2
Overview of Managed Underground Storage Systems

This chapter provides the background information necessary to understand the detailed science, technology, and institutional issues related to managed underground storage (MUS) systems that are presented in subsequent chapters. The chapter begins with an overview of the components of MUS systems, as they are used throughout the report, and briefly explains some of the issues associated with each component. A condensed history of the evolution of MUS systems, focusing on the development and use of these systems within the United States, follows. Next, a review of the types of uses for which MUS systems have been or are being developed, and of other drivers behind the development of these systems, including agency-sponsored programs, is provided.

COMPONENTS OF MANAGED UNDERGROUND STORAGE SYSTEMS

Throughout this report, MUS systems are discussed in terms of five major components:

  1. Source of water to be stored

  2. Recharge method

  3. Storage method and management approach

  4. Recovery method

  5. End use of recovered water

Opportunities and issues related to the selection, development, use, and regulation of MUS systems are typically tied to these components, and subsequent discussions regarding hydrogeology and hydraulics (Chapter 3), water quality (Chapter 4), legal, regulatory and economic issues (Chapter 5), and management of systems (Chapter 6) are usually tied to one or more of these components. While issues related to water sources and end uses may be common to both underground and surface storage of water, many of these issues are unique to underground storage systems, such as the potential interactions between the stored water and the native water in the surrounding aquifer.

Figure 2-1 illustrates some of the categories of issues encountered in MUS systems. The words in italics represent some of the criteria associated with each component that affect system selection and design. Note that many MUS systems contain some form of pretreatment before recharge and posttreatment during recovery. Monitoring of the stored water is often required. A source of



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2 Overview of Managed Underground Storage Systems This chapter provides the background information necessary to understand the detailed science, technology, and institutional issues related to managed un- derground storage (MUS) systems that are presented in subsequent chapters. The chapter begins with an overview of the components of MUS systems, as they are used throughout the report, and briefly explains some of the issues as- sociated with each component. A condensed history of the evolution of MUS systems, focusing on the development and use of these systems within the United States, follows. Next, a review of the types of uses for which MUS sys- tems have been or are being developed, and of other drivers behind the devel- opment of these systems, including agency-sponsored programs, is provided. COMPONENTS OF MANAGED UNDERGROUND STORAGE SYSTEMS Throughout this report, MUS systems are discussed in terms of five major components: 1. Source of water to be stored 2. Recharge method 3. Storage method and management approach 4. Recovery method 5. End use of recovered water Opportunities and issues related to the selection, development, use, and regulation of MUS systems are typically tied to these components, and subse- quent discussions regarding hydrogeology and hydraulics (Chapter 3), water quality (Chapter 4), legal, regulatory and economic issues (Chapter 5), and man- agement of systems (Chapter 6) are usually tied to one or more of these compo- nents. While issues related to water sources and end uses may be common to both underground and surface storage of water, many of these issues are unique to underground storage systems, such as the potential interactions between the stored water and the native water in the surrounding aquifer. Figure 2-1 illustrates some of the categories of issues encountered in MUS systems. The words in italics represent some of the criteria associated with each component that affect system selection and design. Note that many MUS sys- tems contain some form of pretreatment before recharge and posttreatment dur- ing recovery. Monitoring of the stored water is often required. A source of 25

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26 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER END USE SOURCE WATER Quality Quality Quantity Quantity Duration Duration Reliability Reliability STORAGE ZONE Duration RECOVERY METHOD RECHARGE METHOD Capacity Capacity Land Availability Water Quality Efficient Recovery of Aquifer Types Changes Recharge Water Location Water Management/ Flood Control FIGURE 2-1 The five major technical components of MUS systems and some major de- sign criteria. water is required for all systems, but selection of the source is tied to the end use (particularly with respect to whether that end use is to be potable or not), as are the treatment and management during recharge, storage, and recovery. Major factors that impact the selection of recharge methods include aquifer type, land availability, and proximity to the water source. These and other factors are described in the sections that follow. Source Water A variety of source waters may be used for underground storage, such as surface water, groundwater, stormwater, treated effluent, and (rarely) produced water. Waters from difference sources may have very different water quality characteristics. The water source used for recharge depends on availability, quality, duration, and reliability, as well as regulatory constraints. When considering the end use of the water, a suitable water quality source must be selected. However, variations in source water quality and quantity may be mitigated during storage provided adequate storage time and capacity are available. Water quality improvements may occur during pretreatment prior to recharge, during storage, and during posttreatment prior to use. Ideally, the se- lection of source waters will minimize pre- and posttreatment requirements since these increase overall system cost. Pretreatment may be required to maintain infiltration rates, prevent negative interactions with aquifer materials, and pre- vent degradation of existing groundwater quality. Pretreatment requirements for recharge basins may be as simple as a stilling basin to remove heavy loads of solids prior to application. Stormwaters and surface waters are typically applied to recharge basins without pretreatment. High quality source waters such as treated drinking water may suffer from

