Summary

Pressure on freshwater supplies will increase to meet anticipated needs for municipal and industrial uses, agricultural irrigation, and environment protection in the coming years. Certain conditions such as increasing population, changing land use, reallocation of existing water resources, reduction of snowpack, and overdrafting of aquifers will require tapping into other non-traditional sources of water. While other water management strategies have been used to increase freshwater supply through importation or desalination, improving water efficiency through technology and conservation, and reuse of treated wastewater, the potential for managed underground systems to sustain future water supplies is considerable.

With or without the other strategies, there is already a need for temporary detention and storage of water during times of abundance and recovery that water in times of scarcity. The traditional practice of storing water aboveground has been met with several challenges such as evaporative losses, sediment accumulation, land consumption, high cost, and ecological impact. Because of these factors there is increasing interest in storing recoverable water underground as part of a larger water management strategy. This has brought with it, however, its own set of challenges, such as costs to design, construct, and monitor the system; loss of some percentage of the water; chemical reactions with aquifer materials; ownership issues; and environmental impacts.

The source water for underground storage may come from streams or groundwater, water reclamation plants, or other sources and be recharged through different methods. After recovery, it may be used for potable, industrial, agricultural, environmental, and other purposes. For this report, the term managed underground storage (MUS) is used to refer to this purposeful recharge of water into an aquifer system for intended recovery and use as component of long-term water resource management.

The growing importance of the topic emphasized the need to study the state of the knowledge and identify the research and education needs and priorities for Managed Underground Storage of Recoverable Water. In 2003, the National Research Council (NRC) along with the AWWA Research Foundation (AwwaRF) organized a planning meeting for a consensus study on the topic. The feedback received during this planning meeting was instrumental in formulating a statement of task (Box S-1) for a follow-on study, whose results are summarized in this report.

In early 2005, the authoring committee for this report met for the first time to identify research and education needs and priorities in underground storage technology and implementation. Members represented multidisciplinary



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Summary Pressure on freshwater supplies will increase to meet anticipated needs for municipal and industrial uses, agricultural irrigation, and environment protection in the coming years. Certain conditions such as increasing population, changing land use, reallocation of existing water resources, reduction of snowpack, and overdrafting of aquifers will require tapping into other non-traditional sources of water. While other water management strategies have been used to increase freshwater supply through importation or desalination, improving water effi- ciency through technology and conservation, and reuse of treated wastewater, the potential for managed underground systems to sustain future water supplies is considerable. With or without the other strategies, there is already a need for temporary detention and storage of water during times of abundance and recovery that wa- ter in times of scarcity. The traditional practice of storing water aboveground has been met with several challenges such as evaporative losses, sediment ac- cumulation, land consumption, high cost, and ecological impact. Because of these factors there is increasing interest in storing recoverable water under- ground as part of a larger water management strategy. This has brought with it, however, its own set of challenges, such as costs to design, construct, and moni- tor the system; loss of some percentage of the water; chemical reactions with aquifer materials; ownership issues; and environmental impacts. The source water for underground storage may come from streams or groundwater, water reclamation plants, or other sources and be recharged through different methods. After recovery, it may be used for potable, indus- trial, agricultural, environmental, and other purposes. For this report, the term managed underground storage (MUS) is used to refer to this purposeful recharge of water into an aquifer system for intended recovery and use as component of long-term water resource management. The growing importance of the topic emphasized the need to study the state of the knowledge and identify the research and education needs and priorities for Managed Underground Storage of Recoverable Water. In 2003, the National Research Council (NRC) along with the AWWA Research Foundation (AwwaRF) organized a planning meeting for a consensus study on the topic. The feedback received during this planning meeting was instrumental in formu- lating a statement of task (Box S-1) for a follow-on study, whose results are summarized in this report. In early 2005, the authoring committee for this report met for the first time to identify research and education needs and priorities in underground stor- age technology and implementation. Members represented multidisciplinary 1

