To accomplish this, a pumping well located in the adjacent aquifer induces surface water infiltration through the bed and bank. Bank filtration differs significantly from artificial recharge and from aquifer storage and recovery, both of which rely on engineering works to move water into specially constructed and maintained recharge basins or wells for infiltration into or replenishment of the aquifer. Although microorganism removal can occur in such engineered systems, they are not bank filtration. This is because bank filtration relies solely on the natural properties of the surface water bed and aquifer, unmodified by engineered works or activity, except for the recovery of ground water via a pumping well.

A significant proportion of microorganisms and other contaminants are removed by contact with the aquifer material as the water travels to the well through the subsurface. Flow to the well may be horizontal or vertical, but more typically will take a variable path with both horizontal and vertical components. The water which has been induced to infiltrate through the river's bed and bank is known as " bank filtrate." It will be mixed with ambient ground water that has taken a different and typically longer path to the well. The ambient ground water may have originated as bed or bank infiltration from an upstream portion of the river or from a lake. It may have originated from infiltrating precipitation. Regardless, ambient ground water is likely to contain different contaminants and contaminant concentrations than bank filtrate because its origin and flow pathways differ significantly. Ambient ground water should not be assumed to be uncontaminated.

Aquifers suitable for bank filtration are composed of unconsolidated, granular material ( i. e., grains) and have open, interconnected pores that allow ground water to flow. Pathogen removal is enhanced when fine grained sediment is present along the flow path. Geologic units consisting primarily of fine grained ( e. g., clay sized) materials will have higher removal but will be incapable of yielding economically significant water flow rates. In aquifers containing both sandsized and finer grains, the presence of fine grains increases the possibility that pathogens will encounter a grain surface. This is because flow is slower and flow paths are longer than they would be in aquifers without such fine grains. Microorganisms will be removed from flow as they contact and attach to grain surfaces. Although microorganism ( e. g., Cryptosporidium) detachment can occur, it usually does so at slow rates ( Harter et al., 2000). When little or no detachment occurs or when detachment is slow, microorganisms can become non viable while attached to grain surfaces. Thus, bank filtration provides physical removal, and in some cases, inactivation, to remove pathogens from water supplies. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 2 The purposes of this chapter are: 1) to clarify the requirements of the LT2ESWTR related to receiving Cryptosporidium removal credit for the use of bank filtration systems 2) to present the current state of the science, advantages and disadvantages of Cryptosporidum removal by bank filtration; 3) to explain how local geologic and hydrologic conditions affect the functioning and effectiveness of bank filtration systems; and 4) to provide suggestions for optimal operation of bank filtration systems.

This chapter is organized as follows:

4.2 LT2ESWTR Compliance Requirements  describes requirements for receiving Cryptosporidium removal credits related to the proposed installation of bank filtration wells.

4.3 Toolbox Selection Considerations  describes the advantages and disadvantages of using bank filtration as a pretreatment technology.

4.4 Site Selection and Aquifer Requirements  characterizes surface water and aquifer types that are suitable for bank filtration.

4.5 Design and Construction  describes the types of wells eligible for bank filtration credits and the locations at which such wells are best placed.

4.6 Operational Considerations  describes issues relevant to the optimal operation of bank filtration systems in order to protect public health.

4.2 LT2ESWTR Compliance Requirements Systems that propose to install bank filtration wells to meet any additional treatment requirements imposed by the LT2ESWTR may be eligible for 0.5 or 1.0 log Cryptosporidium removal credit ( 40 CFR 141.726( c)). Systems meeting all regulatory requirements ( e. g. systems with conventional or direct filtration that meet the well siting requirements) receive Cryptosporidium log removal credit prior to construction of the production wells. For those systems which already use bank filtration as a component of their treatment process and which also have existing conventional or direct filtration treatment, the LT2ESWTR requires source water monitoring of produced water from the bank filtration well. This will determine the initial bin classification for these systems. Because their source water monitoring accounts for any bank filtration treatment, these systems are not eligible for subsequent additional bank filtration credits ( 40 CFR 141.704).

Systems using ground water under the direct influence ( GWUDI) of surface water or bank filtered water without additional filtration must take source water samples in the surface water to determine bin classification ( 40 CFR 141.704). This applies to systems using an alternative Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 3 filtration demonstration to meet the Cryptosporidium removal requirements of the IESWTR or LT1ESWTR ( 40 CFR 141.173( b) and 141.552( a)). As a result, the requirements and guidance provided in this chapter do not apply to existing primacy agency actions providing alternative filtration Cryptosporidium removal credit for IESWTR or LT1ESWTR compliance.

4.2.1 Credits The LT2ESWTR specifies the following design requirements for systems to receive log removal credit for bank filtration ( 40 CFR 141, Subpart W, Appendix A):

° Wells must draw from granular aquifers that are comprised of clay, silt, sand, or pebbles or larger particles. Minor cement may be present.

° The aquifer material must be unconsolidated, with subsurface samples friable upon touch.

° Granular aquifers formed by alluvial or glacial processes are eligible for bank filtration credit.

° Granular aquifers, either unconsolidated or partially consolidated, and mapped as earlier than Quaternary alluvium, must be considered on a case by case basis by the state to determine if they are too cemented, and therefore too fractured, to provide sufficient natural filtration.

° Wells located in consolidated clastic aquifers ( e. g., conglomerates), fractured bedrock aquifers, and karst limestone aquifers are not eligible for bank filtration credit.

° Only horizontal and vertical wells are eligible for bank filtration log removal credit.

° Other ground water collection devices such as infiltration galleries and spring boxes are ineligible.

° Systems using horizontal or vertical wells located at least 25 feet from the surface water source are eligible for a 0.5 log removal credit and those located at least 50 feet from the surface water source are eligible for a 1.0 log removal credit.

° Systems with vertical wells must identify the distance to surface water using the floodway boundary or 100 year flood elevation boundary as delineated on Federal Emergency Management Agency ( FEMA) Flood Insurance Rate maps. If the floodway boundary or 100 year flood elevation boundary is not already delineated, systems must determine the floodway or 100 year flood elevation boundary using Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 4 methods substantially similar to those used in preparing FEMA Flood Insurance Rate maps.

° Systems with horizontal wells must measure the distance from the normal flow stream bed to the closest horizontal well lateral.

° Systems must characterize the aquifer at the proposed production well site to determine aquifer properties.

° At a minimum, the aquifer characterization must include the collection of relatively undisturbed continuous core samples from the surface to a depth at least equal to the projected bottom of the well screen for the proposed production well.

° The recovered core length must be at least 90 percent of the total depth to the projected bottom of the well screen and each sampled interval must be a composite of no more than 2 feet in length.

° Each composite sample must be examined to determine if at least 10 percent of the grains in that interval are less than 1.0 mm in diameter. Each composite sample with at least 10 percent of the grains less than 1.0 mm in diameter is considered an interval with sufficient fine grained material to provide adequate removal.

° An aquifer is eligible for removal credit if at least 90% of the composited intervals contain sufficient fine grained material as defined previously.

4.2.2 Monitoring Requirements The LT2ESWTR requires systems to monitor turbidity in bank filtration wells to provide assurance that the assigned log removal credit is appropriate. The LT2ESWTR specifically requires the following monitoring ( 40 CFR 141.726( c)( 1)):

Turbidity measurements must be performed on representative water samples from each wellhead every four hours that the bank filtration system is in operation or more frequently if required by the state.

Continuous turbidity monitoring at each wellhead may be used if the system validates the continuous measurement for accuracy on a regular basis using a protocol approved by the state.

If the monthly average of daily maximum turbidity values at any well exceeds 1 NTU, the system must report this finding to the state within 30 days. In addition, within 30 days of the exceedance the system must conduct an assessment to determine the cause Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 5 of the high turbidity levels and submit that assessment to the state for a determination of whether any previously allowed credit is still appropriate.

4.3 Toolbox Selection Considerations Bank filtration is best suited to systems that are located adjacent to rivers with reasonably good surface water quality and that plan to use bank filtration as one component of their treatment process. For systems that can meet the aquifer requirements ( section 4.4) and the design criteria ( section 4.5), bank filtration can be an efficient, cost effective pretreatment option to improve water quality ( Berger, 2002). Medema et al. ( 2000) and Wang et al ( 2000, 2002) documented high removal of Cryptosporidium indicator organisms at production well sites in The Netherlands and in Louisville, Kentucky. There was very little occurrence of Cryptosporidium in river water at the Kentucky site and no Cryptosporidium was found in the well water at either site. The amount of Cryptosporidium removal at either site is unknown.

The efficient removal of indicator organisms at the Netherlands site was likely due to the relatively impermeable, fine grained layer of river sediment present, as well as the effect of pyrite oxidizing to ironhydroxides. Ironhydroxides may enhance the attachment of microorganisms to riverbed sediments ( Medema et al, 2000). In Louisville, Kentucky, an alluvial aquifer was chosen for the bank filtration site. Wang et al ( 2000, 2002) found that removal of particles increased with filtration distance of the riverbank filtration process, although most of the removal occurred at the surface of the riverbed, within the first two feet of filtration. Wang et al ( 2002) attributed the removal in their bank filtration system to a combination of mechanical filtering and biological activity ( e. g., biofiltering) at the surface of the riverbed.

As discussed in section 4.4, only certain sites are suitable for bank filtration. It is important to understand the type of bed and aquifer material present, the dynamics of groundwater flow, and the potential for scouring of riverbed materials at a potential bank filtration site. The degree to which the bed and banks of surface water bodies may effectively filter Cryptosporidium may vary not only only from site to site, but also at a single site over time.