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 27 water quality deterioration during storage, although subsurface storage may also provide protection for drinking water quality. Water quality sources such as reclaimed water typically tend to improve during subsurface storage since there is a large potential for improvement. When reclaimed water is used and the final use is drinking water, the MUS system is referred to as indirect potable reuse (IPR; NRC, 1998) and special pretreatment or post-treatment requirements often apply to ensure that drinking water standards are not compromised and the receiving aquifer is not contami- nated. When IPR systems use recharge basins, conventional water reclamation technologies are often sufficient to prevent significant deterioration of existing groundwater quality and water quality improvements are observed during sub- surface transport. When IPR systems use recharge wells, advanced treatment technologies such as reverse osmosis are often used to prevent clogging and deterioration of groundwater quality. Stormwater is often captured in retention basins that serve the dual purpose of capturing it and recharging it into the ground. Stormwater quality and quan- tity can be highly variable, and consistent with the National Research Council (NRC, 1994) report caution is needed in determining that the water is of accept- able quality for recharge. As noted in Chapter 4, limited data exist on the use of stormwater for MUS, and research is needed to determine the true potential of this little utilized but potentially important source water. Produced water is a by-product of oil and gas production, and its disposal water is often a problem. Recharge of the produced water for future recovery is an option, provided the water quality of the receiving aquifer is not compro- mised. However, produced water is usually not suitable for placement in drink- ing water aquifers due to high salinity and the presence of organic contaminants, and it would generally require extensive treatment prior to recharge. Recharge Method The major methods that have been developed for accomplishing recharge are through recharge basins or through wells. With recharge wells, dual-purpose aquifer storage and recovery (ASR) wells may be used for both recharging and recovering stored water, or the water may be recovered through a separate well. Although not as often used for MUS systems, subsurface infiltration methods such as vadose zone wells can also be applied in unconfined aquifers, combining some of the advantages of both surface recharge and well recharge. Figure 2-2 illustrates the difference in location of recharge basins, vadose zone wells, and recharge wells with respect to the saturated zone of an aquifer. The selection of recharge method will depend on aquifer type and depth and aquifer characteristics, which impact the ability to recharge water into the stor- age zone and recover that water later. The use of recharge basins and vadose zone wells is restricted to unconfined aquifers, while direct recharge and ASR wells may be used in both unconfined and deeper confined aquifer systems.

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28 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER Methods for Aquifer Recharge RECHARGE BASIN VADOSE ZONE RECHARGE WELLS WELL Vadose Zone Unconfined Aquifer Aquitard Confined Aquifer FIGURE 2-2 Major methods for aquifer recharge. Recharge basins are the most common type of surface spreading (see Box 1-1), which includes recharging water at the surface through recharge basins, ponds, pits, trenches, constructed wetlands, or other systems. Consistent with the figure, recharge wells can be used in either confined or thick, uncon- fined aquifers. Vadose zone wells are the least common of the methods shown. Regulatory considerations may also come into play; for example, U.S. Environ- mental Protection Agency (EPA) Underground Injection Control (UIC) regula- tions regarding nonendangerment of groundwater are a particular concern for systems using well recharge (see Chapter 5). Subsurface infiltration methods have been used to protect recharged water from evaporation losses or contamination, or where the land surface is not suit- able for surface infiltration due to lack of land ownership and control, pavement of land surface, or other land uses that may cause surface infiltration methods to be infeasible. If there is a relatively thin layer of impermeable soil or man-made land cover (e.g., pavement) above the aquifer or vadose zone and the aquifer or vadose zone is relatively close to the land surface, an infiltration pit, trench, or shaft may be used. If the impermeable layer is thicker or farther below the land surface, recharge wells must be used.

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 29 Pretreatment requirements for recharge basins to maintain hydraulic capac- ity are not extensive since the accumulation of biological growth and suspended solids in the upper layer of soil is expected. Hydraulic capacity is normally maintained by drying and scraping to remove the clogging layer near the soil surface. Extensive pretreatment is generally required for vadose zone wells to prevent biofouling and clogging from solids. Vadose zone wells are a relatively new technology and no effective tech- niques have yet been demonstrated for backwashing or cleaning them after they have clogged. Therefore, clogging must be prevented through the use of careful pretreatment. Direct recharge wells must also be treated to prevent biofouling and clog- ging from solids. Introduction of a disinfectant (e.g., chlorine) may be effective at preventing biofouling. Reversal of flow in direct recharge wells may prevent the accumulation of solids and mitigate problems with biofouling; the ability to develop and minimize fouling of dual-purpose wells is one of the advantages that have been identified for ASR wells. The hydraulic capacity of recharge basins depends on the local soil charac- teristics and the clogging potential of the water to be recharged. The capacity of vadose zone wells depends on the hydraulic conductivity of the vadose zone soils. Similarly, the hydraulic capacity of recharge wells depends on the charac- teristics of the receiving aquifer. Vadose zone wells have hydraulic capacities that are comparable to recharge wells. A hectare (2.5 acres) of recharge basins might be equivalent to a single recharge well. Therefore, the extensive land re- quirements for the use of recharge basins have made wells a more popular choice for groundwater recharge in urban areas. The cost of wells depends pri- marily on the depth of the well while land is the primary cost associated with recharge basins. The selection of recharge method is a key consideration in determining the costs, issues, and operation and maintenance requirements for the MUS system. Some of the key characteristics associated with each recharge method are sum- marized in Table 2-1. Storage Zone The one component of MUS systems that may not be effectively engineered is the actual aquifer system used for storage. The capacity of the aquifer to store water is one of the most critical factors in selecting a site for underground stor- age systems. A second consideration is water quality improvement and/or dete- rioration that may occur during storage as a consequence of complex biogeo- chemical reactions. A third is impacts on the aquifer, such as clogging of aquifer pore spaces. Recharge water may be stored in confined and unconfined aquifers. Other methods of subsurface storage, such as underground caves or abandoned mines, have also been used but are not considered in this study, which focuses on

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30 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER TABLE 2-1 Major Characteristics of Aquifer Recharge Methodologies Vadose Zone Recharge Wells Recharge Basins Wells (including ASR) Aquifer type Unconfined Unconfined Unconfined or confined Prevention of Pretreatment Low/minimal Prevention of clogging and requirements technology clogging and biofouling biofouling $100,000- Estimated major Land and $100,000- 1,000,000 + per capital costs distribution 250,000 per well US$ system well Capacity 1000-20,000 1,000-3,000 2000-6000 M3/ha-d 3 3 m per well m per well Maintenance Drying and Drying and Dis- Disinfection and requirements Scraping infection flow reversal Estimated life >100 years 5-20 years 25-50 years cycle Saturated zone Location of Vadose zone and Vadose zone aquifer-water Saturated zone and Saturated contact zone storage in an aquifer. Selection of an appropriate storage zone is an important consideration, impacting costs, the physical ability to get water into and out of the storage zone, and the potential for water quality impacts (negative and posi- tive) on both the storage and the native water. Specific aquifers may also be protected by regulatory programs that must be considered in selecting and man- aging MUS systems. During storage in unconfined aquifers, the groundwater table may rise and distinct mounds of water may develop below recharge basins or vadose zone wells. While increasing groundwater levels is often a goal of groundwater re- charge, rising groundwater levels may have negative impacts if landfills or structures are located adjacent to groundwater recharge facilities. Storage in confined aquifers will increase the pressure in the aquifer, but actual groundwa- ter levels may rise only if the aquifer is partially unconfined. ASR wells provide the ability to store water in aquifers with poor-quality water such as brackish aquifers. The ASR wells produce a zone of stored water around the well that displaces poor-quality water and creates a storage zone of high-quality water. The stored water may be recovered from the storage zone, and the recovery efficiency (see “Recovery Efficiency and Target Storage Vol- ume,” Chapter 3) depends on the blending of injected water with the existing poor-quality water. The efficiency of recovery may be highly variable depend-

