Conclusions and Recommendations
As demand for water increases, water managers and planners 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 in 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occurs throughout the nation and world wherever treated wastewater is discharged into a water course or underground and withdrawn downstream or downgradient for potable purposes, such sources are in general less desirable than using a higher quality source for potable purposes.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 conclusions and recommendations emerged from the committee's deliberations:
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 purposes but for potable purposes under appropriate conditions—and at the same time provides an avenue for wastewater management.
Artificial recharge with waters of impaired quality has been practiced successfully in various pans of the United States and elsewhere for many years. Source water options include treated municipal wastewater, stormwater runoff, and irrigation return flow. Treated municipal wastewater and stormwater runoff are the two most commonly used sources; experience with the intentional use of irrigation return flow is scarce and not well documented. Recharge can be accomplished either through surface infiltration methods or through injection directly into the aquifer by wells. Hydrogeologic conditions, land availability, and the purpose of the recharge dictate the method of recharge, which in turn dictates the required pre-recharge treatment of the source water. Surface infiltration methods are used far more frequently than wells because of economic and operational considerations. However, wen recharge is increasing because suitable sites for surface infiltration are not always available.
Ground water recovered from aquifers recharged with waters of impaired quality has been used for various purposes, ranging from landscape irrigation to potable supply. The desirability of using such waters for various purposes depends on the quality, availability, and cost of alternative sources of supply and varies considerably by site and source. One advantage of nonpotable reuse is that it releases other, higher quality sources for potable use.
A fundamental conclusion of this report is that impaired quality waters used to recharge ground water aquifers must receive a sufficiently high degree of pretreatment (prior to recharge) to minimize the extent of any degradation of ground water quality, as well as to minimize the need for any extensive post-treatment at the point of recovery. With surface infiltration systems, considerable quality improvements can be obtained as the water flows through the unsaturated zone to the aquifer, this soil-aquifer treatment (SAT) reduces pretreatment requirements.
Although some impacts of artificial recharge of ground water with source waters of impaired quality are not understood with complete certainty, experience with recharge projects has failed to show (within the limitations of toxicological testing) that water recovered from the aquifer poses greater health risks than currently acceptable potable water supplies. The state of our knowledge
about artificial recharge using waters of impaired quality is more than sufficient to indicate that this technology offers particularly significant potential for all nonpotable uses. With proper pretreatment and posttreatment or dilution with native ground water, potable use also can be a viable option. These statements must, of course, be qualified by the fact that conditions vary from site to site, and thus the appropriateness of all recharge is site-specific. In particular, the quality of source waters, the hydrogeological setting, the costs of recharge facilities, and the availability and costs of alternative sources of supply will differ from situation to situation, and these factors must be considered when evaluating the feasibility of a specific artificial recharge project.
Once artificial 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 waters 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 con-
taminated, 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.
Treated municipal wastewater is by far the most consistent impaired-quality water source, both spatially and temporally and in terms of quality and quantity. One exception to this generalization is municipal wastewater and stormwater commingled in a combined wastewater collection system, and another occurs when industrial wastewater is discharged to the municipal wastewater collection system. The quality of treated municipal wastewater has been characterized for various levels of treatment to meet regulations pertaining to the disposal of sewage effluent and to allow use of the effluent for recharge and other purposes. The characterization of the quality of stormwater runoff and irrigation return flow is far less comprehensive because general assessments of stormwater and irrigation flow quality must be drawn from a much less systematic and comprehensive database than is available for treated municipal wastewater.
Attempts to use impaired-quality water sources for artificial recharge should be conservative. For this reason, the choice of source water and the degree of treatment necessary for the intended use are critical. Although soil-aquifer treatment improves water quality, the precise level of soil-aquifer treatment achieved often is unpredictable and very difficult to monitor, suggesting that the most reasonable course is to require the best possible source water and use impaired-quality sources only in appropriate circumstances. Municipal wastewater that has undergone at least secondary treatment provides a widely available source water that contains levels of many contaminants within the treatment and removal capability of well-designed and well-managed soil-aquifer treatment systems.