4.3.1 Advantages and Disadvantages <IP_ADDRESS> Removal of additional contaminants The two research sites with published data ( Medema et al., 2000; Wang et al., 2000; Wang et al., 2002; Berger, 2002) have reported that bank filtration is effective at removing Cryptosporidium. Bank filtration has also been shown at some sites to be an effective technology for attenuating a variety of additional microorganisms as well as particulates, ammonia, nitrate, pesticides ( e. g., atrazine), heavy metals, ethylenediamine tetra acetic acid ( EDTA), alkylated and chlorinated benzenes and other organic contaminants, and disinfection by product precursors ( DBPs) in the form of natural organic matter ( NOM) ( Schijven et al., 2003; Tufenkji et al., 2002; Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 6 Ray et al, 2002; Kuehn and Mueller, 2000). Bank filtration achieves the removal of these diverse contaminants by facilitating or enhancing physical and chemical filtering, sorption, reduction/ oxidation, precipitation, ion exchange, and biodegradation ( Schijven et al., 2003; Ray et al., 2002; Tufenkji et al., 2002). Bank filtration further reduces contaminant concentrations and especially shock contaminant loads from spills and intentional acts by providing for the multidimensional dispersion and dilution of contaminants ( Ray et al., 2002).

The degree to which any particular contaminant will be removed via bank filtration depends on site specific conditions. For example, under aerobic conditions, ammonia is often completely transformed, whereas such removal may not occur under more reducing conditions. Oxygen is usually significantly depleted within 5 15 feet of the riverbed, due to microbial activity in this zone. As infiltrating water becomes increasingly depleted of organic matter due to degradation, microbial activity diminishes, and the aquifer may be reaerated at a certain distance from the riverbed ( Tufenkji et al., 2002). The anaerobic part of the aquifer was observed to remove up to 99% of polar organic contaminants at a site in central Germany ( Juttner, 1995). Miettinen et al ( 1994) found that almost 90% of the high molecular weight fraction of NOM had been removed at a bank filtration site in Finland.

The reduction in some treatment costs made possible by bank filtration results from a reduced need for other treatment technologies. When bank filtration decreases the concentration of dissolved organic carbon reaching a treatment plant, costs are lowered because a decreased proportion of dissolved contaminants needs to be adsorbed onto activated carbon filters. Thus, each filter is capable of operating for a longer period of time, and fewer replacement filters are needed. Particle and microorganism removal during bank filtration allows for more efficient filtration, use of membranes, and disinfection during subsequent treatment steps. The removal of ammonia means that the additional treatment step of oxidizing ammonia with chlorine may be unnecessary. The removal of nitrate when water is induced to flow through anaerobic areas may eliminate the need for expensive ion exchange or reverse osmosis treatment processes ( Kuehn and Mueller, 2000). Finally, because it is effective at biodegrading many contaminants, bank filtration reduces the need for adding large quantities of flocculants to drinking water, thereby reducing both costs and the unhealthful effects of water treatment residuals ( Kuehn and Mueller, 2000).

Another advantage of bank filtration as a pretreatment technology is that it acts to equalize fluctuations in contaminant concentrations observed in surface waters. This is due to the effects of dilution and dispersion which serve to spread peaks in contaminant concentrations over space and time by the time they reach wells. Contaminant concentration peaks may be due to variations in river water levels, seasonal effects, and runoff, in addition to spills, terrorist acts and emissions by municipal and industrial institutions ( Kuehn and Mueller, 2000). Bank filtration also smooths out fluctuations in water temperature. Bank filtration is continuously active, and the decreased amplitude of the contaminant peak by the time it reaches a well ( an inherent result of subsurface transport through porous material) allows for easier and less expensive treatment by utilities with limited capabilities. In addition, the time lag between contamination of surface water and arrival of contaminant at a well would give utilities more of an opportunity to respond to a Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 7 threat or an accidental spill. Kuehn and Mueller ( 2000) estimate that in many modern bank filtration systems bank filtrate spends anywhere from 5 to 15 days in the subsurface before reaching supply wells. At one site in the Netherlands, bank filtrate was estimated to spend 45 65 days in the subsurface before reaching the supply well ( Medema, et al., 2000). Residence time depends on site specific hydrogeology as well as bank filtration system design.

The removal of NOM during bank filtration is useful because NOM occurrence can result in the production of harmful disinfection byproducts, as discussed above. In addition, moderate to high concentrations of NOM in drinking water can result in unpleasant taste and odor. Finally, NOM removal via bank filtration can also aid in the removal of a large variety of additional organic and inorganic contaminants. These contaminants are sometimes made more mobile in surface and ground waters due to a partitioning process whereby they are attached to NOM, which is relatively mobile, and thereby carried along a flow path. The removal of NOM and associated contaminants prior to above ground treatment is likely to lessen the overall cost of water treatment at a given facility.

<IP_ADDRESS> Clogging of pores Clogging of the surface water  ground water interface has the potential to be a problem with any riverbank filtration system, and results from physical, chemical, and biological processes. Partial clogging during riverbank filtration system operation is likely to be unavoidable ( Wang et al., 2001), however its effects are not always deleterious. The disadvantage of clogging is that it can reduce hydraulic conductivity of the local riverbed and the aquifer, thereby temporarily or permanently reducing well yields. On the other hand, a limited accumulation of fine grained sediments and the accompanying development of a biologically active zone can enhance pathogen removal. Indeed, this enhanced removal is a basic principle behind riverbank filtration as a water treatment technology. An optimal amount of clogging is beneficial because it can reduce the size of large pores or reduce entrances to pores in a stream bed or aquifer. Pore size reduction and decreased hydraulic conductivity also result in longer travel times which can result in additional pathogen inactivation. Transport of fewer pathogens is also likely because there are more opportunities for pathogen contact with aquifer grain surfaces.

Physical clogging of the surface water  ground water interface results from the deposition of fine grained, suspended sediment at the interface and in the near surface pores. The deposition and growth of microorganisms also contribute to physical clogging. This clogging may be exacerbated during periods of low surface water discharge, and is most apparent near the river's edge where flow velocities are generally lower than at the center of the river. Chemical clogging can result from precipitation of dissolved surface water constituents and may occur near the interface or anywhere along the flowpath. This is due to the change in geochemical conditions as infiltrating water enters the riverbed and aquifer. Factors to be considered when evaluating the potential for chemical clogging include electrolyte concentration, pH, redox potential, presence of dissolved or colloidal organic matter, and the mineralogy and surface characteristics of stream bed and aquifer solids. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 8 Finally, biological or microbial clogging can result from the accumulation of bacterial cells in pore spaces, the production of extra cellular polymers, the release of gaseous byproducts from denitrifying bacteria and methanogens, and the microbially mediated accumulation of insoluble precipitates ( Vandevivere et al., 1995; Baveye et al., 1998). Biogenic gas bubbles have the effect of blocking or partially blocking water flow through pores in much the same way that solid particles do ( Orlob and Radhakrishna 1958; Oberdorfer and Peterson 1985; Sanchez de Lozada et al., 1994). Insoluble sulfide salts can cause clogging due to the activity of sulfate reducing bacteria, whereas iron hydroxide and manganese oxide deposition can be brought on by bacterial iron metabolism ( Vandevivere et al., 1995; Baveye et al., 1998). Biological clogging is most likely to occur near the surface water  ground water interface where nutrients are most available.

Some or all of these processes may act at a particular site to lower hydraulic conductivity and thus decrease flow velocities. For example, several months of pumping from a new riverbank filtration well in Louisville, Kentucky resulted in a significant decline in well production, presumably due to a 70% reduction in leakance from the river to the adjacent aquifer. The reduced well yields were attributed to the physical clogging of riverbed sediments ( Schafer, 2000). The disadvantage of reduced well yields accompanies the advantages of increased microbial inactivation rates due to lower flow velocities ( and thus longer residence times in the aquifer) as well as increased removal of pathogens due to smaller pores.

<IP_ADDRESS> Scour Both the positive and negative effects of clogging on riverbank filtration system performance may be diminished following periodic flooding. Scour refers to the erosion of the river's bed and banks, and depends on both flood conditions and the resistance of the bed and bank material that has been deposited at a particular site. During flooding the river channel may be scoured, and fine sediments at the surface water  ground water interface mobilized.

Much of the removal of the contaminants and microbes discussed above occurs during the first few centimeters of the flow path, due to the significant filtering and sorptive capabilities of sediments in the riverbed. These sediments are often organic rich, highly biologically active, and fine grained. The effectiveness of bank filtration, however, may be temporarily threatened during high flows if this active layer is washed away or scoured. EPA suggests the potential for stream channel scour be evaluated during riverbank filtration site selection ( section 4.4). Section 4.5 provides further discussion of scour and its implications for riverbank filtration system operation.

<IP_ADDRESS> Additional Treatment Steps In addition to clogging and scour, there are several disadvantages to bank filtration which utilities may wish to consider and balance against the advantages and cost savings described in section 4.3.1. One disadvantage is that an additional aeration step may be required during water treatment due to the possible depletion of oxygen as biological activity consumes oxygen during riverbank filtration pretreatment ( Kuehn, et al., 2000). This oxygen depletion may lead to Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 9 extremely anaerobic conditions over a portion of the flow path, which may sometimes result in the release of iron and manganese from the bank sediment into the flowing water. This process occurs due to a redox reaction which reduces iron and manganese to their water soluble forms. This condition may necessitate the removal of these metals during subsequent treatment steps ( Kuehn, et al., 2000; Tufenkji et al., 2002).

On the other hand, if the flow path between the riverbank and the well is long enough, iron and manganese may precipitate onto the sediments in the subsurface before ever reaching the well ( Tufenkji et al., 2002). The aquifer becomes reaerated with increasing distance from the riverbed. This is one reason for locating riverbank filtration wells greater than 25 or 50 feet from the river, as discussed in section <IP_ADDRESS>. Even though most contaminant removal occurs during the first few centimeters of subsurface transport, the reaeration and associated precipitation reactions in the aquifer may significantly improve water quality before it reaches the well ( Tufenkji et al., 2002). The location of the aerated and anaerobic portions of the aquifer vary seasonally due to variable microbial activity and changing pumping rates.