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 31 ing on the hydrogeology of the aquifer system, and the efficiency will determine the economics of such a system. During aquifer storage, the time that water is stored in the subsurface is controlled by the design of the recharge method and recovery system. Systems that contain separate recharge locations and recovery locations often have de- fined flow paths and residence times between the point of recharge and the point of recovery. However, in the case of ASR systems, the last water to be re- charged will likely be the first to be recovered and the residence time of stored water is highly variable. Consequently, if water quality changes occur during ASR, the water quality changes may also be variable. Recovery wells are lo- cated primarily for practical reasons such as proximity to point of use or con- veyance system. When separate recharge systems and recovery wells are used, the recovery wells do not necessarily recover the same water that was recharged, however, the stored water remains available for future use. After the water is recovered, treatment prior to use may be required depending on the specific use requirements. During storage, a variety of water quality transformations may occur de- pending on biogeochemical processes. Transformations because of changes in redox conditions and chemical interactions often occur rapidly and may impact the hydraulic capacity of recharge wells in addition to changing water quality. Transformations that depend primarily on biological reactions such as the bio- degradation of organic compounds often occur slowly, and longer storage times are often necessary to achieve the full effects of water quality transformations during storage. Aquifers consisting of alluvial materials such as sand and gravel have a large amount of surface area that may contact the water traveling through the aquifer. This surface area mediates many biogeochemical reactions that may improve water quality during subsurface transport. Fractured and karst aquifers may have flow paths through fissures and conduits where surface area contact between the water and aquifer materials is limited. Since most biogeochemical reactions are surface mediated, water quality transformations that occur in allu- vial materials may not be expected in nonalluvial aquifers where preferential flow paths exist. During storage, both water quality improvements and deterioration may oc- cur (Chapter 4). Improvements often come from the same natural processes that attenuate naturally occurring contaminants; many groundwaters do not require any treatment for potable purposes. Water quality improvements that are micro- bially mediated, such as the biodegradation of organic compounds, tend to corre- late with longer storage times. When water is left in an aquifer for a long time it may approach native groundwater quality. Water quality deterioration often occurs due to geochemical interactions re- sulting from redox changes as the injected water and the native aquifer water mix and continue to flow through the subsurface. The resulting dissolution of minerals may cause inorganic contamination of the recharged water. Water quality deterioration is most commonly associated with recharge wells for sev- eral reasons. Chlorination of the injected water to prevent biofouling is often

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32 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER necessary, and the resulting disinfection by-products may persist and/or actually increase in concentration during subsurface storage. Also, when water is in- jected, there are rapidly changing velocities as water moves away from the point of recharge, and if redox gradients occur, the potential for widespread geo- chemical interactions with negative consequences exists. When recharge basins are used for groundwater recharge, chlorination is not necessary unless it is a regulatory requirement, and disinfection by-products have not been observed to be a problem with recharge basins. While redox changes may occur below recharge basins as a consequence of wetting and dry- ing cycles, these changes occur slowly and rapidly changing velocities are not associated with most recharge basins. A plume of recharged water below re- charge basins may become anoxic if sufficient oxygen demand was present in the recharged water. This commonly occurs with bank filtration systems in Europe, and the recovered water must be treated for dissolved iron and manga- nese. Since bank filtration systems do not have a vadose zone, there is no op- portunity for aeration of the water during subsurface transport while most re- charge basins have some opportunity for aeration during vadose zone transport. Development of redox gradients is not the only potential cause of water quality changes during storage. Dissolution and precipitation reactions are caused by chemical differences, most commonly differences in the acidity or alkalinity of the waters. Salinity differences can also lead to interactions be- tween the water and the aquifer materials. Chapter 4 discusses water quality issues in detail. When water is stored in an aquifer, there is considerable uncertainty about the flow path of the stored water and the potential changes in water quality. These uncertainties may be reduced by monitoring (Chapter 6). Monitoring of flow paths may be accomplished by measuring water levels and/or pressures, and the measurements may be input into groundwater flow models to assess groundwater movement. Monitoring flow paths is relatively inexpensive, how- ever, and capital costs will increase as the depth of required monitoring wells increases. Monitoring water quality transformations requires obtaining water samples and analyzing the samples either on-site or in an analytical laboratory. The level of difficulty and cost depends on the type of sampling equipment re- quired and the analyses that must be performed. When emerging contaminants such as pharmaceuticals and personal care products are of concern, the costs for analysis of a single sample may exceed several thousand dollars. For recharge basins and vadose zone wells, suction lysimeters are necessary to obtain vadose zone samples. It may take more than 24 hours to obtain a vadose zone sample and sample volumes often limit the analyses that may be completed. Especially for projects using reclaimed water where monitoring requirements are stringent, monitoring may be the largest cost associated with the project.

