Ground water is a far more important resource than is often realized—excluding the water locked in glaciers and icecaps, about 97 percent of the world's fresh water is ground water, while streams, rivers, and lakes hold only about 3 percent (Bouwer, 1978). In the United States, about one-half of the population and three-fourths of the public water supply systems rely on ground water. Ground water also provides a critical source of water for agricultural irrigation and industries. But as is often the case with critical resources, ground water is not always available when and where needed, especially in water-short areas where heavy use has depleted underground reserves.
The growing competition for water in the United States and elsewhere around the world are leading to even greater use of this enormous water resource. As part of this trend, there has been increased interest in the use of artificial recharge to augment ground water supplies, especially in the western United States and other water-short areas such as Florida. Stated simply, artificial recharge is a process by which excess surface water is directed into the ground—either by spreading on the surface, by using recharge wells, or by altering natural conditions to increase infiltration—to replenish an aquifer. The most common purpose of artificial recharge (also called planned recharge) is to store water underground in times of surplus to meet water demand in times of shortage. Recovered recharge water is especially well-suited to nonpotable uses such as landscape irrigation (which, in turn, frees other supplies for higher uses); under appropriate conditions and where better sources are not available, recovered recharge water also can be an option for potable use. Artificial recharge can be used to control sea water intrusion in coastal aquifers, control land subsidence
caused by declining ground water levels, maintain base flow in some streams, and raise ground water levels to reduce the cost of ground water pumping.
As the benefits of artificial recharge of ground water have become evident, water planners have sought alternative sources of water for recharge projects. The National Research Council's Committee on Ground Water Recharge was established to study issues associated with the recharge of ground water using source waters of impaired quality, specifically treated municipal wastewater, stormwater runoff, and irrigation return flow, and issues associated with the use of recovered recharge water for potable as well as nonpotable purposes. (This report does not address industrial wastewater, which can contain too wide and too different an array of possible constituents to be dealt with in this same volume.) The committee was asked to address a range of topics such as source water characteristics, pretreatment and recharge technologies, public health, and the nature of the physical, chemical, and biological processes and transformations that occur during transport through the subsurface. Economic, institutional, and regulatory issues were also to be considered.
The committee is aware that much work has been done in the past to understand the opportunities and potential problems related to artificial recharge of ground water, and we have been careful to try to build on this strong foundation. The committee also recognizes that artificial recharge using waters of impaired quality is one of many strategies that can be used, alone or in conjunction with other strategies, to augment water supplies, such as reducing water consumption or creating secondary water systems that deliver certain wastewaters directly to nonpotable uses (e.g., the use of gray water for landscape irrigation). This report summarizes the state of our knowledge about artificial recharge using source waters of impaired quality and its usefulness and makes recommendations to help the nation use this water management tool more effectively.
SOURCE WATERS AND THEIR TREATMENT
The quality of the source waters used to recharge ground water has a direct bearing on operational aspects of the recharge facilities and also on the ultimate use to be made of the recovered water. In general, the source water characteristics that affect the operational aspects of recharge facilities include suspended solids, dissolved gases, nutrients, biochemical oxygen demand, microorganisms, and the sodium adsorption ratio (which affects soil permeability). The constituents that have the greatest potential effects when potable reuse is being considered include organic and inorganic toxicants, nitrogen compounds, and pathogens.
Of the three types of impaired quality water considered in this report—treated municipal wastewater, stormwater runoff, and irrigation return flow—municipal wastewater is by far the most consistent spatially and temporally and in terms of both quantity and quality. Exceptions to this generalization are
where raw municipal wastewater and stormwater are cornmingled or where there are variable industrial contributions. The major constituents of municipal waste-water have been studied extensively, but less is known about trace constituents. Possible constituents of concern in municipal wastewater effluent include organic compounds, nitrogen species, phosphorus, pathogenic organisms, and suspended solids. Treatment processes are available to bring municipal wastewater effluent to levels acceptable for various recharge applications; however, even when it has been treated to a high degree, effluent that has been disinfected with chlorine will contain disinfection by-products that can be of concern if the recovered water is to be used for potable purposes.
The quality of stormwater runoff is affected by several factors, including rainfall quantity and intensity, the natural and anthropogenic characteristics of the drainage basin, time since the last runoff event, and, in northern areas, time of year. Constituents of concern in stormwater runoff include trace metals, organic compounds, pathogenic organisms, suspended solids, and in northern climates in the winter, dissolved solids such as sodium chloride contributed by road deicing practices. Generally, stormwater runoff from residential areas is of good quality but its quantity may be extremely erratic.