Finally, riverbank filtration is ineffective at removing a few persistent compounds, primarily non polar organic compounds and highly soluble chemical contaminants such as methyltertiarybutylether ( MTBE) and trichloroethylene ( TCE), which would need to be addressed during subsequent treatment steps. In addition, when bank filtration is used to induce infiltration of highly contaminated surface water, it may be important to include additional adsorption steps during later treatment ( Kuehn, et al., 2000).

4.4 Site Selection and Aquifer Requirements Unconsolidated, granular aquifers with sufficient amounts of fine grained material ( see section 4.4.2) are eligible for Cryptosporidium removal credits under the LT2ESWTR. Partially consolidated, granular aquifers may also be eligible for removal credits. Each granular aquifer proposed as a bank filtration site is to be evaluated on a case by case basis with regard to its grain size distribution and degree of cementation. For example, a partially consolidated, granular aquifer may be too cemented, and thus perhaps too fractured, to provide adequate pathogen removal. Geophysical methods, discussed in section <IP_ADDRESS>, may be helpful in determining the degree of fracturing of such aquifers.

This section characterizes river and aquifer types that may be suitable for bank filtration surface water treatment. A list of selected sites in the United States and Europe which have used bank filtration is provided for reference. No information is available for these sites, however, regarding whether they would meet the siting criteria in the LT2ESWTR. Some common aquifer types that are clearly not appropriate for this technology are described as well. Finally, sitespecific aquifer criteria which shall be met in order for systems to receive Cryptosporidium removal credits are outlined in section 4.4.3. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 10 Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 11 4.4.1 Selected Bank Filtration Sites Table 4.1 Selected Bank Filtration Systems in Europe and the United States Site Location Well Type* Number of Wells Maximum Capacity mgd ( m3/ s) River System Europe Torgau, Germany V 42 39.7 ( 1.737) Elbe Mockritz, Germany V 74 28.8 ( 1.260) Elbe United States Cincinnati, OH V 10 40.0 ( 1.750) Great Miami Columbus, OH H 4 40.0 ( 1.750) Scioto/ Big Walnut Louisville, KY H 1+ 20.0 ( 0.875) Ohio Terra Haute, IN H 1 12.0 ( 0.525) Wabash Jacksonville, IL H 1 8.0 ( 0.350) Illinois Galesburg, IL H 1 10.0 ( 0.438) Mississippi Henry, IL V 1 0.7 ( 0.030) Illinois Mt. Carmel, IL V 1 1.0 ( 0.044) Wabash Quincy, IL H 1+ 10.0 ( 0.438) Mississippi Sacramento, CA H 1 10.0 ( 0.438) Sacramento Sonoma County, CA H, V 5 ( H) + 7 ( V) 85.0 ( 3.727) Russian Independence, MO H * 1 15.0 ( 0.656) Missouri Lincoln, NB H, V 2 ( H) + 44 ( V) 35.0 ( H) ( 1.530) Platte Kennewick, WA H 1 3.0 ( 0.130) Columbia Kalama, WA H 1 2.6 ( 0.110) Kalama St. Helens, OR H 3 5.0 ( 0.219) Columbia Kansas City, KS H 1 40.0 ( 1.750) Missouri Sioux Falls, OK H 1+ 40.0 ( 1.750) Missouri * H   horizontal, V   vertical * Gravel packed Laterals Reprinted from Journal AWWA, Vol. 94, No. 4 ( April 2002), by permission. Copyright © 2002, American Water Works Association. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 12 4.4.2 Aquifer Type <IP_ADDRESS> Unconsolidated, Granular Aquifers Unconsolidated, granular aquifers can be composed of a wide range of sediment sizes including clay, silt, sand, and larger particles. They may also exhibit minor cementation, but subsurface samples are typically friable ( readily crumbled by hand). To be eligible for bank filtration credits under the LT2ESWTR, unconsolidated granular aquifers are expected to contain a sufficient amount of fine grained sediments to achieve adequate pathogen removal and/ or inactivation ( section 4.4.3 prescribes the amount deemed sufficient). In aquifers with these characteristics, the flow path is tortuous at the micro scale ( Figure 4.3), providing many opportunities for removal of microorganisms by straining or by their attachment to grain surfaces.

Many alluvial aquifers contain significant amounts of well sorted, fine grained sediments. Alluvial aquifers are produced by fluvial depositional processes and are adjacent to modern streams. Aquifers formed in glacial deposits may also contain sufficient amounts of fine grained material. These may be " till" deposits, which have a wide range of poorly sorted sediment sizes, or glacial outwash deposits that are formed by meltwater and often contain well sorted, sandsized sediments. Any of these alluvial or till aquifers would be likely to be suitable for a bank filtration system. On the other hand, coarse gravel aquifers produced by the rapid drainage of glacial lakes, or in outwash environments that deposit little fine grained material, may not be eligible for bank filtration credits unless sieve analysis shows sufficient fine grained material as discussed in section <IP_ADDRESS>.

Alluvial aquifers may be identified on detailed hydrogeologic maps simply as " Quaternary alluvium", indicating both their genesis and relative age. Glacial deposits are documented on surficial geology maps and, where aquifer forming, may be identified on large scale hydrogeologic maps.

<IP_ADDRESS> Karst, Consolidated Clastic, and Fractured Bedrock Aquifers In karst, consolidated clastic, and fractured bedrock aquifers, ground water velocities are fast, and flow paths may be direct, allowing microbial contaminants to travel rapidly to a well with little removal or inactivation. Therefore, these aquifer types are not eligible for bank filtration treatment credits.

Karst may be broadly defined as a region where the dissolution of calcitic or other soluble bedrock, primarily limestone ( calcium carbonate), produces a unique subsurface drainage network and associated surface landforms. Ground water movement in karst aquifers differs from that in porous, granular aquifers in that flow in the former occurs predominantly in conduits and dissolution enlarged fractures. Consequently, there is little physical removal of microbes and other particles by filtration and few opportunities for microbes to come in contact with the Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 13 surfaces of aquifer materials. Furthermore, rapid flow creates conditions where inactivation is less likely to occur before ground water reaches a well.

Although fractures have a role in ground water movement through any aquifer, fractures provide the dominant flow path in fractured consolidated clastic and fractured bedrock aquifers. Most consolidated aquifers can be presumed to be fractured. Similar to solution conduits in karst aquifers, fractures in consolidated aquifers provide preferential flow paths that may transmit ground water at high velocities, and in a relatively direct flow path to a well, with little time or opportunity for inactivation or removal of microbial pathogens. Wells located in these aquifers would not be eligible for bank filtration credit.

<IP_ADDRESS> Partially Consolidated, Granular Aquifers Granular aquifers formed by marine processes earlier than Quaternary alluvial or glacial deposition may be partially consolidated by natural cement that fills pores, connects grains, and makes the aquifer material less friable. Partially consolidated, granular aquifers are present within the Atlantic Coastal Plain, Gulf Coast Lowland, Texas Coastal Upland, and Mississippi Embayment aquifer systems ( USGS 1998). When significant proportions of cement are present, fractures are more likely to exist. As in consolidated aquifers, fractures in partially consolidated, granular aquifers create direct paths for microbial contamination that minimize the natural filtration capabilities of the aquifer system. EPA suggests that partially consolidated aquifers be evaluated at the proposed well location to determine if they may be too cemented, and thus perhaps too fractured, to provide sufficient natural filtration.

The degree of cementation can be evaluated by a variety of methods. Geologic material collected from below the aquifer's weathered zone that is friable upon touch is likely to be adequate for bank filtration purposes. Another test for the degree of cementation includes the slaking test, which involves alternate wetting and drying of the sample in water, or in salt or alcohol solutions. Finally, a triaxial compression test can be used to measure strain in three mutually perpendicular directions. Less cemented samples will be more deformable during such tests.

4.4.3 Aquifer Characterization Systems seeking Cryptosporidium removal credit are required to characterize the aquifer properties between their surface water source and their well. The aquifer characterization will include, at a minimum, core sampling to determine grain size distribution. This data will establish whether enough fine grained sediment is present to provide adequate filtration. The following procedure outlines the steps necessary to perform such a characterization, which will ultimately determine eligibility for bank filtration treatment credits under the LT2ESWTR. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 14 16) Collect relatively undisturbed continuous core samples from the surface to a depth at least equal to the projected bottom of the well screen for the proposed production well.

17) Determine if recovered core consists of at least 90% of the interval from the surface to the planned location of the well screen bottom. If core recovery is insufficient, another well core must be obtained.

18) Examine each 2 foot long composite sample of recovered core in a laboratory using sieve analysis to determine grain size distribution. Core intervals are typically 2 feet long for a conventional split spoon sampler and 3 or 4 feet long for soil probes ( e. g., a Giddings type soil probe).

19) If more than 10 percent of the sediments in each 2 foot long composite sample are less than 1.0 mm in diameter ( very coarse sand), then the core interval from which it was taken is noted as containing a sufficient quantity of fine grained material to provide adequate pathogen removal.

20) To receive Cryptosporidium removal credit, at least 90 percent of the analyzed composited core intervals from the sampled aquifer will meet criterion number ( 4) above.

<IP_ADDRESS> Coring The collection of relatively undisturbed cores in unconsolidated aquifers can be quite difficult, especially when gravel sized clasts are present. The two most important criteria for successful test drilling to obtain a core are sample accuracy and drilling speed. Borehole stability is a major problem in drilling in an unconsolidated gravelly formation. Rotary core drilling is particularly suited to drilling in unconsolidated formations because the drilling fluid, which cools the drill bit and carries up the core, also acts to stabilize the borehole ( Driscoll, 1986).

Other drilling methods require the installation of a casing to stabilize the borehole, a process which slows down the speed of drilling. Rotary core drilling is the fastest method for drilling in an unconsolidated formation. One disadvantage to rotary core drilling is the separation of different sized core particles as they rise ( smaller particles rise faster) and cross contamination by overlying borehole material. An experienced driller can avoid cross contamination by using the dual wall method of rotary core drilling. In the dual wall method, the core is pushed up the inner pipe of the drill rather than traveling in the space between the drill and the borehole wall ( Driscoll, 1986). Shallow wells will have fewer particle size separation problems than deeper wells.