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2 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER BOX S-1 Statement of Task The proposed study will provide an overview of some of the research and education 1 needs and priorities concerning sustainable underground storage technology and imple- mentation. It will also assess geological, geochemical, biological, engineering, and institu- tional factors that may affect the performance of such projects, based in part on a review and evaluation of existing projects. Specifically, the study will assess and make recommendations with respect to re- search and education needs on the following questions: • What research needs to be done to develop predictors of performance for under- ground storage projects based on the character of the recharge water in terms of contaminants, disinfectants, and microbes, the hydrogeology and major ion geo- chemistry of the source water and the aquifer, and the well or basin characteris- tics? • What are the long-term impacts of underground storage on aquifer use— hydraulic, geotechnical, geochemical, adsorptive capacity of contaminants—at wellhead and regional scales, and can these impacts be ameliorated? • What physical, chemical, and geological factors associated with underground storage of water may increase or decrease human and environmental health risks concerning microbes, inorganic contaminants such as nitrite, disinfectant by-products, endocrine disruptors, personal care products, pharmaceuticals, and other trace organic compounds? • Are there any chemical markers or surrogates that can be used to help assure regulators and the public of the safety of water for groundwater recharge? What should we monitor and at what spatial and temporal scales? • What are the challenges and potential for incorporating sustainable underground storage projects into current systems approaches to water management for solv- ing public and environmental water needs? • How do the institutional, regulatory and legal environments at federal, state, and local levels encourage or discourage sustainable underground storage? expertise in groundwater and surface water hydrology, inorganic and organic hydrogeochemistry and biogeochemistry, risk assessment, environmental and water resources engineering, water reuse, and natural resource economics and law. The potentially widespread implication of the study is apparent in its spon- sors, which represent water utilities, water associations, federal and state agen- cies, and science organizations: the American Water Works Association Re- search Foundation, the WateReuse Foundation, the U.S. Geological Survey (USGS), the CALFED Bay-Delta Program and the California Department of Water Resources Conjunctive Water Management Branch, the City of Phoenix, the Inland Empire Utilities Agency, the Sanitation Districts of Los Angeles County, the Chino Basin Watermaster, and the NRC President's Committee of the National Academies. 1 In this report the term “managed underground storage” is used instead of “sustainable underground storage.”

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SUMMARY 3 An overall evaluation and a summary of the key conclusions and recom- mendations of the study follow. OVERALL EVALUATION Conclusion: The challenges to sustaining present and future water supplies are great and growing. The present overdrafting of aquifers and overallocation of rivers in many regions is a clear indication of these challenges, but the former also creates in many cases the underground storage potential needed to accom- modate MUS systems. Thus, demand for water management tools such as MUS is likely to continue to grow (Chapter 1). Conclusion: Some simple forms of MUS have been used for millennia, and even the most recent development–aquifer storage and recovery–now has about four decades of history behind it. These systems use water from a variety of sources such as surface water, groundwater, treated effluent, and occasionally stormwater. They recharge groundwater through recharge basins, vadose zone wells, and direct recharge wells. The water is stored in a wide spectrum of con- fined and unconfined aquifer types, from unconsolidated alluvial deposits to limestones and fractured volcanic rocks. Recovery typically is achieved through either extraction wells or dual-purpose recharge and recovery wells, but occa- sionally is achieved via natural discharge of the water to surface waterbodies. Finally, the recovered water is used for drinking water, irrigation, industrial cooling, and environmental and other purposes. There is, therefore, adequate experience from which to draw some general conclusions about the degree to which MUS systems are successful in meeting their stated goals and the chal- lenges and difficulties that some of them face (Chapter 2). Conclusion: Although failures have occurred and the potential for con- taminating groundwater is a considerable risk, most MUS systems have success- fully achieved their stated purposes. In fact, there are MUS systems that have functioned without major problems for decades. However, increasing efforts to use karst and fractured aquifers for storage will increase the potential for fail- ures. Chemical reactivity of the aquifer in the former case and uncertainty over flow paths in either case are much greater and the treatment potential is lower compared to alluvial aquifers. Learning from past positive and negative per- formance will help guide development of the many new MUS systems that are under consideration (Chapter 7). Recommendation: Given the growing complexity of the nation’s water management challenges, and the generally successful track record of managed underground storage in a variety of forms and environments, MUS should be seriously considered as a tool in a water manager’s arsenal (Chapters 1-7). Conclusion: In the future, multiple strategies are likely to be needed to manage water supplies and meet demands for water in the face of scarcity.