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 33 Recovery Method The location of recovery wells may affect several key factors associated with underground storage systems. Recovery wells may be located to direct the recovery of stored water toward proximity to the final use of the stored water. For systems that use recharge basins or vadose zone wells, the screened depth of the recovery well can have an important impact on storage time. As water per- colates through the vadose zone, it accumulates in the uppermost portion of the aquifer and travels primarily in the horizontal direction under saturated condi- tions. By locating the screened interval below the top of the aquifer, the storage time before recovery can effectively be increased since vertical groundwater velocities are typically orders of magnitude lower than horizontal groundwater velocities. Of course, a well pumped in this zone would be continuously draw- ing antecedent groundwater, so such a strategy would not be appropriate for aq- uifers containing brackish or saline water. For direct recharge wells, the screened interval does not tend to affect travel time since the primary component of flow is horizontal unless the recharge and recovery wells are very close or the recharge zone is very thick. For dual- purpose wells, the recharge and recovery well are the same, resulting in variable storage times. Land ownership and zoning are also considerations impacting the location of recovery wells. Not all recovery occurs with wells. Along the Platte River in Colorado, wa- ter is taken from the river during high-flow, low-demand periods to offset im- pacts of well withdrawals from alluvial aquifers on more senior surface water rights. The water is placed in recharge ponds or ditches during nonirrigation seasons, where it seeps into the aquifer. It then flows in the subsurface back to the stream, which “recovers” it directly as seepage. Box 2-2 describes this crea- tive system in more detail. End Use The final use of recovered water is the most important factor driving the economics of MUS systems, as well as many of the decisions regarding site se- lection; recharge and recovery methods; timing and duration of storage; avail- able options for source waters; requirements for pre- and posttreatment; and permitting and regulatory constraints. This includes type of use (e.g., drinking water, irrigation water, industrial cooling water, environmental water) and tim- ing of use (e.g., long-term storage for emergency use vs. operations to address seasonal variations in water availability and demand). The final use of the stored water will dictate the desired final water quality. In many cases, treatment prior to the final use is primarily to prevent undesirable interaction with the distribution systems. For example, if potable reuse is de- sired, disinfection is often a standard practice to prevent biofilm development in the distribution system. When stored water is used for irrigation, posttreatment

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34 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER is typically unnecessary because the water quality is often comparable to or bet- ter than alternative surface water supplies with respect to pathogens and solids. HISTORY OF MANAGED UNDERGROUND STORAGE SYSTEMS Groundwater recharge for the purpose of storing water for future use has a very long history, with examples of surface recharge for water storage dating back over millennia. In the KaraKum Plain Desert of Turkmenistan, layers of clay with low hydraulic conductivity hold water at shallow depths underneath sand dunes. Consequently, nomadic tribes in Turkmenistan were known to dig trenches radially from sand dunes. The trenches were graded toward the dunes to collect rainwater that could be stored below the dunes. This simple form of groundwater recharge allowed the water to be stored for future use by simply excavating the sand dune (United Nations, 1975). Bank filtration systems have been employed dating back to the nineteenth century. During bank filtration, river water is extracted indirectly by drawing it through the subsurface prior to use. While bank filtration systems do not pro- vide storage of surface water underground, they do demonstrate the potential for water quality improvements during subsurface transport. Some bank filtration systems have been in operation for over 100 years (Grischek et al., 2002), and although they are not defined as MUS systems, many of the data on water qual- ity transport during bank filtration are applicable to other underground storage systems. During the twentieth century, advances in the science of groundwater hy- drology led to the integration of deliberate and managed storage of water sup- plies underground into the development and integrated management of water supplies for various uses. While many of the issues associated with surface re- charge and well recharge systems are similar, the technologies have evolved somewhat separately. The histories of surface recharge and of well recharge are presented separately below. The history of groundwater recharge for the purposes of underground stor- age of water to be recovered for later use is closely tied to the history of other types of artificial recharge to conserve or enhance aquifers, prevent saltwater intrusion, induce bank filtration, prevent land subsidence, or other purposes. History of Surface Recharge MUS Systems Many underground storage systems have consisted of recharge basins where excess surface waters were retained and allowed to percolate to a receiving aqui- fer. The use of recharge basins was a logical extension of flood retention basins where excess drainage waters in urban areas were stored. Since there was no use for the water stored in the retention basins, subsurface storage would prevent water losses from evaporation and allow the water to be used in the future.

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 35 When floodplains were used for locating recharge basins, the underground stor- age systems also provided the benefit of diverting floodwaters and maintaining floodplains. The use of recharge basins is limited primarily to storage of water in unconfined aquifers, where no impermeable layer separates the recharge basin surface from the aquifer. Recharge rates can be enhanced by various means. Often times, the ground is excavated to increase percolation rates by removing less permeable surface soils. In addition to recharge basins, pits, trenches and shafts may be excavated for purpose of enhancing the recharge of unconfined aquifers. In the United States, attempts at artificial recharge began in the late nine- teenth and early twentieth centuries. Many of these projects were oriented less toward augmenting groundwater supplies than toward draining surface water for agriculture. However, there were exceptions. For example, water from Mill Creek and the Santa Ana River in Southern California was used to recharge the Bunker Hill Basin beginning in the 1890s and 1911, respectively (California Regional Water Quality Control Board, 1995). In nearly all cases, this recharge water was untreated. Therefore, success was most commonly achieved in highly porous and permeable aquifers such as limestones and fractured basalts where bacterial growth and suspended sediment deposition had relatively less impact (Weeks, 2002). Long Island, New York, and Southern California were the foci of more sci- entific efforts to use artificial recharge to conserve or enhance groundwater stor- age beginning around the 1930s. For example, stormwater runoff collection basins were built on Long Island to collect water and permit it to infiltrate to the unconfined aquifer. Their number has increased from 14 basins in 1950 to more than 3,000 today (Ku and Simmons, 1986). The first large-scale planned opera- tion of groundwater recharge using municipal wastewater in the United States was implemented by the Sanitation Districts of Los Angeles County in 1962, using secondary effluent as source water and recharging via recharge basins (NRC, 1994). Artificial recharge in the fast-growing State of Arizona did not begin at a large scale until the Granite Reef Underground Storage Project was permitted in 1994. History of Recharge Wells As groundwater withdrawal and water supply problems became more criti- cal in the twentieth century, techniques to store water in confined aquifers using recharge wells were developed. Interest in using wells for groundwater recharge specifically to store water supplies increased after World War II, tied in part to concerns regarding potential attacks on water supply facilities. The U.S. Geo- logical Survey (USGS) was involved with a number of early well recharge in- vestigations with western cities, including Walla Walla, Washington (Price, 1960); Salem, Oregon (Foxworthy, 1969); Portland, Oregon (Brown, 1963); and Amarillo, Texas (Moulder and Frazor, 1957). Price (1960) noted that the use of