Irrigation return flow exhibits the widest variation in quality of the three potential source waters considered here, and quality characteristics beyond salinity and concentrations of nitrate are not well studied. Salt content can be a problem in arid and semiarid areas, and suspended solids, nutrients, pesticide residues, and trace element concentrations including selenium, uranium, boron, and arsenic may also be of concern. Although treatment processes are available to remove the constituents of concern to acceptable levels, treatment of irrigation return flow generally is not done, and the cost-effectiveness of such treatment is questionable.
SOIL AND AQUIFER PROCESSES
The common assumption that passage of source water through the soil to the aquifer and through the aquifer to the point of withdrawal provides no treatment is overly conservative when applied to most chemicals and microorganisms. The soil and underlying aquifer have a great capacity to remove contaminants and pathogens from recharge water. The ideal soil for a soil-aquifer-treatment (SAT) system balances the need for a high recharge rate, which occurs in coarse-textured soils, with the need for efficient contaminant adsorption and removal, which are better in fine-textured soils.
The unsaturated soil layer (vadose zone) can play an important role in artificial recharge: it can remove or reduce the chemical and biological constituents present in the impaired quality source water as it moves toward the underlying aquifer and thus help reduce potential health risks before the recharge water enters the ground water. Nitrogen, for example, quickly transforms to nitrate,
which is very mobile under normal conditions in the soil but which can be removed only by denitrification under anaerobic conditions. Phosphorus levels are reduced by sorption and precipitation, although not completely. Trace metals, with the exception of boron, are strongly attenuated and precipitated in the soil, especially under aerobic and alkaline conditions. Organic chemicals are removed to varying degrees by volatilization or chemical or biological degradation during passage through the vadose zone. Some pathogen removal by filtration occurs for larger organisms, and there is some sorption of bacteria and viruses. Unfortunately, the processes by which removal occurs are not completely efficient in a natural setting, and not all constituents are retained or degraded to the same extent. Moreover, management strategies that may enhance hydraulic capacity or removal of one chemical or pathopen may actually decrease the efficiency of removal of another.
With adequate management and monitoring, an SAT system may reduce pretreatment and posttreatment costs. With proper management, an SAT operation employing periodic drying to reduce clogging should be sustainable indefinitely. Slow trace element migration remains a concern, however, the presence of such constituents necessitates careful monitoring during SAT use and regulation during closure of a facility. Although near-surface monitoring is desirable for proper vigilance, soil variability makes it very difficult to provide complete coverage with existing devices. A combination of near-surface and distant monitoring thus provides the best compromise.
PUBLIC HEALTH ISSUES
A major consideration in the use of impaired-quality waters for artificial recharge is the possible presence of chemical and microbiological agents in the source waters that may be hazardous to human health. Such concerns apply both to potable use and to indirect human exposures that might occur from nonpotable uses, although the possible exposure and thus the risk is significantly less for nonpotable reuse. While a vast body of knowledge exists about relatively uncontaminated, conventional source waters, there is still some uncertainty about the risks associated with impaired sources, principally related to the presence of synthetic organic chemicals, disinfection by-products, and some pathogenic organisms.
The challenge in considering the health risks from recharge systems is to assess and understand the relative risks and develop strategies for the use and operation of recharge systems to minimize those risks. A principal issue is the extent to which impaired-quality source waters require treatment prior to recharge. Another key issue, which is equally important with conventional water supplies, is the need to minimize not only the potential exposures to pathogenic microorganisms, but also the concentrations of possible disinfection by-products. The behavior and fate of microorganisms, disinfection by-products, and
other chemical toxicants in the ground water system will affect their concentrations at the point of recovery. Therefore, an understanding of the chemical and microbiological composition of the source water and the changes it can undergo in the complex underground environment is key to the optimal use of impaired-quality water for recharge.