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4 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER Various water conservation and management strategies, including transfers and water recycling, can be used to stretch available water supplies. However, each of these has its rate of delivery limits. Water storage facilities will continue to be an essential component of water management, particularly in areas where water availability varies greatly over seasons or years, such as the arid South- west. Integrated strategies will be needed in which all measures for improving water quality and managing water scarcity are considered and, if appropriate, employed in a balanced, systematic fashion. Seasonal to multi-year storage of water will often be a necessary component of such strategies. Recommendation: In anticipating, planning for, and developing MUS pro- jects, water managers should consider them in a watershed and regionally based context and as part of the overall water management strategies (Chapter 7). HYDROGEOLOGICAL ISSUES Conclusion: To facilitate the siting and implementation of MUS systems, maps of favorable aquifers and hydrogeological characteristics can be prepared using three-dimensional (3-D) capable geographical information systems (GIS). At a regional or statewide scale, such GIS maps can help visualize and charac- terize major aquifers for future development of MUS systems, map and analyze regional changes in head and flow patterns, and facilitate comprehensive, re- gional water resources management. At a project scale, they can aid in estab- lishing the design, spacing, orientation, and capacity of wells and recharge ba- sins, evaluating their impact on the environment and existing users, estimating the critical pressure for rock fracturing, visualizing the movement of stored wa- ter throughout the system (especially useful for systems with waters of varying density or quality), and evaluating the extent of potential water quality changes in the aquifer during storage and movement. Recommendation: States, counties, and water authorities considering MUS should consider incorporating 3-D capable geographical information sys- tems along with existing hydrogeologic, geochemical, cadastral, and other data in (1) regional mapping efforts to identify areas that are, or are not, likely to be favorable for development of various kinds of MUS systems, and (2) project conception, design, pilot testing, and adaptive management (Chapter 3). Conclusion: Long-term local and regional impacts of MUS systems on both native groundwater and surface water have been recognized, including changes in groundwater recharge, flow, and discharge, and effects on aquifer matrix such as compaction of confining layers or clay interlayers during re- charge and recovery cycles. Recommendation: Monitoring and modeling should be performed to pre- dict likely effects–positive or negative–of MUS systems on the physical system, including inflows, storage, and outflows. Appropriate measures can and should be taken to minimize negative effects during operations (Chapter 3).

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SUMMARY 5 Conclusion: Groundwater numerical modeling at regional and/or high- resolution local scales provides a cost-effective tool for planning, design, and operation of a MUS system. Recommendation: Analyses using groundwater flow and solute transport modeling should become a routine part of planning for, designing, and adap- tively operating MUS systems. Uncertainty analysis should also be incorporated into prediction of a system’s short- and long-term performance, especially re- garding the expected values of recovery efficiency and storage capacity (Chapter 3 and 4). Specific Research Recommendations: In addition to the topics above, re- search is particularly needed, and should be conducted, in the following areas (Chapter 3): • Hydrologic feasibility. This includes (1) a lack of knowledge about storage zones and areas favorable for recharge for major aquifers in the United States; (2) limited understanding of how aquifer heterogeneity, scale effects, and other physical, chemical, and biological properties impact recharge rate and recovery efficiency of the MUS system; (3) a lack of understanding of matrix behavior, especially fractured aquifers, during injection vs. withdrawal tests (e.g., expansion vs. compaction) to prevent or limit artificially induced deformation of the aquifer matrix; (4) a need to develop tools to analyze non-Darcian flow around re- charge wells to avoid poor design of recharge wells; and (5) need for overall characterization, system recovery efficiency, optimum place- ment of monitoring wells, recharge and pumping impacts, and hydrau- lic fracturing in an aquifer with dual porosity. • Impacts of MUS systems on surface water. How, in terms of both quantity and timing, might a surface spreading or well recharge facility affect the flow of neighboring streams? What would be the hydrologic, ecological, and legal consequences of this interaction between the MUS system and surface water? An integrated or system approach should be developed and employed for assessing such impacts. • Technology enhancement and methodology development for determin- ing hydrological properties of the aquifers and their impacts on per- formance of the MUS system. These include (1) surface and borehole geophysical methods to determine hydrological properties and the ex- tent of recharge water volumes during cycle testing, (2) optimization of cycle test design (frequency, duration, and intensity) to improve per- formance of MUS systems for various hydrological settings, (3) better conceptual models for delineation of storage zone and recovery zone, and (4) better understanding of non-Darcian flow near wells through experimental study and field monitoring, and further development of