Auger drilling is another method for drilling test wells. In this method an earth auger is screwed into the earth by rotation. Auger drilling in an unconsolidated formation is slower than rotary core drilling, due to the necessary installation of casing to support the borehole. Sampling Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 15 with augers can provide reliable samples from any depth. A split spoon sampler can be used wherein a split spoon is driven to the bottom of the hole. The depth to which an auger can drill is dependent on the size of the rig. The maximum drilling depth possible for a small drill rig is approximately 250 ft. ( Driscoll, 1986). Information about drilling and finding a driller can be found through the National Groundwater Association ( NGWA) website: http:// www. ngwa. org/. In addition, the EnviroDirectory   provides listings for laboratories and drillers in New England, the Mid Atlantic, and the Great Lakes regions ( www. envirodirectory. com).

<IP_ADDRESS> Sieve Analysis The American Society for Testing and Materials ( ASTM) has a published standard for conducting sieve analysis, the Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates: Standard C 136 1 ( ASTM, 2003).

Sieve analysis is used to determine the particle size distribution of a sample of dry aggregate of known mass by passing the sample through a series of sieves with progressively smaller openings. Sieve analysis requires the following equipment:

1. A balance, accurate to 0.1g or 0.1% of test load for fine aggregate, or accurate to 0.5g or 0.1% of test load for a mixture of fine and coarse aggregate 2. Stackable sieves 3. A mechanical sieve shaker ( for sample sizes greater than 20kg) 4. An oven capable of maintaining 110 ± 5oC ( 230 ± 9oF)

In the first step of sieve analysis the sample is dried using the oven. Once dry, its weight is measured and recorded. While the sample dries, sieves are selected with suitable openings to furnish the information required. For bank filtration related sieve analyses, it is only necessary to determine what percentage of the sample is less than 1.0mm; however, it is recommended that sieves covering a range of sizes be used so as to prevent the overloading of any one sieve. Once the sample is dry and the sieves are stacked in order of decreasing mesh size, the sample is placed in the top sieve and sieving either by machine or hand begins. Sieving should be continued until no more than 1% by mass of the material retained on an individual sieve will pass through that sieve during 1 minute of continuous hand sieving. Finally the mass on each sieve is weighed. The total mass of the material after sieving should correspond closely with the original mass of the sample. Using the mass for each size increment and the total mass of the sample, the size distribution of the sample can be determined ( ASTM, 2003).

Further information about sieve analysis can be found at the ASTM web site ( www. astm. org). A multi media sieve analysis demonstration can be found at Geotechnical, Rock and Water Resources Library ( GROW) ( http:// www. grow. arizona. edu/ geotechnical/ virtual_ labs/ sieveanalysis/ sieveanalysisexp. shtml). Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 16 ASTM also provides a search engine which allows the user to search for laboratories that perform sieve analyses ( http:// astm. 365media. com/ astm/ labs/). The EnviroDirectory   provides listings for laboratories and drillers in New England, the Mid Atlantic, and the Great Lakes regions ( www. envirodirectory. com).

4.4.4 Site Selection as it Relates to Scour Stream channel scour may often be an important consideration in choosing sites that are suitable for riverbank filtration. This section discusses stream channel erosional processes in general, as well as reasons sites with certain characteristics may be unsuitable for riverbank filtration. Section 4.6 discusses the implications of periodic scour for riverbank filtration system operations. Detailed information on fluvial erosional processes can be obtained from any of a number of texts on fluvial geomorphology, hydrology, and river hydraulics ( e. g., Leopold et al., 1964; Ritter et al., 1995; Chow 1964).

<IP_ADDRESS> Stream Channel Erosional Processes This discussion focuses on the dominant erosional processes of alluvial rivers because, given the LT2ESWTR's aquifer requirements, such rivers may be among the most suitable for bank filtration credits. Although many lake banks are also suitable sites for bank filtration, lakes will not be discussed in detail in this section. Lake bank filtration settings typically do not change rapidly with time and climate. Their hydrologic properties are not highly variable and thus do not require the detailed evaluation discussed here for riverbank filtration settings.

The width, depth, and gradient of an undisturbed alluvial river has typically adjusted to prevailing discharge conditions and sediment loads such that no net erosion or deposition occurs over long time periods ( Mackin 1948; Leopold and Maddock 1953). The quasi equilibrium condition of such rivers, which are referred to as " graded streams" ( Mackin1948), may be disrupted over short time periods ( e. g., due to floods), when erosion or deposition may be considerable.

The dominant scouring process in alluvial rivers is lateral migration. This process is responsible for the stream meanders visible on many floodplains, and is accomplished by the progressive erosion of the outside bank of a river bend with concurrent deposition on the inside bank. Because erosion is generally matched by deposition in this process, channel dimensions do not change significantly over time, and the net result is migration of the channel across the floodplain ( Figure 4.1). Stream channel meanders are characteristic of many alluvial rivers and are indicative of a graded stream.

Downcutting, another type of scour that can occur in fluvial environments, is the vertical erosion of the streambed. Downcutting is fairly uncommon in alluvial rivers except during floods or if the stream is not graded. The long term dynamic equilibrium of a graded stream can be Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 17 disrupted by a variety of changing hydrologic and geologic conditions and especially by anthropogenic activity. Human activities in a watershed or river channel may alter the conditions to which an alluvial river has become adjusted, initiating a period of readjustment marked by either progressive downcutting or aggradation ( deposition).

Urbanization generally increases the proportion of impervious surface in a watershed, increasing flood volumes during precipitation events because less water is able to infiltrate the land surface and recharge ground water ( Jacobsen et al., 2001). Increased flood volumes may cause higher water levels in a river channel, increasing the shear stress on the channel bed and causing scour ( Booth, 1990). Downcutting may continue until the channel gradient, and/ or channel dimensions, become adjusted to the new flooding regime.

Impoundment is another activity that may disrupt the quasi equilibrium state of a graded river and initiate readjustment of the channel. The sharp decrease in sediment supply, which commonly occurs subsequent to dam and reservoir construction, may initiate downcutting in the reach immediately downstream until the channel adjusts to the lightened sediment load. This has been observed downstream of many dams throughout the world. One of the most dramatic examples is the 7.5 meters of channel bed degradation that occurred twelve kilometers downstream of the Hoover Dam after its completion in 1935 ( Williams and Wolman, 1984).

The construction of artificial levees ( raised banks along a stream channel) also may result in flooding downstream. Levees allow greater quantities of water to be carried by the stream, thus decreasing the probability of flooding in the vicinity of the levee, but increasing flood hazards downstream ( Montgomery, 2000). Even if flooding downstream does not result, the high flows downstream may cause downcutting of the river, removal of fine grained bed material, and thus a threat to the protectiveness of a riverbank filtration system. Another possible effect of levees is an increase in sedimentation in the channel. Sediment that would otherwise be deposited on the floodplain may be trapped within the channel. This can raise the channel bottom and thus raise stream stage or the elevation of the water surface in the channel ( Montgomery, 2000). The consequences of this for a riverbank filtration system are variable. Increased sedimentation may lead to clogging and/ or decreased well yields. On the other hand, higher stream stages may result in flooding and scour along certain portions of the river as the channel adjusts to a new equilibrium condition. Understanding the impact of current or planned upstream activities can be an important part of site selection for a riverbank filtration system.

<IP_ADDRESS> Unsuitable Sites As discussed in section <IP_ADDRESS>, some sites may be ineligible for bank filtration credit due to the type of aquifer adjacent to the river. The following section, however, focuses on importance of understanding the nature of the surface water ground water interface at a potential bank filtration site. In some localities, frequent scour, the absence of a sufficiently fine grained interface, and/ or the coarse grained or fractured aquifer materials may suggest that a certain river or reach is not the best possible location for a bank filtration system. A system may choose to Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 18 evaluate such situations on a site by site basis, however, except as specified in the LT2ESWTR, EPA does not require such evaluations or any particular decisions made on the basis of such evaluations. EPA recommends, however, that this information be considered in order to ensure that bank filtration systems are protective of public health.

Data from studies of aerobic and anaerobic spore ( indicator organism) removal in bank filtration systems indicate that much of the removal of spores between a surface water body and a bank filtration pumping well takes place at the surface water   ground water interface and in the aquifer material proximal to that interface. The interface, which lines the bottom of the riverbed, is typically comprised of very fine grained, biologically active material a few inches to a foot in thickness. Some rivers, however, ( especially many in the western United States) lack this finegrained bed and the important organic rich materials or silts usually associated with the surface water  ground water interface ( Ray et al., 2002). For this reason, such rivers should be evaluated carefully to determine if bank filtration is a suitable treatment technology.

Other rivers may sometimes possess this important layer of fine grained sediment for the first few inches or feet from the surface water source, but may at other times be subject to periodic scour. The nature and performance of the surface water   ground water interface on such rivers may be altered temporarily by flood scour, specifically during the high river stages that occur periodically throughout the year. This situation is most likely to occur on uncontrolled rivers. Higher flow velocities in the river and increased bedload transport at such times mobilizes fine sediments ( which were deposited when discharges were lower). ( Note that bedload transport is the carrying of heavy, coarse sediments by saltation along the stream bed rather than by suspension. Saltation is the process in which particles jump from one point to the next along the stream bed.) Lower log removals are thus expected to occur during floods. If such situations are expected to occur very frequently, and if a system cannot envision a way of managing the system so as to adequately protect its water supply during such events, sites on such rivers may be inappropriate for riverbank filtration. On the other hand, through careful management it may be feasible to protect drinking water wells from the potentially negative consequences of occasional scour, as discussed in section 4.6.2.