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36 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER untreated surface water with high suspended solids (2 mg/L total suspended sol- ids) resulted in significantly degraded well efficiencies. Walla Walla, Salem, and Portland have all since developed operational aquifer storage and recovery facilities using basaltic aquifers in the same vicinity as these early well recharge experiments (Shrier, 2004). The need to control seawater intrusion into aquifers motivated the develop- ment of recharge well systems to provide a hydraulic barrier between seawater and inland freshwater aquifers. In Orange County, California (see Box 2-1), Water Factory 21 began injecting water into the coastal barrier in 1976. Several alternative sources of water were evaluated for the recharge program including imported water, deep well water, reclaimed municipal wastewater, and desalted BOX 2-1 CASE STUDY: Orange County Water District, Fountain Valley, California The Orange County Water District (OCWD) began pilot studies in 1965 to determine the feasibility of injecting effluent from an advanced wastewater treatment facility into aqui- fers in the Talbert Gap at the mouth of the Santa Ana River to create a freshwater mound 3 3 that prevents seawater intrusion. The 15 million gallons per day (Mgal/d; 57 x 10 m /d) facility, known as Water Factory 21 (WF-21), began recharge of treated wastewater in 1976 via 23 multiple-cased recharge wells. Additional wells have been constructed in recent years. WF-21 operated from 1976 until 2004, when it was decommissioned to begin con- struction of a new water purification system. WF-21 received activated sludge secondary effluent from the adjacent Orange County Sanitation District (OCSD) Plant No. 1. The treatment processes at WF-21 changed through the years. In its final configuration, it consisted of microfiltration, reverse osmosis, and advanced oxidation using hydrogen peroxide and ultraviolet radiation. Although origi- nally intended as a seawater intrusion barrier, the bulk of the injected water flows inland to augment groundwater used as a potable supply source. Extensive monitoring at WF-21 verified that the treatment provided is capable of producing water that meets all regulatory requirements for indirect potable reuse, including those related to xenobiotics and other trace organic contaminants. 3 In the 1990s, OCWD estimated that an additional 45 to 70 Mgal/d (170 x 10 to 265 x 3 3 10 m /d) could be recharged using existing recharge basins in the Orange County re- charge area in Anaheim and Orange. A recharge project called the Groundwater Replen- ishment (GWR) System was conceived by OCWD and the Orange County Sanitation Dis- trict to provide a new reliable drought-proof water supply, prevent seawater intrusion, im- prove groundwater quality, reduce ocean discharge, and defer the need for a new ocean 3 3 outfall. In the first phase of the project, 70 Mgal/d (265 x 10 m /d) of purified water will be used for recharge. The GWR System is expected to become operational in November 2007. The source water and treatment processes for the GWR System will be the same as those used in WF-21’s final configuration. The majority of the treated water will be pumped approximately 14 miles (23 km) through a 78-inch (198-cm) pipeline through the Santa Ana River corridor to Kraemer Basin in Anaheim, one of the deep recharge basins used in the 3 Orange County inland recharge area. Some of the water, 15 to 40 Mgal/d (57 x 10 to 150 3 3 x 10 m /d) depending on time of year, will be diverted to an expanded Talbert Gap Sea- water Intrusion Barrier previously served by WF-21. The estimated capital cost of the GWR System is $480 million, and the estimated an- nual operating and maintenance cost is $22 million.

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 37 seawater. The water supply selected for recharge was a blend of deep well water and reclaimed water. The creation of hydraulic barriers with recharge wells could be done in both confined and unconfined aquifers. The development of hydraulic barriers protected an important source of groundwater from contami- nation while simultaneously replenishing the existing groundwater supply with a source that would have been discharged to the ocean. As discussed in this case study (Box 2-1), this system began primarily for seawater intrusion prevention, but evolved to include groundwater replenishment as well. ASR systems, in which wells designed for the dual purpose of both recharge and recovery of water were integrated into a water supply system, were also being developed around this time. ASR systems were attractive to water supply agencies because an existing distribution system could be used for both water supply and storage. For example, a surface water supply could be treated to drinking water standards at a surface water treatment plant and distributed for both direct use and aquifer recharge. The first ASR system was implemented in Wildwood, New Jersey in 1968. In the late 1960s, California passed its State Water Plan with significant plans for underground storage of water, to be im- ported from Northern California to Southern California, through artificial re- charge. Sites were subsequently developed in California in the 1970s. The ear- liest use of ASR in California was at the Goleta Water District, operational since 1978. Other early users of ASR in California include sites operated by the City of Oxnard and the City of Camarillo, both of which began operations in the late 1970s. New Jersey, California, and Florida (whose first ASR well was in Mana- tee County in 1978; Pyne, 2005) continued to be the only states with operational ASR facilities through the mid-1980s. In all three states, ASR was used as a form of water storage, but ASR facilities in these states were often also used as a tool for groundwater management in aquifers that were experiencing declining water levels and saltwater intrusion. During the 1990s, the City of Scottsdale, Arizona, embraced the concept of using vadose zone wells for groundwater recharge. As real estate prices in- creased toward the end of the twentieth century and appropriate locations for surface recharge basins became scarcer, the need to develop a cost-effective method to recharge deep unconfined aquifers led to the development of vadose zone wells. Vadose zone wells are essentially shafts that are engineered to inject water efficiently into the ground. In Scottsdale, direct recharge wells would have to be 500 feet deep and vadose zone wells were determined to be economi- cal even if the life cycle of the wells was only five years. The vadose zone wells were relatively inexpensive compared to direct recharge wells, did not require the extensive space of recharge basins, and could be placed in a variety of loca- tions. The Scottsdale experience is also an example of where overdraft has cre- ated a tremendous storage zone.

