Aquifer Storage and Recovery

INTRODUCTION
Aquifer storage and recovery (ASR) is a water resources management technique for actively storing water underground for recovery and use when needed. "Conjunctive use" and "artificial recharge" are closely related management practices, and the terms are sometimes used interchangeably. Conjunctive use is a combination of practices to make the best use of surface water during wet periods and ground water during dry periods, but does not necessarily imply active water storage practices used in ASR. Artificial recharge (AR) is actively moving water into ground-water systems. AR can be seen as the storage part of aquifer storage and recovery.

As California’s population continues to grow, so too will demands on California’s water resources. Used in combination with other practices including more efficient irrigation technologies, urban conservation, water recycling and desalination, ASR will become increasingly important.

Few large dams are likely to be built in the foreseeable future. Ground-water storage is often more attractive than surface-water reservoirs in terms of construction costs, environmental effects, evaporative loss of water, water eutrophication, reservoir induced earthquakes, potential for catastrophic failure, and proximity to users. Potential problems with ASR include aquifer characteristics, water quality, required infrastructure, and unanticipated off-site effects. These are discussed below.

Ground-water pumping in many of California’s aquifers has exceeded aquifer recharge for many decades. This pattern, often called “overdrafting,” lowers water tables, causes land subsidence and infrastructure damage, impairs water quality, increases pumping costs, and, in coastal aquifers, induces sea water intrusion. ASR can be used to mitigate the effects of overdrafting.

ASR SCIENTIFIC AND TECHNICAL ISSUES
The development of ASR programs requires comprehensive information on the hydrogeologic characteristics of recharged aquifers, and of existing ground-water quality and recharge water quality from multiple sources

Historically and currently, spreading basins are the primary facilities used for artificial recharge. Ideally, basins are located in or adjacent to natural streams, have sand or gravel beds, and good hydrologic connections to well-defined, high storage capacity aquifers. Ideal conditions are rare. Recharge techniques continue to develop and evolve, enabling water managers to recharge water at higher rates in areas with geologic materials that often inhibit relatively rapid recharge.

Spreading basins are conceptually simple and can be operated with conventional equipment, but may not be adaptable to many locations where ASR is needed. At the opposite end of the AR technology spectrum are injection wells that require specialized knowledge of the receiving aquifers and an understanding of geochemical conditions and processes. In general, water quality requirements are highest for aquifer injection.

WATER QUALITY
The quality of water used for ASR purposes should be consistent with existing and anticipated ground-water uses. This can mean that stored water must often meet drinking-water standards prior to recharge. The U.S. Environmental Protection Agency (EPA) sets maximum contaminant levels for trace elements, different types of organic carbon, microbial (biological) contaminants, trihalomethanes (THMs), and many other potential contaminants to ensure that the water is safe for human consumption.

THMs are disinfection by-products formed by the reaction of dissolved organic carbon in water that has been chlorinated to meet microbial drinking-water standards. Water may also be chlorinated prior to injection to control "biofouling" or plugging of wells by bacterial growth. The injection of treated surface water has resulted in the recovery of water with THM concentrations that exceed drinking-water standards because of continued reaction between dissolved organic carbon and free chlorine in the recharged water.

One of the most common water quality problems associated with ASR projects is elevated concentrations of dissolved solids, or salts. The major soluble cations (calcium, magnesium, and sodium) and anions (sulfate, chloride, and bicarbonate) are often higher in recharge water than in native ground water. This is usually not a health issue, but changes in taste, scaling in household appliances, and "hardness" may cause complaints from water users.

Reactions between ground water and recharge water can create other problems such as mineral precipitation or mobilization of trace elements. If the potential for mineral precipitation is identified, it can sometimes be avoided by adjusting pH or other properties of the recharge water. Study of the aquifer system matrix materials and water can identify trace elements or other contaminants that might be mobilized by AR. In Yucca Valley, California, a potential source of nitrate contamination of an aquifer was shown to occur from septic tank seepage. Seepage can cause high nitrate levels in the unsaturated soils between the septic systems and the water table. When AR was used in the Yucca Valley ground-water basin, rising ground water intercepted the nitrates, in some cases causing nitrate concentrations to exceed the EPA’s maximum contaminant level. Knowledge of the presence and distribution of anthropogenic and natural contaminants in an ASR project area is needed to anticipate and manage contaminate mobilization.

Although spreading basins are less prone to serious plugging than injection wells, recharge water should be of an adequate quality to avoid clogging the infiltrating surface. Clogging can be caused by precipitation of minerals on and in the soil, entrapment of gases in the soil, formation of biofilms and biomass on and in the soil, and by deposition and accumulation of suspended algae and sediment. Pretreatment of the water can greatly reduce suspended solids and nutrients, but the infiltrating surfaces usually require periodic cleaning to maintain infiltration rates.