EPA recommends that the potential for scouring be considered during site selection. If a site that undergoes occasional scour is selected for riverbank filtration, the system may wish to locate its wells at greater than the required separation distance from the surface water body, as discussed in section <IP_ADDRESS>. Such a solution helps to ensure the protection of public health. The drawback of this solution to the problem of scour, however, is that wells located at very great distances from surface water sources are drawing in more ambient ground water and less riverbank filtrate than wells located closer to the river or lake. One result of this is that the yield to the wells is likely to be smaller when the wells are located far from the surface water source.

The potential for scour can be evaluated initially by examining the past frequency of high flow and flood events. Data on flood history and discharge is typically available from the US Geological Survey, the Army Corps of Engineers, the US Bureau of Reclamation and the Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 19 Department of Homeland Security ( formerly FEMA). State and county highway and transportation departments typically evaluate river scour to determine the safety of bridge supports. A more comprehensive evaluation of the potential for scour can be conducted when the effect of past and current human activities ( as discussed in section <IP_ADDRESS>) is considered in comparison to the history of flood events.

Sources of high flow and flood data USGS Main Page: http:// water. usgs. gov The National Flood Frequency ( NFF) Program: http:// water. usgs. gov/ pubs/ wri/ wri024168/ pdf/ entirereport. pdf ° A computer program developed by the USGS for estimating the magnitude and frequency of floods for ungaged sites. Since 1993, updated equations have been developed by the USGS for various areas of the nation. These new equations have been incorporated into an updated version of the NFF Program. USGS Fact Sheets ( listed by state): http:// water. usgs. gov/ wid/ index state. html ° Includes NFF program methods for estimating flood magnitude and frequency ( in rural and urban areas) for: AL, AZ, AR, CA, CN, HI, LA, MD, MO, NV, NM, NC, OK, SC, SD, TX UT, VT, VA, and WA. These fact sheets describe the application of the updated NFF Program to various waterways within the specific State. Includes maps of each of the above state's hydrologic regions, as well as regression equations and statistics. WaterWatch: http:// water. usgs. gov/ cgi bin/ dailyMainW? state= us& map_ type= flood& web_ type= map ° Map of current flood and high flow conditions in the United States. The map shows the location of streamgages where the water level is currently at or above flood stage (

) or at high flow (  ) . The high flow conditions are expressed as percentiles that compare the current ( i. e., within the past several hours) flow value to historical daily mean flow values for all days of the year. The real time data used to produce the maps have not been evaluated or edited.

Army Corps of Engineers Main Page: http:// www. usace. army. mil/ ° Flood control and management pages. For example, river and reservoir reports including flood level data are available for the St. Louis district of Missouri ( see example below) ( http:// mvs wc. mvs. usace. army. mil/ dresriv. html).

Mississippi River:

River Mile Gage Station 6 am Levels 24 hr Change National Weather Service River Forecasr Flood Level Gage Zero Record Level Record Date Next 3 Days Crest Date 309.0 301.2 Hannibal Dam 22 tw 16.9 15.9  0.2  0.3 16.6 16.1 15.6 15.8 15.3 14.7 16.0 16.0 449.3 446.1 31.80 29.58 07/ 10/ 93 07/ 16/ 93 Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 20 US Bureau of Reclamation Main Page: http:// www. usbr. gov/ main/ Dams and Reservoirs Page: http:// www. usbr. gov/ dataweb/ html/ dam_ selection. html ° The project DataWeb provides the most current information on the bureau's projects, facilities, and programs including dam and reservoir information for western states. This data can be obtained by selecting a dam or from the State and Region maps.

The Department of Homeland Security ( formerly FEMA) Main Page: http:// www. fema. gov/ Flood Hazard Mapping: http:// www. fema. gov/ fhm/ ° The flood maps describe where the flood risks are, based on local hydrology, topology, precipitation, flood protection measures such as levees, and other scientific data. Fee to obtain maps. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 21 Figure 4.1 Generalized Depiction of Stream Channel Lateral Migration ( a) Map of a Stream Meander; ( b) Cross section of the Channel from A A

with Channel Positions at 3 Successive Times ( t 0, t 1, and t 2); ( c) Map of Stream Meander Showing Location After Migration Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 22 4.5 Design and Construction This section describes the type of wells eligible for bank filtration credits under the LT2ESWTR. Because specific well construction requirements ( e. g., casing depths) vary by state and with geologic conditions, this guidance will address these issues only briefly where appropriate. Readers are referred to the agency within their state that makes regulations or recommendations regarding well construction for details on issues such as casing depths, annular seals, drilling methods, filter packs, etc. Other good general references on well construction include Driscoll ( 1986) and USEPA ( 1975).

Figure 4.2 Taking a Water Level Reading The pump house for the horizontal collector well caisson is in the background. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 23 4.5.1 Well Type Only vertical and horizontal wells are eligible for bank filtration credits. Other types of ground water collection devices may not provide adequate filtration of pathogens. For example, a spring box is a ground water collection device located at the ground surface and is designed to contain spring outflow and protect it from surface contamination until the water is used. Spring boxes are found where local hydrogeologic conditions have focused ground water discharge into a smaller area ( i. e., a spring) and at a faster volumetric flow rate than elsewhere. Often, localized fracturing or dissolution enhanced channels are the cause of the focused discharge to the spring. As noted in section <IP_ADDRESS>, fractures and dissolution channels have significant potential to transport microbial contaminants. Thus, spring boxes are not eligible for bank filtration credit.

Infiltration galleries ( or filter cribs) are also not eligible for bank filtration credits. Infiltration galleries are designed to collect water infiltrating from the surface, or to intercept ground water flowing naturally toward surface water, using a slotted pipe installed horizontally in a trench and backfilled with granular material ( Symons et al., 2000). An infiltration gallery is not bank filtration because the material overlying an infiltration gallery may be engineered to optimize oocyst removal. Bank filtration systems are defined as relying solely on the natural properties of the system to remove microbial contaminants. An infiltration gallery may, however, be eligible for Cryptosporidium removal credit as an alternative treatment technology [ 40 CFR 141.73( d)].

Horizontal and vertical wells are both eligible for bank filtration credits. They are distinguished from each other by the orientations of their well screens, and the important implications this has for their well hydraulics ( Figure 4.3 and 4.4). Collector horizontal wells are constructed by the excavation of a central vertical caisson or pipe. One or more laterals ( i. e., collector lateral well screens) extend horizontally from the caisson bottom and may be very long. Laterals may extend radially in all directions  resulting in a radial collector well or primarily in the direction of the river ( Driscoll, 1986; Ray, 2001a). The lateral well screens are often installed near the bottom of the formation, allowing a greater proportion of the saturated thickness of the aquifer to be used. A greater proportion of pathogens and other contaminants are removed when the distance between the surface water body and the laterals is increased ( Ray, 2001a). Section <IP_ADDRESS> contains a discussion of when it may be appropriate to locate wells at separation distances greater than those required by the LT2ESWTR. Laterals may extend underneath a surface water body in the United States. This is generally not how horizontal wells are placed in Europe ( Ray 2001a) because in Europe such wells are required to meet a 55 60 day average travel time requirement. An example of a pump house for a horizontal collector well in Louisville, KY is shown in Figure 4.2. It is elevated to prevent flood waters from entering it.

The choice between using a vertical or horizontal well for a bank filtration system depends on the site hydrogeology and the pumping requirements. For systems with large production requirements ( e. g., many Public Water Systems) or for pumping in shallow alluvial aquifers, horizontal wells may be preferred because they are designed to capture large volumes of surface water recharge with little drawdown ( Driscoll, 1986). Vertical wells with large production Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 24 requirements are not well suited to shallow alluvial aquifers because the necessary low drawdown cannot be sustained ( Ray, 2001a).

Finally, a comparison of construction expense with the costs of well maintenance may play a role in the choice of well type. Horizontal collector wells are substantially more costly than vertical wells ( Driscoll, 1986). However, moderately large utilities may need many smaller capacity vertical wells to match the capacity of a horizontal well. The maintenance of these vertical wells may require significant effort and expense ( Ray, 2001a). In such cases, horizontal collector wells may be preferred.

Figure 4.3 Schematic Showing Generalized Flow and Required Separation Distance to a Vertical Well ( Inset shows tortuous ground water flow at the micro scale.) Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 25 Figure 4.4 Schematic Showing Generalized Flow and Required Separation Distance to a Horizontal Well With Three Laterals 4.5.2 Filtrate Flow Path and Well Location For systems to receive Cryptosporidium log removal credits, the ground water flow path length between the edge of the surface water body and the well is expected to be sufficient for effective oocyst removal. This section discusses EPA's requirements for appropriate flow path lengths, and associated well locations, for the log removal credits available under the LT2ESWTR. The ground water flow path length necessary to receive credits is specified for both vertical and horizontal wells. A discussion of how to obtain information necessary to define the edge of the surface water body is also included.

<IP_ADDRESS> Required separation distance between a well and the surface water source Cryptosporidium oocyst removal may vary significantly throughout the year in many bank filtration systems. At most typical bank filtration locations, high log removal rates ( e. g. 3.5 log Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 26 removal over 13 m) may be expected with the surface water discharges that predominate during most of the year. During short flood periods, however, there may be substantially lower removal ( e. g. 0.5 to 1.0 log removal over 13 m) due to scouring of the surface water   ground water interface, as discussed below in section 4.6.2. In summary, a number of different factors may contribute to increased risk of Cryptosporidium reaching wells. These factors include the presence of coarse grained aquifer or stream bed sediments, high river velocities, and frequent scouring of riverbeds. Given the need to protect water supplies during periods of high surface water discharge with their potentially lower log removal capabilities, the LT2ESWTR rule language ( 40 CFR 141.726( c)) provides 0.5 log removal credit for systems with bank filtration wells located greater than 25 feet from a surface water source and 1.0 log removal credit for wells located greater than 50 feet from a surface water source.