For conventional and impaired-quality sources alike, disinfection by-products and their precursors are of potential concern when the water is intended to support potable uses. The nature and toxicity of disinfection by-products have been studied most widely for chlorine disinfection. The by-products of other disinfectants, such as ozone and chloramine, are not as well characterized. Also, the nature of the disinfection by-products and their precursors in disinfected wastewater is uncertain, as is their behavior in and their effects on ground water aquifers. Recent studies have indicated that disinfection by-products like trihalomethanes and haloacetic acids can be removed by biological degradation in aquifers. Reduction of disinfection by-product precursors during pretreatment will help reduce associated risks. In balancing the risks in using chemical disinfectants to reduce pathogenic microorganisms with those associated with the disinfection by-products formed in the process, it should be emphasized that effective disinfection is critical. The probability of mortality induced by improperly disinfected drinking water exceeds the carcinogenic risks posed by disinfection by-products associated with chlorine by as much as 1,000-fold (Bull et al., 1990).
The public health implications of nonpotable reuse, except where intended for market crops to be eaten raw, have not been addressed as extensively as the implications of potable reuse because nonpotable reuse has been practiced widely for decades without public concern and because the exposure from nonpotable reuse, and thus the risk, is limited.
The health implications of using reclaimed water for potable purposes have been investigated at a number of projects: for example, studies of direct potable reuse have been conducted in Denver; indirect potable reuse via surface sources has been studied in San Diego and Tampa; and indirect reuse via injection has been practiced and studied in Orange County, California. Such studies employ state-of-the-art methodologies to measure toxicological effects and determine the identifies of inorganic and organic chemical compounds. None of the studies found significant effects from chemical toxicants or infectious disease agents, although methodological limitations and the limited extent of testing prevent us from interpreting these results as showing with complete certainty that there are no health effects associated with human consumption of recharged water from impaired quality sources, especially over the long term. Thus, whale studies to date fail to show that professionally managed recharge projects produce extracted water of lower quality from a health perspective than water from historically acceptable sources, the methodologies used in such assessments have limitations and, accordingly, uncertainties remain. Also, although various treatment tech-
nologies are available, there is always some inherent uncertainty and risk of failure because of the possibility of human error or equipment malfunction.
The information available from on-site and laboratory studies do not indicate that the health risks from recovered water are greater than those from existing water supplies or that the concentrations of chemicals or microorganisms are likely to be higher than those established in drinking water standards set by the Environmental Protection Agency (EPA). There are uncertainties, however, such as limited chemical and toxicological characterizations of source waters, the absence of water quality standards for some of the chemicals, and the uncertain environmental fates of chemicals and microorganisms in the recharge system. Furthermore, it should be remembered that the EPA standards are based on water sampled from high quality sources. Such standards are often years behind current knowledge and current knowledge at any time is limited. (For example, health effects are determined for each organic compound separately, not for the inevitable mixtures of organics.) Accordingly, monitoring of potentially toxic constituents and pathogenic microorganisms should be required in using water extracted from recharge systems.
ECONOMIC, LEGAL, AND INSTITUTIONAL CONSIDERATIONS
The future of artificial recharge using waters of impaired quality will be crucially affected by the economic, legal, and institutional setting. Indeed, the institutional barriers may prove to be more problematic than the remaining technical constraints.
From an economic perspective, recharge with waters of impaired quality may be a more attractive option in the future both because of the increasing scarcity of new surface water sources and because of increasingly stringent wastewater discharge regulations. Although the economic feasibility of recharge varies from site to site, in general, recharge will be economically attractive whenever it is the least cost source of supplemental water. The cost of treatment over and above what is required to meet wastewater discharge standards will be particularly important. Cost is, of course, sensitive to the distance the recovered waters must be transported for spreading or injection and to the recharge techniques used. Economic feasibility also will depend on the benefits ultimately provided by the recovered water.
From both an economic and a legal perspective, the need to define rights to both source waters and recovered waters is paramount. Failure to define water rights clearly makes recharge with waters of impaired quality a far less attractive option than otherwise might be the case. From a strictly legal standpoint, the central question is how to formulate policy to protect public health and the environment, while not imposing inappropriate or unnecessarily burdensome controls on this potentially important form of water development. State laws governing recharge vary greatly. California is developing a comprehensive set of
laws and regulations related to recharge, but most states have not addressed this regulatory problem adequately. Although there are federal laws that govern certain aspects of the recharge process, the federal government has not exercised strong leadership in developing appropriate institutions to govern wastewater discharge and reuse.
Another potential constraint is public perception. Indeed, the importance of the attitudes of the public cannot be discounted because the public is ultimately the recipient of the recovered water and it ultimately, albeit often indirectly, bears the burden of the costs of such operations. People's attitudes about the reuse of water in general depend on the source and the intended purpose of the reuse; nonpotable reuse is relatively well accepted, but potable reuse and other high-contact uses are not favored because of perceived health risks and water quality problems. Where artificial recharge is planned, especially where impaired quality sources are used, early public involvement can help develop the public's understanding of the issues.