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6 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER theories and numerical models to assess the interaction of stored water (especially urban runoff) with native groundwater. WATER QUALITY CONSIDERATIONS Conclusion: There is a substantial body of work documenting improve- ments in water quality that can occur in an MUS system, particularly those that involve surface spreading. The subsurface has, to a greater or lesser extent, the capacity to attenuate many chemical constituents and pathogens via physical (e.g., filtration and sorption), chemical, and biological processes. In places where the groundwater quality is saline or otherwise poor, the implementation of MUS will likely improve overall groundwater quality and provide a benefit to the aquifer. However, the type of source water used for recharge along with subsurface properties and conditions influences the extent of treatment and the effects on native groundwater quality. Therefore, a thorough knowledge of the source wa- ter chemistry and mineralogy of the aquifer is requisite to embarking on any MUS project. It is important to establish whether the mixing of source water and native groundwater, as well as chemical interaction with aquifer materials, yields compatible and acceptable effects on water quality. Recommendation: A thorough program of aquifer and source water sam- pling, combined with geochemical modeling, is needed for any MUS system to understand and predict its medium- and long-term chemical behavior and help determine the safety and reliability of the system (Chapter 4). Conclusion: A better understanding of the contaminants that might be pre- sent in each of the potential sources of recharge water is needed, especially for underutilized sources of water for MUS, such as stormwater runoff from resi- dential areas. Limited data exist on the use of urban stormwater for MUS sys- tems. Consistent with an earlier NRC report (1994), urban stormwater quality is highly variable and caution is needed in determining that the water is of accept- able quality for recharge. Recommendation: Research should be conducted to evaluate the variabil- ity of chemical and microbial constituents in urban stormwater and their behav- ior during infiltration and subsurface storage to establish the suitability of com- bining MUS with stormwater runoff (Chapter 4). Conclusion: The presence and behavior of “emerging” contaminants (e.g., endocrine disrupting compounds, pharmaceuticals, and personal care products) is of concern, especially with reclaimed wastewater. However, the concern about these compounds is not unique to MUS systems. Surface waters and groundwaters around the nation carry the same kind of chemicals, and surface water treatment systems are not normally designed to address them.