Based on current information, municipal wastewater used 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, industrial wastewater, and stormwater runoff from industrial areas, 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 flows, salt-laden snowmelt flows, and flows 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 only be considered when better quality sources are unavailable.
Recharge projects in the United States and elsewhere have provided analytical data on the chemicals and microorganisms found in treated municipal waste-water before and after recharge and soil-aquifer treatment. The concentrations of these constituents are highly variable and are dependent on the source of water and the specific sites involved. Although the database is not large, the available information does provide a basis for a limited assessment of the potential adverse health impacts when the recovered water is used as a potable supply.
All methodologies have inherent limitations, but on the basis of available information there is no indication that the health risks from water recovered after recharge of treated municipal wastewater are greater than those from existing water supplies, or that the concentrations of chemicals or microorganisms are higher than those established in drinking water standards by the Environmental Protection Agency (EPA). Uncertainties exist, however, where data are not available.
Drinking water from ground water recharge is regulated by EPA in the same fashion as drinking water from other ground water sources. Comparison with existing drinking water standards is one common and convenient approach for evaluating the quality of the recovered water. Other criteria to estimate the health risks from extracted ground water, such as health advisories developed by EPA, are useful. Artificially recharged ground water used for potable supplies need not be subject to stricter water quality requirements than conventional water supplies; however, as stated elsewhere in this report, water quality monitoring and operations management should be more stringent for recharge systems intended for potable reuse.
Assessing the risk to an individual from pathogens in ground water that has been recharged with impaired-quality water is difficult. Bacteria and parasites generally are removed to a greater extent than are enteric viruses during infiltra-
tion through soils; thus viruses may be of greater concern when there is human exposure to recovered water.
Disinfection by-products (DBPs) are of concern in artificially recharged ground water systems used for potable water, as they are in water supplies drawn from surface or naturally recharged ground waters. The nature and toxicity of such DBPs have been most widely studied for chlorine disinfection of potable water supplies. However, the possible differences in the nature and quantifies of DBPs resulting from the disinfection of impaired-quality waters that may be used for ground water recharge have not been studied thoroughly.
A key issue in developing any potable water supply, including ground water recharge systems, is the need to balance the risks in using chemical disinfectants to reduce the number of pathogenic microorganisms with those associated with the DBPs formed in the process. As a crude comparison, it has been estimated that the probability of mortality from pathogenic microorganisms in improperly disinfected drinking water would exceed the carcinogenic risks introduced by chlorine as much as 1000-fold. Chlorination has been the most widely used disinfectant of highly treated municipal wastewater for ground water recharge and other uses, but other disinfection processes, including the use of ultraviolet radiation, are increasingly being assessed. Although the mix of DBPs formed from the use of these other disinfection processes requires more study, these alternatives may be more efficacious than chlorination in minimizing the health risks from ground water recharge due to pathogenic microorganisms and DBPs.
Although the health risks associated with potable use of recovered water are likely to be minimal and may be mitigated by sound design and operation of treatment facilities, this committee believes that the best available water sources should be used for potable purposes whenever possible in preference to ground water recharged with impaired-quality source water. Under conditions of increasing water scarcity, however, economic and practical considerations may dictate the use of lesser-quality source waters in recharge of ground water that ultimately serves potable purposes.
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 and formation 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 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.
The protection of both human health and the environment are goals in any recharge system, and both require careful attention to system management and monitoring. The use of recharge technologies may have impacts on the environment, and the presence of these impacts as well as their magnitude will vary from situation to situation. The careful operation of wastewater treatment systems for ground water recharge has shown that the use of various processes can reduce the concentrations of nitrogen, phosphorus, heavy metals, organic chemicals, suspended solids, and pathogenic microorganisms in the effluent used for recharge. However, even when treated to a high degree, effluent disinfected with high chlorine doses may contain disinfection by-products (DBPs). If the
water is to be used for drinking, it is critical that the formation of DBPs be minimized by focusing on the nature and location of the disinfection process and by ensuring the optimal combination of pre-and post-disinfection.