As demand for water increases, water managers and planners will need to look widely for ways to improve water management and augment water supplies. The Committee on Ground Water Recharge concludes that artificial recharge can be one option in an integrated strategy to optimize total water resource management, and it believes that with pretreatment, soil-aquifer treatment, and posttreatment as appropriate for the source and site, impaired-quality water can be used as a source for artificial recharge of ground water aquifers.
Artificial recharge using source waters of impaired quality is a sound option where recharge is intended to control saltwater intrusion, reduce land subsidence, maintain stream baseflows, or similar in-ground functions. It is particularly well-suited for nonpotable purposes, such as landscape irrigation, because health risks are minimal and public acceptance is high. Where the recharged water is to be used for potable purposes, the health risks and uncertainties are greater. In the past, the development of potable supplies has been guided by the principle that water supply should be taken from the most desirable source feasible, and the rationale for this dictate remains valid. Thus, although indirect potable reuse occurs throughout the nation and world wherever treated wastewater is discharged into a water course or underground and withdrawn downstream or downgradient for potable purposes, such sources are in general less desirable than using a higher quality source for potable purposes. However, when higher-quality, economically feasible sources are unavailable or insufficient, artificially recharged ground water may be an alternative for potable use.
The following recommendations emerged from the committee's deliberations; an expanded discussion of these points appears in chapter 7.
Artificial Recharge: A Viable Option
Artificial recharge of ground water using source waters of impaired quality can be a viable way to augment regional water supplies—primarily for nonpotable uses but sometimes for potable uses under appropriate conditions—and at the same time provide an avenue for wastewater management.
Once recharge has been deemed feasible as part of an integrated approach to regional water supply planning, the method of recharge chosen should be based on hydrogeologic conditions and the specific benefits sought from the recharge. In general, surface spreading offers the greatest engineering and operational advantages. Surface methods can accommodate waters of poorer quality and are simpler to design and operate than recharge wells, although certain conditions may require use of wells. Because surface spreading requires large amounts of land with permeable soil, it may not be feasible in densely populated areas or elsewhere where suitable land is expensive or unavailable. Injection wells require high quality source water to avoid clogging problems and also because aquifers alone do not provide the same degree of treatment as soil-aquifer systems. Although there are indications of some water quality improvements within aquifers, considerable pretreatment is necessary if the source water to be used in wells is of impaired quality.
Artificial recharge using water of impaired quality offers particularly significant potential for nonpotable uses. Nonpotable reuse can help reduce demand on limited fresh water sources at minimal health risk; it is widely practiced and achieves good public acceptance. Potable reuse is equally possible to engineer, but the health risks may be greater and public acceptance is less certain. In either approach, but especially where potable reuse is considered, careful pre-project study and planning is required.
Potential Impaired Quality Sources
Three main types of impaired quality waters are potentially available for ground water recharge—treated municipal. wastewater. stormwater runoff, and irrigation return flow. Of these, treated municipal wastewater is usually the most consistent in terms of quality and availability. Stormwater runoff from residential areas generally is of acceptable quality for most recharge operations, but at some times and places it may be heavily contaminated, and its availability is variable and unpredictable. Irrigation return flow exhibits wide variations in quality and is sometimes seriously contaminated, and thus usually is not a desirable source of water for recharge.
Based on current information, treated municipal wastewater intended as a
source for artificial recharge should receive at least secondary treatment. Municipal wastewater that has received only primary treatment may be adequate for the recharge of nonpotable ground water in certain areas, but use of primary effluent should not be considered without implementation of a site-specific demonstration study.
Certain impaired quality waters, such as irrigation return flow, stormwater runoff from industrial areas, and industrial wastewater, generally should not be regarded as suitable sources for artificial recharge. Exceptions might be identified, but only after careful characterization of source water quality on a case-by-case basis. Other types of stormwater runoff to avoid include most dry weather storm drainage flow, salt-laden snowmelt flow, and flow originating from certain commercial facilities, such as vehicle service areas. Construction site runoff also should be avoided to prevent clogging of recharge facilities with eroded soil and other debris.