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SUMMARY 7 Recommendation: Basic and applied research on emerging contaminants that has begun at a national scale should be encouraged, and MUS programs will be among the many beneficiaries of such investigations (Chapter 4). Conclusion: A better understanding is needed of potential removal proc- esses for microbes and contaminants in the different types of aquifer systems being considered for MUS. These studies are necessary to assess spatial and temporal behavior during operation of an MUS system. This research could reduce uncertainty regarding the extent of chemical and microbial removal in MUS systems. In addition, this information could help reduce impediments to public acceptance of a wide variety of source waters for MUS. Conclusion: In particular, changes in reduction-oxidation (redox) condi- tions in the subsurface are common and often important outcomes of MUS op- eration. These changes can have both positive and negative influences on the physical properties and the chemical and biological reactivity of aquifer materi- als. For example, the existence of both oxidizing and reducing conditions might enhance the biodegradation of a suite of trace organic compounds of concern or, conversely, lead to accumulation of an intermediate product of concern. Redox changes can cause dissolution-precipitation or sorption-desorption reactions that lead to adverse impacts on water quality or clogging of the aquifer; however, such precipitation reactions can also sequester dissolved contaminants. Recommendation: Additional research should be conducted to understand potential removal processes for various contaminants and microbes and, particu- larly, to determine how changes in redox conditions influence the movement and reactions for many inorganic and organic constituents. Specific areas of re- search that are recommended include (1) bench-scale and pilot studies along with geochemical modeling to address potential changes in water quality with variable physical water conditions (pH, oxidation potential [Eh], and dissolved oxygen [DO]); and (2) examination of the influence of sequential aerobic and anaerobic conditions or alternating oxidizing and reducing conditions on the behavior of trace organic compounds in MUS systems, especially during storage zone conditioning (Chapter 4). Conclusion: Molecular biology methods have the potential for rapid iden- tification of pathogens in water supplies. These noncultivable techniques have not been tested in a meaningful way to address background and significance of the findings. False negatives and false positives remain an issue that needs to be addressed. Recommendation: Research should be conducted to address the ap- proaches and specific applicability of molecular biology methods for pathogen identification, particularly interpretation of results that cannot determine viabil- ity, for the different types of source waters and aquifer systems being considered for MUS (Chapter 4).

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8 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER Conclusion: Pathogen removal or disinfection is often required prior to storing water underground. If primary disinfection is achieved via chlorination, disinfection by-products (DBPs) such as trihalomethanes and haloacetic acids are formed. These have been observed to persist in some MUS systems. How- ever, chlorine is the most cost-effective agent for control of biofouling in re- charge wells; hence, it may not be possible to eliminate entirely the use of chlo- rine in MUS systems (e.g., periodic pulses of chlorine to maintain injection rates). Recommendation: To minimize formation of halogenated DBPs, alterna- tives to chlorination should be considered for primary disinfection requirements, such as ultraviolet, ozone, or membrane filtration (Chapter 4). LEGAL, ECONOMIC, AND OTHER INSTITUTIONAL CONSIDERATIONS Conclusion: Some states have created statutory schemes that are tailored to MUS projects, This approach is desirable because of the novel questions posed. For example, a state may find it desirable that withdrawals from an MUS project be done over a longer period than a traditional water right might provide, or that MUS be allowed despite the “junior” status of the right’s holder. States can an- ticipate these adjustments to traditional water rights as appropriate. Recommendation: While a comprehensive approach has advantages, at a minimum states should define property rights in water used for recharge, aquifer storage, and withdrawn water, to provide clarity and assurance to MUS projects (Chapter 5). Conclusion: The federal regulatory requirements for MUS are inconsistent with respect to treatment of similar projects. Federal Underground Injection Control (UIC) regulation addresses only projects that recharge or dispose of water directly to the subsurface through recharge wells, while infiltration pro- jects are regulated by state governments whose regulatory standards may vary. The appropriateness of regulation through the UIC program has been questioned by states with active aquifer storage and recovery (ASR) regulatory programs. Also, there are inconsistencies between the Clean Water Act and the Safe Drink- ing Water Act that impact MUS systems. For example, some jurisdictions try to control surface water contamination problems by diverting polluted water from aboveground to groundwater systems. This approach may undermine MUS programs by putting contaminants underground without appropriate controls. Recommendation: Federal and state regulatory programs should be exam- ined with respect to the need for continued federal involvement in regulation, the necessity of a federal baseline for regulation, and the risks presented by in- adequate state regulation. A model state code should be drafted that would assist states in developing comprehensive regulatory programs that reflect a scientific approach to risk (Chapter 5).