<IP_ADDRESS> Locating wells at greater than required distances from the surface water source Given the dynamic nature of riverbanks and aquifer systems, including scouring processes, as discussed in section <IP_ADDRESS>, it may sometimes be advisable to place bank filtration wells at distances greater than 25 or 50 feet from a surface water source. This extra precaution may also be advisable when a system is uncertain as to whether the riverbed and bank contain sufficient fine grained material to provide adequate removal of Cryptosporidium oocysts. That is, EPA is requiring the separation distances of 25 feet and 50 feet for the log removal credits discussed above, but greater separation distances may result in additional public health protection at some sites. The disadvantage of using greater separation distances between the surface water source and the bank filtration well is that water yields to the well will be decreased. When a system makes a decision to place wells at a greater distance from a surface water source than EPA requires, it will need to balance the sacrifice in well yield with the added public health protection.

The remainder of section <IP_ADDRESS> discusses geophysical methods which may be of use in constructing a conceptual model of subsurface flow conditions in riverbank filtration systems. By obtaining hydrogeologic information through geophysical or other means ( e. g., pre existing hydrogeologic or geologic maps), systems can determine the degree to which local conditions may affect Cryptosporidium removal at the bank filtration site. For example, if mapping the bedrock alluvial interface and the water table at a particular site indicates that the aquifer is fairly thin, it is unlikely that infiltrating river water will be diluted by much ambient ground water. In such a case it may be advisable to locate wells at greater than the required distance from the surface water source. On the other hand, if detailed hydrogeologic investigations indicate that the aquifer contains a large proportion of fine grained sediments, it would not be advisable to locate the well at greater than the required distance from the surface water source, because the aquifer is already likely to be an efficient pathogen filter, and it would be inadvisable to further sacrifice well yields.

When the aquifer contains fine grained material, it is possible that well over pumping may break the hydraulic connection between ground water and surface water, yielding a variably Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 27 saturated zone underneath a perched stream, as shown below in Figure 4.5. Formation of such a variably saturated zone during periods of high pumping can greatly alter the existing ground water flow paths. New ground water flow paths could result in marked changes in water quality. For example, surface water infiltration could occur further upstream, resulting in a longer ground water flow path for infiltrating surface water flowing towards the well. The increase in flow pathlength could improve water quality. Alternatively, the result of over pumping could be decreased water quality. This may occur because the decreased thickness of the saturated aquifer  due to the formation of a large variably saturated zone  may cause faster ground water flow ( assuming pumping rates remain constant). Faster ground water flow provides less time for contaminant attenuation within the aquifer. Finally, the variably saturated zone itself, to the extent that it transmits water, can improve water quality because contaminant attenuation is usually increased under variably saturated conditions. If possible, the potential for formation of a variably saturated zone can be investigated in order to provide additional information regarding the desirability of locating wells at greater than required distances from the surface water source.

Figure 4.5 The Streambed of a Perched Stream Is Well above the Water Table Geophysical methods generally do not disturb subsurface materials. They are often less expensive than labor intensive digging of trial pits or drilling of boreholes. Furthermore, the useful information gleaned by using geophysical methods can aid in choosing the best locations for wells ( Reynolds, 1997). Geophysical methods include gravity and magnetic methods, seismic methods, electrical methods, and ground penetrating radar.

Hydrogeophysical methods can be used in pre existing boreholes, thereby providing high resolution data for a very localized area around the borehole. Alternatively, surface geophysical methods can be used to obtain more generalized information over a large area, including information on the depth to the water table, the depth to bedrock, and stratigraphy ( Hubbard, 2003). The discussion below provides only a generalized overview of currently available geophysical methods. More detailed information can be obtained from texts by Hearst ( 2000), Reynolds ( 1997), Rubin and Hubbard ( 2003), Keys ( 1990) and Burger ( 1992). Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 28 Gravity surveying measures variations in the acceleration due to the Earth's gravitational field which are caused by density variations in subsurface rocks. Subsurface cavities can be detected with this technology, however sites with such cavities would not be suitable for bank filtration. Reynolds ( 1997) states that gravity methods are fairly uncommon in hydrogeological work compared to electrical methods. On the other hand, in the Arizona district of the United States Geological Survey, gravity methods have been in use for over 15 years to evaluate changes in water storage in aquifers. These methods can detect water table elevation changes of as little as a few inches ( Callegary, 2003). Thus, gravity methods may be useful at riverbank filtration sites for assessing the depth to water table, aquifer thickness, and seasonal effects on the dilution of infiltrating river water with ambient groundwater. Magnetic surveying or magnetic anomolies can also be used in hydrogeologic investigations. For example, clay infilling bedrock cavities can be detected due to slight changes in the magnetic susceptibility of clay and most bedrock ( Reynolds, 1997).

Seismic methods are widely used in hydrogeologic investigations. Applied seismology involves generating a signal through an explosion or other method at a specific time. The generated seismic waves travel through the subsurface, are reflected and refracted back to the surface, and the return signals are detected on monitoring instruments. The amount of time that elapses is the basis for determining the nature of subsurface layers/ materials ( Reynolds, 1997). Reynolds ( 1997) provides a detailed example of the use of seismic refraction surveying for locating the bedrock/ alluvial interface at one particular site.

Seismic methods can be used to:

° estimate depth to bedrock ( ideal for riverbank filtration applications) ° determine the nature of bedrock ( e. g., cavernous) or location of cavities. Note that karst buried by alluvium may contain unexpected ground water flowpaths. ° determine the location of faults that may juxtapose bedrock against alluvial material ° determine stratigraphy ( useful where sands and clays may be interlayered) ° determine porosity ° determine ground water particle velocities ( an important parameter for riverbank filtration systems)

Electrical resistivity methods are used extensively in downhole logging to identify hydrogeologic units that will produce high flow rates. Electrokinetic surveying makes use of electrodes implanted at the ground surface to identify the location of the water table. This may be useful at riverbank filtration sites, where water table layer and depth to bedrock can be used to determine aquifer thickness  an important parameter in determining how much dilution of bank filtrate with ambient groundwater is occurring. A more recent development is the use of electrokinetic methods to measure flowrates in boreholes ( Reynolds, 1997).

The spontaneous polarisation or self potential ( SP) method is conducted by measuring differences in ground electrical potential at different locations, but is still fairly uncommon. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 29 Another electrical method, the induced polarisation ( IP) method can be used to detect ground water and water tables, however electromagnetic induction methods are generally considered more practical for these purposes in the field. Contaminated ground water within subsurface clays can also sometimes be detected with the IP method ( Reynolds, 1997).

Electromagnetic ( EM) methods have been used in groundwater investigations to delineate contaminant plumes, and thus can be useful in conceptualizing flow systems in a riverbank filtration context when the quality of infiltrating river water is especially poor. Pulse transient EM ( TEM) surveys ( a type of EM method) may be useful in conceptualizing flow for riverbank filtration systems where infiltrating water quality is poor. It may also be useful in monitoring the quality of infiltrating water. When data is available from both borehole and surface instruments, EM and electrical methods can be used to map subsurface geology such as the locations of coarse grained and fine grained units.

Ground penetrating radar ( GPR) has been used as a surface method for contaminant plume mapping and monitoring pollutants in groundwater. To operate such a system, a signal generator, transmitting and receiving antennae, and a receiver must be used. Radiowaves are generated, which travel in a broad beam at high speeds. Energy is lost or attenuated depending on the subsurface materials through which the waves travel. GPR has proven valuable in mapping sediment sequences, and can be used to investigate sediments through freshwater up to 27 m deep ( Reynolds, 1997). Thus, it may be of use in gaining information about the composition of riverbeds, and for monitoring the effects of scour on riverbed composition. GPR can also be used to locate water tables, delineate sedimentary structures which may contain pockets of coarsegrained alluvium, and determine the spatial extent and continuity of buried clay and peat layers within subsurface deposits. Borehole radar can also be used for hydrogeologic investigations.

Before choosing a specific geophysical method it may be important to consider the following: desired level of resolution, area of coverage, site specific conditions and their influence on the applicability of the method, possible non uniqueness of the geophysical attribute, resources needed to interpret the geophysical data, and possible integration with direct measurements. In general, mapping the water table and finding the depth to bedrock are considered standard hydrogeophysical procedures. Other applications such as estimating permeabilities or porosities are at an earlier stage of development and may not yet be appropriate for routine use at riverbank filtration sites ( Hubbard, 2003).

<IP_ADDRESS> Delineating the edge of the surface water source The flow paths due to induced infiltration to a vertical well have both vertical and horizontal components, and are tortuous at the micro scale ( Figure 4.3). Such flow will typically have a significant horizontal component, especially if the vertical well is screened in a shallow, unconsolidated, alluvial aquifer that is eligible for bank filtration credits. Therefore, for the purpose of receiving log removal credits, the flow path length to a vertical well is to be determined using the measured horizontal distance from the edge of the surface water body to the Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 30 well intake. The edge of the surface water body is defined as the edge of either the 100 year floodplain or the floodway, discussed below. The 100 year floodplain is defined by its boundary  the flood elevation that has a one percent chance of being equaled or exceeded each year.

As a first step, utilities may use the online maps available at the following website to get a general idea of the mapped extent of the 100 year floodplain in their area: http:// www. esri. com/ hazards/ makemap. html. In order to satisfy the requirements of the LT2ESWTR for the location of the wells of a bank filtration system, however, an official Federal Emergency Management Agency ( FEMA) ( now part of the Department of Homeland Security) flood hazard map must be used. Such maps can be ordered in either paper or digital formats from FEMA. The following website can be used to order these maps: http:// msc. fema. gov/ MSC/.

Although in some areas of the United States the mapped extent of the 100 year floodplain may be more easily accessible than the mapped extent of the floodway, some utilities may choose to use the edge of the floodway as a starting point for measuring separations distances to wells because it typically allows wells to be placed slightly closer to the river and is thus a somewhat less restrictive requirement. The floodway is a regulatory concept, and is defined as that portion of the overbanks that must be kept free from encroachment to discharge the one percent annual chance flood ( i. e. the 100 year flood) without increasing flood levels by more than 1.0 foot. It is determined by specified methods according to FEMA guidelines, as described below.