Human Health Concerns
The principal concern with regard to artificial recharge using waters of impaired quality for potable purposes is the protection of human health. Several major studies employing state-of-the-art methods for organic analysis and toxicological testing show that well managed recharge projects produce recovered water of essentially the same quality from a health perspective as water from other acceptable sources. However, there are uncertainties in identifying potentially toxic constituents and pathogenic agents in the methodologies used in these studies, and thus potable reuse should be considered only when better quality sources are unavailable.
Disinfection of treated municipal wastewater prior to recharge should be managed so as to minimize the formation of disinfection by-products. Alternatives to chlorination include disinfection with ultraviolet radiation and the use of other chemical disinfectants. However, additional research should be undertaken on pathogen removal, formation of disinfection by-products, and removal of disinfection by-products before alternative disinfectants can be classified as conclusively superior to chlorine.
Recovered water must be monitored carefully to provide assurance that pathogenic microorganisms and toxic chemicals do not occur at concentrations that might exceed drinking water standards or other water quality parameters established specifically for reclaimed water which consider the nature of the source water. The outcomes of existing studies of potable use of recovered ground water recharged with treated municipal wastewater suggest that additional epidemiological, in-vivo, or short-term toxicological studies would be of marginal value. As long as the recovered water meets drinking water standards and other water quality limits specified for the site, and there is no evidence from
monitoring of constituents that pose undesirable health risks, additional toxicological testing is unnecessary. If the quality of the extracted water is uncertain for any reason, it should not be considered for potable reuse.
There are significant uncertainties associated with the transport and fate of viruses in recharged aquifers. These uncertainties make it difficult to determine the levels of risk of any infectious agents still contained in the disinfected wastewater. Thus, additional research should be undertaken on the transport and fate of viruses in recharged aquifers to allow improved assessments of the possible health risks and needs for post-extraction disinfection associated with such systems.
Artificial recharge of ground water with waters of impaired quality should be used to augment water supplies for potable uses only when better-quality sources are not available, subject to thorough consideration of health effects and depending on economic and practical considerations.
System Management and Monitoring
Protecting public health and the sustainability of soil-aquifer systems will require careful planning, operation, and management of recharge systems. Under appropriate conditions, the soil-aquifer system has the capacity to remove certain chemicals and pathogens and can therefore be an effective component in ground water recharge and water reuse systems. However, the processes through which removal occurs are not completely efficient in natural settings, and not all constituents are retained or degraded to the same extent. In addition, strategies that may enhance the removal of one chemical or pathogen can decrease the efficiency of removal of another.
Assessments of the feasibility of any recharge technology should include analyses of the possible impacts of the use of the system on the environment.
Monitoring of recharge water should be undertaken as it moves toward points of recovery. This is critical to help ensure that water quality is maintained, to provide early warning of unexpected problems, and to help maintain the long-term viability of the treatment system.
Artificial recharge opportunities need to be evaluated within the overall context of available water supplies, existing and projected water demands, and related costs and benefits to ensure that the opportunity is economically justified.
The price of recovered water should reflect the true cost of making the
water available to ensure that the water is used efficiently. The cost of recharge operations should not be subsidized to make this water source more attractive than it would otherwise be.
Legal and Institutional Considerations
The development of institutional arrangements governing artificial recharge is critical in determining the extent to which water supplies will ultimately be available from recharge with waters of impaired quality. Institutions need to be capable of formulating policies to protect public health and safety and environmental amenities while not imposing inappropriate or inefficient controls on this potentially important form of water resource management. Federal leadership will be needed if the full promise of artificial ground water recharge is to be realized.
As a first step in developing institutional arrangements that will foster artificial recharge as a means of augmenting water supplies, states should move to clarify the legal rights to source waters and recovered waters for artificial recharge operations.
In addition to ensuring the protection of public health related to the consumption of recovered water, when developing regulatory policies states should make explicit provision for the evaluation of project sustainability and environmental impacts of artificial recharge projects.
Regulatory processes should ensure that environmental impacts and other third party effects are adequately accounted for in the design and operation of artificial recharge projects.
The federal government should assume leadership in supporting the development of artificial recharge with treated municipal wastewater and other suitable impaired-quality water sources by providing technical assistance to the states and by developing model statutes and guidelines.
Bouwer, H. 1978. Ground Water Hydrology. New York: McCraw Hill. 480 pp.
Bull, R. J., C. Gerba, and R. R. Trussell. 1990. Evaluation of the health risks associated with disinfection. Critical Reviews in Environmental Control 20:77-113.