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SUMMARY 9 Conclusion: Regulations are, quite properly, being developed at the state level that will require a certain residence time, travel time, or travel distance for recharge water prior to withdrawal for subsequent use. However, regulations based on attenuation of a single constituent or aquifer type, such as pathogen attenuation in a homogeneous sand aquifer, may not be appropriate for a system concerned with trace organics and metals in a fractured limestone, and vice versa. Such regulations are particularly pertinent for MUS with reclaimed water. Recommendation: Science-based criteria for residence time, travel time, or travel distance regulations for recharge water recovery should be developed. These criteria should consider biological, chemical, and physical characteristics of an MUS system and should incorporate criteria for adequate monitoring. The regulations should allow for the effects of site-specific conditions (e.g., tempera- ture, dissolved oxygen, pH, organic matter, mineralogy) on microbial survival time or inactivation rates and on contaminant attenuation. They should also con- sider the time needed to detect and respond to any water quality problems that may arise (Chapter 5). Conclusion: MUS projects can exhibit numerous and complementary eco- nomic benefits, but they also entail costs. Some of those benefits and costs are unlikely to be incorporated in the calculations of individual water users—that is, there may be spillover costs to third parties or spillover benefits that are not given market valuations. Failure to account for all benefits and costs, including ones that may not be reflected in market prices for water, can lead to underin- vestment in groundwater recharge, overconsumption of water supplies, or both. Recommendation: An economic analysis of an MUS project should cap- ture the multiple benefits and costs of the project. MUS projects invariably en- tail the achievement of multiple objectives. Third party impacts, such as the en- vironmental consequences of utilizing source water, should be included (Chapter 5). Conclusion: Water resources development has been characterized by sub- stantial federal and state subsidies. As water shortages intensify, the political pressure for investment in new technologies will increase. Recommendation: To ensure optimal investment in MUS and other tech- nologies, subsidies should be provided only when there are values that cannot be fully reflected in the price of recovered waters. An example of such a value would be an environmental benefit that accrues to the public at large. In particu- lar, simply lowering costs should not be the justification for providing subsidies for MUS projects (Chapter 5). Conclusion: Antidegradation is often the stated goal of water quality poli- cies, including policies that apply to underground storage of water. For any MUS project–including storage of potable water, stormwater, and recycled wa- ter–it is important to understand how water quality differences between native groundwater and the stored water will be viewed by regulators, who are charged

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10 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER with satisfying those regulatory mandates. In addition to water quality factors, a broader consideration of benefits, costs, and risks would provide a more desir- able regulatory approach. Therefore, weighing water quality considerations together with water supply concerns, conservation, and public health and safety needs is an essential plan of action. Rigid antidegradation policies2 can impede MUS projects by imposing costly pretreatment requirements and may have the practical effect of prohibiting MUS, even in circumstances where the prospects of endangering human or environmental health are remote and the benefits of water supply augmentation are considerable. Recommendation: State laws and regulations should provide regulatory agencies with discretion to consider weighing the overall benefits of MUS while resolutely protecting groundwater quality (Chapter 5). OPERATIONAL ISSUES Conclusion: The development of an MUS system from project conception to a mature, well functioning system is a complex, multistage operation requir- ing interdisciplinary knowledge of many aspects of science, technology, and institutional issues. Recommendation: A comprehensive decision framework should be de- veloped to assist in moving through the many stages of project development in an organized, rational way. Professionals from many fields, including chemists, geologists, hydrologists, microbiologists, engineers, economists, planners, and other social scientists should be involved in developing this framework (Chapter 6). Conclusion: Growing experience with MUS systems indicates that hydro- geological feasibility analysis including aquifer characterization is one of sev- eral important components in their development and implementation. The bene- fits of doing so include establishing the hydraulic capacity, recharge rates, resi- dence times, and recoverable fraction of the introduced water–all of which help identify the optimum design and viability of the MUS system. Some types of aquifers have matrix, hydrogeologic, and geochemical char- acteristics that are better suited to MUS systems than others. For example, the aquifer characteristics may dictate recharge, storage, and recovery methods. For an unconfined aquifer, source water can be recharged into the aquifer through recharge basins, vadose zone recharge wells, and deep recharge wells. Stored water can be recovered by production wells or ASR wells, or it can enhance baseflow to neighboring streams. For confined aquifers, however, source water can only be injected through deep recharge wells, including ASR wells. The 2 In Chapter 5, the term “rigid antidegradation policies” refers to prohibiting any change whatsoever in groundwater quality, even when both the source water and the aquifer water meet all drinking water standards. Further discussion is found in Chapter 5.