For many areas, the mapped extent of the floodway will also be drawn on the flood hazard map obtained by FEMA. The utility may choose to use the edge of the floodway rather than the edge of the 100 year floodplain for the purpose of determining the required separation distance between a river and a riverbank filtration well. If the mapped extent of the floodway is unavailable, the utility may opt to perform the mapping using one of a number of hydraulic models approved by FEMA. A list of these approved models is available at http:// www. fema. gov/ mit/ tsd/ en_ hydra. htm. EPA recommends using the US Army Corps of Engineers' HEC RAS model for mapping floodway limits. The HEC RAS software, User's Manual, Applications Guide, and Hydraulic Reference Manual are available for free downloading from http:// www. hec. usace. army. mil/ software/ software_ distrib/ index. html.

When a utility elects to determine the edge of the floodway, and to model the floodway boundaries if they are not available from FEMA, the preferred encroachment method within HECRAS is Method 4. Method 4 can be summarized as follows, according to FEMA's Map Assistance Center ( 2003):

The Method 4 encroachment operates by analyzing the hydraulic conveyance for the unencroached one percent annual chance floodplain at each cross section, then equally reducing the conveyance from both overbank areas by moving toward the stream channel from the edge of the floodplain until the resulting water surface elevation is one foot higher than the unencroached elevation, and the resulting encroached conveyance is approximately equal to the unencroached conveyance. The new left and right cross section limits are Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 31 assumed to be vertical walls. Finally, a backwater energy balance is calculated using the new cross sections, which results in the encroached or floodway water surface profile. The floodway modeling process requires adjustments and rerunning of the model because the final calculation is the backwater energy balance between new cross sections. Many times the 1.0 foot target cannot be achieved exactly at each cross section because of energy balance considerations. Floodplain geometry, constrictions at culvert and bridge crossings, and constrictions from other man made obstructions in the floodplain may require adjustments to the encroachment widths to stay at or below the 1.0 foot maximum water surface increase. Chapter 10 of the HEC RAS User's Manual includes a discussion of performing a floodway encroachment analysis.

In most areas, however, EPA expects that utilities will find it preferable and simpler to use the previously mapped limits of the 100 year floodplain to determine the edge of the river for riverbank filtration separation distances.

<IP_ADDRESS> Measuring separation distances for horizontal wells and wells that are neither horizontal nor vertical As noted in section 4.5.1, horizontal wells may have laterals that extend underneath a surface water body. The flow direction for induced infiltration to a horizontal well that extends under a surface water body is predominately downward. Therefore, the flow path length to a horizontal well is the measured vertical distance from the bed of the river under normal flow conditions to the closest horizontal well lateral's intake ( Figure 4.4).

Some wells may be constructed so that the well is neither truly horizontal nor truly vertical. In these cases, there is greater uncertainty about the definition of separation distance from surface water. For simplicity, if the well if closer to being a vertical well than to being a horizontal well ( i. e. the well is oriented at greater than a 45 degree angle to a horizontal line), the separation distance is defined for the purposes of this toolbox option to be the horizontal distance from the edge of the river to the closest ( in terms of horizontal distance) intake on the well. Similarly, if the well is closer to being a horizontal well as opposed to a vertical well, separation distance is defined as the shortest possible vertical distance from the riverbed to an intake on the well. To ensure that the assigned log removal credit is realized, systems are expected to perform continuous turbidity monitoring for all wells that receive a credit. Continuous turbidity monitoring is discussed in section 4.2.2.

4.6 Operational Considerations 4.6.1 High River Stage When the river stage ( i. e. the elevation of the water surface) is high, the increased head gradient between the river and the adjacent aquifer results in increased infiltration and increased Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 32 ground water flow rates. This condition can be expected to occur periodically throughout the year at many sites, and will generally be associated with reduced log removals ( Gollnitz, 1999; Ray, 2001b). High river stage is often associated with scouring of riverbed sediments. Nevertheless, even when scour does not occur, the high ground water velocities associated with high river stage can be a significant threat to a riverbank filtration system.

One solution to this problem is that pumping rates can be temporarily decreased during periods of high river flow ( Medema et al., 2000). Decreased pumping rates will in turn decrease the head gradient between the river and the well, thereby decreasing subsurface velocities, increasing residence times, and facilitating pathogen inactivation.

4.6.2 Implications of Scour for Bank Filtration System Operations Periodic, short term flood scour can have both negative and positive impacts on the performance of a bank filtration system. As noted in section 4.5.2 above, lower log removals of oocysts are expected during floods because higher river shear velocities and associated increases in bedload transport mobilize fine sediments deposited when discharges were lower.

Removal of fine sediments opens large pore spaces, increasing the hydraulic conductivity across the surface water   ground water interface ( Gollnitz, 1999; Ray, 2001a; Ray, 2001b). Unfortunately, this potentially increases the number of pathogens transported. Furthermore, the microbial activity and unique geochemical environment of the riverbed, which serves to facilitate the removal of pathogens via sorption and other processes, may not be present for short periods following flood scour. Recent work in Germany ( Baveye et al., 2003) suggests that the biologically active zone is re established very quickly after scour, perhaps within 3 days, at least when measured in terms of the ability to degrade certain organic compounds. Limited scour can reduce clogging at the surface water   ground water interface and improve well yields ( Wang et al., 2001). The continuous turbidity monitoring required by the LT2ESWTR for bank filtration credits can be used to help systems manage the threat posed by periodic, short term flood scour.

When high river stages or high turbidity levels indicate that flood scour may be occurring and compromising the effectiveness of a bank filtration system, pumping rates can be decreased. This will lead to lower velocities and longer subsurface residence times, thereby increasing the protectiveness of the system ( Medema et al., 2000; Juhasz Holterman, 2000).

4.6.3 Anticipating high flow events / flooding Many factors are involved in increasing the probability that a flood will occur. Intense rainfall is the most apparent factor, however the geomorphology of a watershed is important in determining how quickly water will enter a stream system after a rainfall event, as well as how quickly water will enter a major river from smaller tributaries. Systems can anticipate that a high Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 33 flow event will occur if a rapid spring thaw follows a winter of unusually heavy snowfall. It is also important to be aware of recent changes in vegetation due to wildfires or urbanization. When vegetation is removed or decreased there are fewer barriers to rapid surface runoff, plant roots no longer keep soil loose and permeable ( thus more compact soils will be less able to decrease surface runoff), and plants themselves will be unavailable to take in a certain proportion of precipitation ( Montgomery, 2000). Therefore, systems may wish to monitor for pathogens more frequently or change pumping regimes in riverbank filtration systems when high flows are anticipated.

4.6.4 Possible responses to spill events and poor surface water quality One response to a serious water quality threat is to stop pumping from all bank filtration production wells. Other pumping regime changes can also be implemented to reduce risks, including decreasing the number of hours the system is in operation each day. For systems that have a number of wells in operation, it may be advisable to increase pumping rates for wells further from the surface water source and decrease pumping rates for wells that are closer ( Juhasz Holterman, 2000). Juhasz Holterman ( 2000) recommended that this kind of change be implemented seasonally at a site in the Netherlands. Her study of the site's hydrology indicated that during the winter months pumping wells were more vulnerable to contamination due to " short circuited" flow paths from the polluted river through the subsurface. Her solution involved both restricting extraction rates to a few hours a day ( which was acceptable due to decreased demand during the winter months) as well as an altered pumping regime which relied more on wells located further from the river.

4.6.5 Maintaining required separation distances Alluvial rivers that are experiencing active, progressive erosion as an adjustment to new flooding regimes or sediment loads, or in relation to natural lateral migration, may pose serious, longer term challenges to bank filtration systems. For example, significant log removal reductions may be more frequent in an urbanizing basin as a consequence of more frequent flooding and associated scouring. In extreme cases, long term degradation of the bed or banks may reduce the threshold separation distances between the surface water source and bank filtration well. Recall that these separation distances  25 feet for 0.5 log removal credit and 50 feet for 1.0 log removal credit  are required to receive log removal credits under the LT2ESWTR.

Systems may wish to assess their sites for active, progressive erosion. Lateral migration rates can be calculated using sequential aerial photography and/ or topographic maps, if available. Systems without such data may need to obtain the needed information by conducting sequential field surveys of the floodplain area proposed for the site. This will require a far more lengthy investigation period. Progressive downcutting could also be measured with sequential field surveys of the channel bed elevation over a period of years. Regardless of the method used, the Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 34 threshold separation distances between the surface water source and the bank filtration well must be maintained. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 35 References ASTM ( American Society for Testing and Materials), 2003. Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates  Standard C 136 1.

Baveye, P., P. Vandevivere, B. L. Hoyle, P. C. DeLeo, and D. Sanchez de Lozada. 1998. Environmental impact and mechanisms of the biological clogging of saturated soils and aquifer materials. Critical Reviews in Environmental Science and Technology. 28( 2): 123 191.

Baveye, P., Berger, P., Schijven, J., and Grischek, T. 2003. Research needs to improve knowledge of bank filtration removal of pathogens, in Riverbank Filtration: Improving Source Water Quality, edited by Ray, C., Melin, G. and Linsky, R., Kluwer, Dordrecht,

Berger, P. 2002. Removal of Cryptosporidium Using Bank Filtration in Riverbank Filtration: Understanding Contaminant Biogeochemistry and Pathogen Removal, C. Ray ( ed.). The Netherlands: Kluwer Academic Publishers. p. 85 121.

Burger, H. R., D. C. Burger, and R. H. Burger, 1992. Exploration Geophysics of the Shallow Subsurface. Upper Saddle River, NJ: Prentice Hall.

Booth, D. B. 1990. Stream channel incision following drainage basin urbanization. Water Resources Bulletin 26( 3): 407   417.