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38 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER REASONS FOR USING MANAGED UNDERGROUND STORAGE The development of various MUS methods and the increasing prevalence of MUS systems are driven by increasing demands for water in general, as well as the advantages that can be gained through use of underground storage versus surface storage. As in any water planning exercise, the use of MUS systems for storage of water supplies is compared with other alternatives for storage (e.g., surface reservoirs or tanks) and reduction of demand (e.g., various conservation measures). In all cases, economics is an important consideration for the selection of any project, although there may also be several other drivers causing project propo- nents to consider underground storage. For example, groundwater withdrawals may be restricted or prohibited unless a project proponent has first recharged that aquifer so that there is little or no net withdrawal (e.g., capacity use area laws in some eastern Coastal Plain states). There may also be environmental drivers or other public benefit objectives such as ecosystem restoration (Ever- glades, south Florida) or maintenance of minimum instream flows for salmon (Washington County, Oregon), brown trout (Squaw Valley, California), or other aquatic habitat. Underground storage may also provide secondary benefits re- lated to maintenance of aquifer integrity and quality, such as helping to prevent aquifer dewatering or saltwater intrusion. More detail on the types of uses that have developed for MUS systems, and the associated issues and constraints, are discussed later in this report. To illus- trate the range of applications of MUS systems, however, a few examples of types of uses are provided below. Seasonal Water Supply Many MUS systems are developed to take advantage of seasonal availabil- ity of water supplies and seasonal demand, most often for municipal water sup- ply, although seasonal irrigation demands are also often a consideration. In a recent survey of ASR facilities, more than half of the facilities surveyed oper- ated their systems primarily for seasonal water use (AWWA, 2002). Multiyear Water Storage or “Water Banking” Several MUS facilities have been developed to provide multiyear storage in case of drought. Dependent upon site conditions and expected losses to the aq- uifer during storage, permitting requirements may reduce the amount of stored water that can be recovered if storage occurs for more than one year. As with seasonal storage of water supplies, MUS systems do not have the evaporation losses of surface water supplies and also require less use of land surface space for water storage that is needed only in drought situations.

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 39 Emergency Water Supplies In addition to storing water for drought, MUS systems have been developed to provide water supplies when surface water storage facilities or treatment plants are impacted by more catastrophic events. For example, Walla Walla, Washington, uses MUS as a means of protection from forest fires. Recent catas- trophic forest fires in the West during the current drought have caused water stored in surface reservoirs to become unusable due to increased sedimentation from post-fire erosion. MUS systems have been developed to ensure water sup- plies in case of hurricanes in South Carolina and floods in Iowa. MUS systems have also been cited as a means of backup storage of water supplies if there are impacts on treatment plants or surface water storage and distribution systems from earthquakes, brownouts, and terrorist attacks. Availability of Water Rights Both seasonal and long-term water availability may be tied to the prior ap- propriation legal system used throughout the western states. While the most senior water rights are typically for mining, followed by agriculture, and cities, the greatest population growth is occurring in the newest suburbs with the most junior water rights. Suburban municipalities in these and other growing western counties may have the financial resources to buy or lease agricultural water rights, but changing the use on these water rights (i.e., moving water from the farms to the cities) can be legally difficult and politically sensitive. The periods of peak demands for municipal uses, during the summer months, are also the periods when agricultural demands, with the more senior water rights, are great- est. Older, more established large cities with existing storage and treatment fa- cilities (e.g., Denver, Los Angeles) may be willing to develop agreements with newer suburban communities to provide water during wet periods, but they are likely to focus on their own service areas during periods of shortage. If ade- quate groundwater storage zones are available, MUS systems provide a means by which municipalities with junior water rights can develop storage facilities relatively quickly to capture water during seasons when those junior water rights are available, or when water rights transfers and exchanges can be arranged with more senior water rights holders, and to recover that water during periods when more junior water rights would be called out. This may be especially important in the future, because in some areas the allocation of water rights has been based on overoptimistic hydrological forecasts and junior water rights holders may be left without water for long periods of time (NRC, 2007).

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40 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER Species Recovery and Habitat Protection Programs Management of water resources to meet species recovery program require- ments, particularly for endangered species, has been a major concern for water managers. In the Pacific Northwest, MUS systems have been developed to re- duce stresses on surface water flows and stream habitat during low-flow periods by enabling water users to recover stored water in lieu of using surface water rights. One MUS system, in Walla Walla, Washington, has also developed a voluntary experimental project to take cooler water that had been stored in the aquifer and place it directly into Mill Creek during low-flow periods, when fish can be impacted by high stream temperatures, as part of a species recovery effort for the endangered steelhead salmon (Shrier, 2004). The largest ASR system currently under development is part of the Comprehensive Everglades Restora- tion Plan (CERP) in Florida. Box 2-2 provides a case study of the use of surface recharge systems in Colorado to provide both recharge and habitat benefits. Groundwater Resources Management (Water Levels and Water Quality) Several MUS systems have been integrated into regional efforts to manage groundwater levels and groundwater quality. By maintaining water levels by offsetting pumping with recharge, rather than mining nonrenewable groundwater resources, water users can reduce well interference and pumping costs, as well as prevent aquifer dewatering, land subsidence, and other impacts from stresses to groundwater resources by withdrawals. Arizona has an aggressive groundwa- ter resources management program and uses ASR to recover groundwater levels in a stressed aquifer. As part of this program, ASR systems in Arizona are re- quired to leave 5 percent of the recharged water in the aquifer. ASR has also been used to prevent potable groundwater from being impacted by saline water or contaminant plumes. The Equus Beds Aquifer Storage and Recovery project in Wichita, Kansas, is also being designed to control movement of a saline plume in addition to providing future water supply. The system will indirectly divert water from the Little Arkansas River through wells completed adjacent to the stream when flow in the river exceeds baseflow (http://ks.water.usgs.gov/ Kansas/studies/equus/). As noted earlier (Box 2-1), some California ASR facili- ties have been located to help prevent seawater intrusion. Industrial and Cooling Water Supply Another use of MUS systems that has developed is for industrial applica- tions. Micron Technology in Boise, Idaho, has been operational since 2001 and uses an MUS system to store surface water for use at a large semiconductor manufacturing operation. The facility owner-operator has cited the benefits of subsurface storage as a method for ensuring a more consistent water temperature