The long-term viability of any soil-aquifer treatment system will depend on the specific nature of the source water and its treatment, the soil, and the receiving aquifer. Some challenges to sustainability, such as clogging caused by suspended material and biological activity, can be managed. Others, such as the attenuation of viruses and other pathogens and the accumulation of metals, phosphorus, organic compounds, and other constituents, may be more problematical. Thus, monitoring of the recharge system is needed to evaluate its long-term behavior and to formulate appropriate actions when needed. Of the two methods of artificial recharge—surface infiltration and well recharge—the latter requires source water of much higher quality.
Artificial recharge is an established technology, and while there is always room for research and improvement in areas such as how to optimize the process, minimize costs, and maximize safety, the greatest remaining uncertainties relate to the potential implications for human health. In particular, research into the fate and transport of chemicals in recharge waters, removal mechanisms for organic constituents in ground water, and use of alternative disinfection techniques offer potential. There is also a need for efforts to synthesize existing performance data.
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 wanting 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.
Artificial recharge is but one option for augmenting water supplies for potable and nonpotable uses. Most communities and regions will have several such options, and it will be important to evaluate all options on the same basis if the least costly option is to be identified. The cost of otherwise disposing of wastewaters is one factor to be taken into account. The benefits associated with
the ultimate uses to which recharge water is put should be equal to or greater than this cost. The temptation to subsidize recharge operations in order to ensure that the water will be used or to make its price more attractive should be resisted. Subsidies result in distorted water allocation and inefficiencies in use.
The price of recovered water should reflect the true cost of making the water available to ensure that the water is used efficiently. The costs of recharge operations should not be subsidized to make this water source more attractive than it would otherwise be.
LEGAL AND INSTITUTIONAL CONSIDERATION
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. The institutions need to be capable of formulating policies to protect public health and environmental amenities while not imposing inappropriate or inefficient controls on this potentially important form of water resource management. Federal leadership is needed if the full promise of artificial ground water recharge is to be realized.
The feasibility of artificial recharge could be improved by appropriate institutional arrangements to govern and regulate the recharge of impaired quality water and the uses of recovered water. If the regulatory process is itself uncertain, there will be general underinvestment in recharge facilities. Underinvestment will also result from processes that are too conservative. Processes that fail to account adequately for public health and welfare concerns and impacts on the environment are also likely to result in the imposition of unnecessary social costs.
Artificial recharge of ground water will clearly be impeded where title to the recovered water is unclear or undefined. Similarly, clear legal rights to source waters will be required if recharge is to occur. Although institutional arrangements may need to be flexible and may vary from situation to situation, a significant first step to establishing an institutional environment that will foster recharge will be to ensure that these rights are clearly defined. This task will virtually always fall to the states.
Interests in environmental amenities and the benefits they provide are widely distributed and diffuse. Often, no one individual or group has enough stake in the preservation of environmental amenities to permit or motivate it to intervene in the regulatory process. The National Environmental Policy Act applies only to federal activities, and not all states have comparable statutes. Inasmuch as
artificial recharge operations can have significant environmental impacts, it is important that regulatory processes be designed so as to ensure that there is adequate assessment, evaluation, and review of the environmental impacts of artificial recharge projects.
In the absence of federal leadership, states will be compelled to develop standards and regulatory schemes independently. Although the needs, economies, and opportunities of states vary widely and thus state-level leadership and flexibility of approach is important, many states lack the resources to develop adequate regulation and guidance. Moreover, the development of 50 independent sets of standards is inefficient and is unlikely to result in optimal development of artificial recharge operations nationwide. The federal government has the capability to provide technical assistance to the states and help develop model statutes and guidelines to assist the states in developing their own policies. As more experience is gained with the regulatory aspects of recharge, the partition of responsibilities between the national and state governments should be reexamined.
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 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 municipal wastewater and other suitable impaired-quality water sources by providing technical assistance to the states and by developing model statutes and guidelines.