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SUMMARY 11 stored water is usually recovered through ASR wells or downgradient produc- tion wells. As another example, water quality benefits are likely to be greater with alluvial systems compared to fractured or dual-porosity systems. Recommendation: Multiple factors should be assessed and monitored dur- ing design, pilot tests, and operations, including spatial and hydrogeological characterization of storage zones; temporal variation in quality and quantity of recharged, stored, and recovered water; and factors that constrain sustainability of the MUS system, including hydrogeochemical, microbiological, and eco- nomic conditions. Uncertainty reduction is the ultimate goal (Chapters 3, 4, and 6). Conclusion: An independent advisory panel can provide objective, third- party guidance and counsel regarding design, operation, maintenance, and moni- toring strategies for an MUS project. An independent panel can increase public acceptance of and confidence in the system if such trust is warranted. It can also be a catalyst for altering a plan if changes appear to be necessary. Recommendation: Water agencies should highly consider the creation of an independent advisory panel or equivalent at an early stage of planning for an MUS system (Chapter 6). Conclusion: Relatively little research has been done to characterize the ex- tent of vertical migration of fine-grained particles into the sediments beneath surface spreading facilities. Likewise, the science and technology of cleaning recharge basins is not well developed. Recommendation: New approaches should be developed to optimize sur- face recharge, including assessing the extent of migration of fine-grained sedi- ment into the subsurface, its impact on the long-term sustainability of surface recharge, and more efficient methods to clean recharge basins after clogging occurs (Chapter 6). Conclusion: Successful MUS involves careful and thorough chemical and microbiological monitoring to document system performance and evaluate the reliability of the process. Each MUS project needs real-time monitoring of the quality of the waters being introduced into underground storage and of waters being extracted from storage for use. Recommendation: Water quality monitoring programs should be designed on a case-by-case basis to assess water quality changes for elements, com- pounds, and microbes of concern, optimizing the potential to document any im- provement in the quality of the source water and to collect samples representing any adverse water quality changes. A proactive monitoring plan is needed to respond to emerging contaminants and increase knowledge about potential risks (Chapters 4 and 6). Conclusion: New surrogates or indicators of pathogen and trace organic contaminant presence are needed for a variety of water quality parameters to

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12 PROSPECTS FOR MANAGED UNDERGROUND STORAGE OF RECOVERABLE WATER increase the certainty of detecting potential water quality problems through monitoring. The categorization of chemicals and microorganisms into groups with similar fate and transport properties and similar behavior in treatment steps is one approach to streamline the list of potential contaminants to be monitored. It is unclear whether we can continue to rely on total coliform and Escherichia coli indicator bacteria to characterize the microbial quality of water as the drink- ing water industry has done for decades. Such methodologies will improve the ability of MUS systems of a variety of sizes to engage in sound monitoring prac- tices. Recommendation: Research should be conducted to understand whether we can rely on monitoring surrogate or indicator parameters as a substitute for analysis of long lists of chemicals and microorganisms (Chapter 6). Conclusion: Surface spreading facilities sometimes require large amounts of land, particularly where large amounts of water are recharged or the geology is not ideal. Recharge well systems require less land but may have as many dif- ferent factors to consider in their placement. Optimization of recharge facility placement is important but not always well understood. Recommendation: If there is some degree of freedom in site selection for recharge wells or basins, a location suitability assessment may be useful in site optimization. Factors such as ecological suitability, existing uses of the aquifer, groundwater quality, aquifer transmissivity, road density, land use and owner- ship, and access to power lines can be weighed in such an analysis (Chapter 6). REFERENCE NRC (National Research Council). 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: National Academy Press.