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 41 BOX 2-2 Case Study: Recharge Ponds for Streamflow Augmentation in Colorado Recharge ponds for streamflow augmentation have been used in Colorado to control streamflows, offsetting the impacts of well withdrawals from alluvial aquifers on more senior surface water rights. Water is taken from the stream during high-flow, low-demand periods, when unappropriated water is available, and placed in a recharge pond or leaky ditch dur- ing nonirrigation seasons. The water seeps from the pond (or ditch) into the aquifer and flows back to the stream at a rate determined by the properties of the aquifer. There is a lag time in the impacts of both the recharge ponds and the well withdrawals. Typically, as allowed by the Colorado State Engineer’s Office (SEO), the stream depletion caused by a well, or stream accretion created by a recharge pond, is calculated by the “Glover Method” (Glover, 1954), which is represented graphically in the Lower South Platte by the U.S. Geo- logical Survey (Jenkins, 1968) and is referred to as the stream depletion factor (SDF). Colorado water law prohibits the use of wells in alluvial aquifers unless there is a streamflow augmentation plan in place to offset the impacts of well withdrawals on surface water rights. The owner or operator of the pond (or ditch) receives augmentation credits that can be used against impacts to surface water flows from well withdrawals. These credits can also be leased to other water users whose groundwater use requires augmen- tation, if the pond is located and operated so that it can offset impacts of the well. The development of new recharge ponds accelerated rapidly during the 1980s and 1990s in response to emerging legal and administrative issues related to the development of perma- nent decrees and plans to augment streamflows to offset well depletions. Habitat partnership programs have been involved with the development of habitat at managed groundwater recharge sites in Colorado since the mid-1990s to develop recharge ponds that also provide benefits for species habitat. The Tamarack Plan Recharge, Min- now Stream & Wetland Habitat Project, a demonstration project on the west side of the Tamarack Ranch State Wildlife Area, is one of the first sites at which a recharge facility has deliberately been designed and operated to maximize both the recharge credits produced for stream augmentation and the habitat benefits for wildlife. This project, developed as part of the Colorado Tamarack Plan, was designed and created cooperatively by South Platte Lower River Group water resources engineers, Colorado Division of Wildlife aquatic and habitat biologists and geomorphologists, and Ducks Unlimited ecologists. There have been a number of recent projects developed in the Lower South Platte of Colorado in which habitat biologists and water resources engineers have worked together to design multipurpose facilities. Typically, habitat partnership programs develop agree- ments for conservation easements with the private landowners in this region. Landowners who are developing recharge ponds, and are interested in working with a habitat partner- ship program and designing the recharge ponds to provide habitat benefits, will typically contact the habitat partnership program to determine whether their sites would meet the eligibility requirements of that program. and water quality than is typically found when using surface water supplies. MUS is also being explored as a means of storing water for industrial cooling purposes. In addition to the other benefits associated with MUS as a means of water storage for water users, industries that need cooling water can withdraw water from underground storage that is at a lower temperature than surface water and thereby use that water for cooling purposes at lower costs than would be incurred if warmer water were used. After that cooling water has been used, exchanges can then be developed with agricultural water users, who may prefer warmer water for use on some crops.

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42 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER ROLE OF REGULATION AND FEDERAL AGENCY PROGRAMS IN MUS SYSTEM DEVELOPMENT In some states, the lack of legal or regulatory mechanisms to address issues related to permitting of MUS systems was a hurdle that needed to be overcome before these projects could be completed. The development of regulatory pro- grams for permitting and oversight of MUS systems facilitated MUS develop- ment in these cases. State programs designating certain aquifers as protected from new withdrawals, where MUS systems could be used to achieve a net zero impact or even a net increase in groundwater levels, also have led to increased use of MUS in some areas. In addition, there have been agency-sponsored pro- grams, such as federal agency demonstration projects and research programs, have also played an important role in the development of MUS systems. Since the 1980s, several states have developed laws or rules specifically ad- dressing some aspect of MUS, particularly related to ASR systems. In states such as Oregon and Washington, development of some ASR facilities was de- layed while new regulatory programs were being established, after which ASR development accelerated rapidly. Oregon and Washington each have more than a half-dozen ASR sites in operational or pilot stages. Arizona also has multiple ASR facilities that developed following the creation of its regulatory program. Several MUS systems developed in eastern Coastal Plain states in response to the designation during the 1960s, 1970s, and 1980s of regions where net groundwater use was restricted to prevent saltwater intrusion, land subsidence, well interference, or other negative impacts. MUS systems could be used to enable well withdrawals during high-demand periods, such as for tourism in the New Jersey shore. Areas with restricted net groundwater withdrawals in New Jersey, Virginia, North Carolina, and South Carolina have all seen development of MUS projects. Widespread use in the western United States of various forms of MUS (through both well recharge and surface recharge) was spurred by the U.S. Bu- reau of Reclamation’s (USBR) High Plains States Groundwater Demonstration Project. This program was begun in response to concerns regarding falling groundwater levels in the High Plains (also known as the Ogallala) Aquifer and to calls for additional water supplies and water management following droughts in the late 1970s and early 1980s. The original High Plains State Groundwater Demonstration Program Act was passed in 1983 and amended to include consideration of projects from all of the 17 western states in the contiguous United States that fall under the purview of USBR programs, rather than being limited to those states overlying the High Plains Aquifer. A total of 14 projects received federal funding under this part- nership program, out of 42 originally proposed. In selecting the projects to be included in this program, USBR considered not only physical aspects of the sites, but also economic, institutional, and legal factors, to ensure that there was a sponsor that could meet cost-sharing requirements and that funding and project development would not be delayed by legal or regulatory impediments.