Callegary, James, United States Geological Survey, personal communication, 3/ 03.

Chow, V. T. 1964. Handbook of Applied Hydrology. New York: McGraw Hill.

Driscoll, F. G. 1986. Groundwater and Wells. 2nd Edition. St. Paul, Minnesota: Johnson Division.

Dunne, T., and Leopold, L. B. 1978. Water in Environmental Planning. New York: W. H. Freeman and Company. 818 pp.

FEMA Map Assistance Center. 2003. Personal communication.

Juttner, F. 1995. Elimination of terpenoid odorous compounds by slow sand and river bank filtration of the Ruhr River, Germany. Water Science & Technology. 31: 211 217.

Goldenberg, L. C., I. Hutcheon, N. Wardlaw, and A. J. Melloul. 1993. Rearrangement of fine particles in porous media causing reduction of permeability and formation of preferred pathways of flow: experimental findings and a conceptual model. Transport in Porous Media 13: 221 237. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 36 Gollnitz, W. D. 1999. Induced infiltration rate variability and water quality sampling issues. Proceedings of the International Riverbank Filtration Conference. November 4 6, 1999. Louisville, Kentucky.

Harter, T., S. Wagner, and E. R. Atwill, 2000. Colloid Transport and Filtration of Cryptosporidium parvum in Sandy Soils and Aquifer Sediments, Environmental Science and Technology, 34( 1), pp. 62 70.

Hearst, J. R., P. H. Nelson, and F. L. Paillet, 2000. Well Logging for Physical Properties: A Handbook for Geophysicists, Geologists, and Engineers, 2nd Edition. New York: Wiley.

Hubbard, Susan M., Lawrence Berkeley National Laboratory, personal communication, 3/ 03.

Hubbs, S., J. Z. Wang, and R. Song. 2001. Use of Riverbank Filtration to Meet the Requirements of SWTR and DBP Rules. Presentation at the American Water Works Association Water Quality Technology Conference, November 11 15, Nashville, TN.

Jacobson, R. B., S. R. Femmer, and R. A. McKenney. 2001. Land use Changes and the Physical Habitat of Streams : a review with emphasis on studies within the U. S. Geological Survey Federal State cooperative program. U. S. Geological Survey Circular 1175. Reston, VA.: U. S. Geological Survey. 63 pp.

Juhasz Hoterman, M. H. A. 2000. Reliable drinking water by bank filtration along the river Maas ( Meuse), by knowledge of the system combined with simple resources, in Proceedings of International Riverbank Filtration Conference, Nov. 2 4, Duesseldorf, W. Julich and J. Schubert ( eds.), International Arbeitgemeinschaft der Wasserwerke im Rheineinzugsgebiet, Amsterdam.

Jüttner, F. 1999. Efficacy of bank filtration for the removal of fragrance compounds and aromatic hydrocarbons. Water Science Technology 40( 6): 123 128.

Keys, W. S., 1990, Borehole geophysics applied to groundwater investigations: U. S. Geological Survey, Techniques of Water Resources Investigations of the U. S. Geological Survey, Book 2, Collection of Environmental Data, Chapter E2, 150 p.

Kuehn, W. and U. Mueller. 2000. Riverbank filtration: an overview. AWWA Journal. 92( 12): 60 69.

Leopold, L. B. and Maddock, T. 1953. The Hydraulic Geometry of Stream Channels and Some Physiographic Implications. U. S. Geological Survey Professional Paper 252. Washington: U. S. Government Printing Office.

Leopold, L. B., M. G. Wolman, and J. P. Miller. 1964. Fluvial Processes in Geomorphology. San Francisco: W H Freeman and Co. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 37 Mackin, J. H. 1948. Concept of the graded river. Geological Society of America Bulletin 59: 463 512.

Medema, G. J., M. H. A. Juhasz Hoterman, and J. A. Luitjen. 2000. Removal of micro organisms by bank filtration in a gravel sand soil, in Proceedings of International Riverbank Filtration Conference, Nov. 2 4, Duesseldorf, W. Julich and J. Schubert ( eds.), International Arbeitgemeinschaft der Wasserwerke im Rheineinzugsgebiet, Amsterdam.

Miettinen, I. T., P. J. Martikainen, T. Vartiainen. 1994. Humus transformation at the bank filtration water plant. Water Science & Technology, 30 ( 10): 179.

Montgomery, Carla W., 2000. Environmental Geology, updated 5th edition. Boston: McGraw Hill.

Oberdorfer, J. A. and F. L. Peterson. 1985. Waste water injection: geochemical and biogeochemical clogging processes. Ground Water 23: 753 761.

Orlob, G. T. and G. N, Radhakrishna. 1958. The effects of entrapped gases on the hydraulic characteristics of porous media. Transactions of the American Geophysical Union 39: 648 659.

Purdue University. 2001. Well Water Location and Condition on the Farm. Available on the Internet at: http:// www. epa. gov/ seahome/ well/ src/ title. htm , accessed November 27, 2002.

Ray, C., T. Grischek, J. Schubert, J. Wang, and T. Speth. 2002. A perspective of riverbank filtration, J. AWWA, 94( 4): 149 160.

Ray, C. 2001a. Riverbank filtration: an analysis of parameters for optimal performance. Proceedings of the Annual Conference of the American Water Works Association. June 17 21, 2001. Washington, DC.

Ray, C. 2001b. Modeling riverbank filtration systems to attenuate shock loads in rivers. Proceedings of the Annual Conference of the American Water Works Association. June 17 21, 2001. Washington, DC.

Reynolds, J. M., 1997. An Introduction to Applied and Environmental Geophysics. New York: Wiley.

Ritter, D. F., C. R. Kochel, and J. R. Miller. 1995. Process Geomorphology. 3rd edition. Dubuque, Iowa: Wm. C. Brown Publishers.

Rubin Y. and S. Hubbard, 2003. Hydrogeophysics. Kluwer. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 38 Sanchez de Lozada, D., P. Vandevivere, P. Baveye, and S. Zinder. 1994. Decrease of the hydraulic conductivity of sand columns by Methanosarcina barkeri. World Journal of Microbiology and Biotechnology 10: 325 333.

Schafer, D. 2000. Groundwater modeling in support of riverbank infiltration for Louisville Water Company, in Proceedings of International Riverbank Filtration Conference, Nov. 2 4, Duesseldorf, W. Julich and J. Schubert ( eds.), International Arbeitgemeinschaft der Wasserwerke im Rheineinzugsgebiet, Amsterdam.

Schijven, J., Berger, P., Miettenen, I. 2003. Removal of viruses, bacteria, protozoa and toxins using bank filtration, in Riverbank Filtration: Improving Source Water Quality, edited by Ray, C., Melin, G. and Linsky, R., Kluwer, Dordrecht,

State Coordinating Committee on Ground Water ( SCCGW). 2000. state of Ohio Technical Guidance for Well Construction and Ground Water Protection. Available on the Internet at: http:// www. dnr. state. oh. us/ water/ pubs/ pdfs/ wellconguide. pdf , accessed November 27, 2002.

Symons, J. M., L. C. Bradley, Jr., T. C. Cleveland, eds. 2000. The Drinking Water Dictionary. American Water Works Association, Denver, CO.

Tufenkji, N., J. N. Ryan, and M. Elimelech. 2002. The promise of bank filtration: a simple technology may inexpensively clean up poor quality raw surface water. Environmental Science and Technology. 36: 422A  428A.

United States Environmental Protection Agency ( USEPA). 2001. Long Term 2 Enhanced Surface Water Treatment Rule ( LT2ESWTR). Pre proposal draft for stakeholder review. November 27, 2001.

USEPA. 1975. Manual of Water Well Construction Practices. Office of Water Supply. EPA/ 570/ 9 75 001. Washington, D. C. 156 pp.

United States Geological Survey ( USGS). 1998. The National Atlas of the United States of America. Principal Aquifers. [ Map]. Reston, VA: U. S. Department of the Interior, U. S. Geological Survey.

Vandevivere, P., P. Baveye, D. Sanchez de Lozada, and P. DeLeo. 1995. Microbial clogging of saturated soils and aquifer materials: Evaluation of mathematical models. Water Resources Research 31( 9): 2173 2180.

Wang, J. Z., S. A. Hubbs, and M. Unthank. 2001. Factors Impacting the Yield of Riverbank Filtration Systems. Presentation at the American Water Works Association Water Quality Technology Conference, November 11 15, Nashville, TN. Chapter 4  Bank Filtration LT2ESWTR Toolbox Guidance Manual Proposal Draft June 2003 4 39 Wang, J. Z., R. Song, and S. A. Hubbs. 2000. Particle removal through riverbank filtration process, in Proceedings of the International Riverbank Filtration Conference, Nov. 2 4, Duesseldorf, W. Julich and J. Schubert ( eds.), International Arbeitgemeinschaft der Wasserwerke im Rheineinzugsgebiet, Amsterdam.

Wang, J. Z., Hubbs, S. A. and Song, R. 2002. Evaluation of Riverbank Filtration as a Drinking Water Treatment Process, American Water Works Association Research Foundation Report 90922, 145 p.

Warner, J. W., T. K. Gates, R., Namvargolian, P. Miller, and G. Comes. 1994. Sediment and microbial fouling of experimental groundwater recharge trenches. Journal of Contaminant Hydrology 15: 321 344.

Weiss, W. J., E. J. Bouwer, W. P. Ball, C. R. O'Melia, H. Arora, T. F. Speth. 2003. Reduction in DBP precursors and pathogens during riverbank filtration at three midwestern drinking water utilities, in Riverbank Filtration for Water Supply, C. Ray and R. Linsky ( eds). Kluwer Academic Publishers.

Williams, G. P., and Wolman, M. G. 1984. Downstream Effects of Dams on Alluvial Rivers. U. S. Geological Survey Professional Paper 1286. Washington: U. S. Government Printing Office.