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 43 A wide range of aquifer recharge approaches were used at the different sites participating in the USBR program, including land use management, surface infiltration, recharge wells, and ASR. Some of the projects were intended for general groundwater replenishment, with no consideration of subsequent uses of the recharged water, while other projects recharged aquifers that were used pri- marily for municipal, industrial, or agricultural uses. The largest federally sponsored ASR project–and by far the largest ASR project in the world–is that associated with the Comprehensive Everglades Res- toration Program. The original restoration plan (USACE and SFWMD, 1999) proposed about 330 ASR wells, each with a capacity of about 5 Mgal/d (a total capacity of 1.65 billion gallons per day). They would have an average annual storage capacity of about 570,000 acre-feet and would represent about 26 per- cent of the new storage capacity for the restoration project (NRC, 2005). Five ASR pilot projects, located in different regions of South Florida, are planned or under way to test the viability of ASR as a large-scale water storage component of the restoration effort. CONCLUSION MUS systems all have five components: (1) source of water to be stored; (2) recharge method; (3) storage method and management approach; (4) recovery method; and (5) end use of recovered water. Issues associated with each com- ponent are discussed in subsequent chapters of this report. These systems use water from a variety of sources such as surface water, groundwater, treated ef- fluent, and occasionally stormwater. They recharge groundwater through re- charge basins, vadose zone wells, direct recharge wells, and ASR wells. The water is stored in a wide spectrum of confined and unconfined aquifer types, from unconsolidated alluvial deposits to limestones and fractured volcanic rocks. Recovery typically is achieved through either discharge wells or dual- purpose recharge and recovery wells, but occasionally is achieved via natural discharge of the water to surface water bodies. Finally, the recovered water is used for drinking water, irrigation, industrial cooling, and environmental pur- poses. Some simple forms of MUS using surface recharge have been applied for millennia. MUS systems using well recharge have a shorter history but have been in use for more than four decades. There is, therefore, adequate experi- ence from which to draw some general conclusions about the degree to which MUS systems are successful in meeting their stated goals and the challenges and difficulties that some of them face. REFERENCES AWWA (American Water Works Association). 2002. Survey and Analysis of Aquifer Storage and Recovery (ASR) Systems and Associated Regulatory

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44 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER Programs in the United States. Denver, CO: AWWA. Brown, S. G. 1963. Problems of utilizing ground water in the west-side business district of Portland, Oregon. USGS Water-Supply Paper 1619-O. Portland, OR: U.S. Geological Survey. California Regional Water Quality Control Board. 1995. Water Quality Control Plan: Santa Ana River Basin. Riverside, CA. Available online at http://www.swrcb.ca.gov/rwqcb8/pdf/R8BPlan.pdf. Accessed on April 2007. Foxworthy, B.L. 1970. Hydrologic conditions and artificial recharge through a well in the Salem Heights area of Salem, Oregon. U.S. Geological Survey Water- Supply Paper 1594-F. Washington, DC: U.S. Government Printing Office. Glover, R. E., and C. G. Balmer. 1954. River depletion resulting from pumping a well near a river. American Geophysical Union Transactions 35(3):468-470. Grischek, T., D. Schoenheinz, E. Worch, and K. Hiscock. 2002. Bank filtration in Europe; and overview of aquifer conditions and hydraulic controls. Pp. 485-488 in Dillon, P. (ed.) Management of Aquifer Recharge for Sustain- ability. Lisse, The Netherlands: A. A. Balkema. Jenkins, C. T. 1968. Computation of rate and volume of stream depletion by wells. In Chapter D1, Book 4, U.S. Geological Survey Techniques of Wa- ter-Resources Investigations. Washington, DC: U.S. Government Printing Office. Ku, H. F. H., and D. L. Simmons. 1983. Use of recharge basins for storm water management on Long Island, New York. Pp. .17-43 In Proceedings of the NWWA Eastern Regional Conference on Ground Water Management. Wor- thington, OH: National Water Well Association. Moulder, E. A., and D. R. Frazor. 1958. Artificial recharge experiments at McDonald well Field. Texas Board of Water Engineers Bulletin 5701. Amarillo, TX: Texas Board of Engineers. NRC (National Research Council). 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: National Academy Press. NRC. 1998. Issues in Potable Reuse: The Viability of Augmenting Drinking Water Supplies with Reclaimed Water. Washington, DC: National Academy Press. NRC. 2005. Re-engineering Water Storage in the Everglades: Risks and Oppor- tunities. Washington, DC: National Academies Press. NRC. 2007. Colorado River Basin Water Management: Evaluating and Adjust- ing to Hydroclimatic Variability. Washington, DC: National Academies Press. Price, C. E. 1961. Artificial recharge through a well tapping basalt aquifers. U.S. Geological Survey Water-Supply Paper 1594-A. Walla Walla, WA: U.S. Geological Survey. Pyne, R. D. G. 2005. Aquifer Storage Recovery: A Guide to Groundwater Re- charge Through Wells. Second Edition. Boca Raton, FL: ASR Press. Shrier, C. 2004. Aquifer storage recovery: Expanding possibilities in the west- ern United States. The Water Report #8: October 15.

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OVERVIEW OF MANAGED UNDERGROUND STORAGE SYSTEMS 45 United Nations. 1975. Ground-water Storage and Artificial Recharge. Natural Resources/Water Series, No. 2. New York: United Nations. USACE and SFWD (U.S. Army Corps of Engineers and South Florida Water Management District). 1999. Central and Southern Florida Comprehensive Review Study Final Integrated Feasibility Report and Programmatic Envi- ronmental Impact Statement. Available online at http://www. evergladesplan.org/pub/restudy_eis.cfm. Accessed December 2006. Weeks, E. P. 2002. A historical overview of hydrologic studies of artificial re- charge in the U.S. Geological Survey. U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4. OFR 02-89. Lakewood, CO: U.S. Geological Survey.

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