PHYSICAL CHARACTERISTICS
Clogging of infiltrating surfaces and injection wells is perhaps the most obvious problem in artificial recharge operations. However, hydrogeologic characteristics of recharged geologic formations may have considerably more influence on a project’s success.

Surface infiltration systems require permeable soils and unsaturated zones to move water into an aquifer. Aquifers recharged from infiltration basins must be unconfined and have sufficient transmissivity to allow lateral flow of the water away from the infiltration sites to prevent excessive ground water mounding. Soils, unsaturated zones, and aquifers should be free of significant contamination. Locations that do not have sufficiently permeable soils and/or available land area may be able to recharge ground water through vertical infiltration systems (trenches, ditches, wells) in the unsaturated zone. For direct injection through wells, water is pumped or gravity-fed into confined and unconfined aquifers.

The presence of permeable aquifer materials is important, but clay lenses, faults, and other features that can significantly retard the movement of recharged ground water can render a seemingly straightforward ASR project only marginally effective at best.

Many coastal aquifers with hydrologic connections to the sea have been overdrafted for decades, resulting in a reversal of the ground-water gradient. This causes salt water to flow inland, and water in the affected aquifers can become unsuitable for most uses.

For example, good quality ground water in the Los Angeles Basin is hundreds of feet deep, extending well below sea level. Under predevelopment conditions, ground water moved through aquifers that discharged under the ocean. Overdrafting has reversed the ground-water gradient, drawing seawater into the aquifers. Although there are large-scale artificial recharge operations along the San Gabriel and Santa Ana Rivers, and numerous other projects as well, these AR activities are not enough to reestablish the ground-water gradient toward the sea.

To minimize seawater intrusion, a series of freshwater barrier mounds have been created along the coast in both Los Angeles and Orange Counties. These mounds were created and are maintained using closely spaced wells to inject large volumes of freshwater into the aquifers that have hydrologic connections with seawater. The effectiveness of these barriers is based on an understanding of the basins’ hydrogeologic characteristics, an understanding that continues to be developed as ground water management becomes ever more refined. Key factors include the location and hydrologic characteristics of the basins’ faults and fault blocks, and the location and behavior of the aquifers and interlayered silts and clays. Ironically, normal aquifer recharge practices would seek to avoid maintaining recharge mounds, but in this situation the mounds are essential for keeping seawater out of the ground water system.

OTHER ISSUES
Throughout much of California, ASR/AR management practices are often implemented where water levels have been lowered significantly. In some of these areas, there may be surface subsidence and associated damage to structures including canals, pipelines, highways and bridges, buildings, and leveled fields. The maximum reported subsidence caused by ground water pumping in California is on the order of 30 feet in the western San Joaquin Valley. Subsidence in the range of 1 to 6 feet is common and almost all is irreversible.

The cause of subsidence is reduced aquifer hydrostatic water pressures with distortion, collapse, and compression of the finer grained silts and clays as water is pumped from an aquifer’s interlayered coarser grained sands and gravels. AR and/or greatly reduced pumping rates can be used to stop or slow subsidence, but if subsequent pumping reduces hydrostatic pressures to previous levels, surface subsidence will resume. Similar issues may emerge if water levels in aquifers that previously have not been subject to overdraft pumping are lowered in anticipation of future artificial recharge, a management technique sometimes referred to as “take-and-put.” More commonly used in ASR are “put-and-take” practices.

A potential hazard that can occur from ASR/AR management activities is liquefaction, caused by creating a very shallow water table in poorly consolidated geologic materials that are subsequently shaken by an earthquake of sufficient magnitude. San Francisco's Marina District, where structures were shaken off their foundations during the 1989 Loma Prieta Earthquake, was a well-publicized example of liquefaction. Such areas are often popular building sites because they tend to be fairly level and may have readily available ground-water supplies. If AR is used for recharge without sufficient understanding of the hydrogeologic conditions and near surface saturation occurs, an earthquake of sufficient magnitude can destabilize foundations and destroy buildings, and cause the loss of lives. In California, earthquakes are an everyday occurrence and liquefaction can be a significant risk.

Water for artificial recharge comes from many sources, including streams, imported water, storm runoff, and wastewater treatment plants. Reclaimed water is becoming an important resource that can be processed to meet or exceed standards and in some instances is the highest quality water available for artificial recharge. Through treatment and AR, reclaimed water can loose its identity and becomes aesthetically more acceptable.

An issue of primary importance is water supply reliability. The relationship between using ASR with related management strategies, and increased effective total water supply, has been a theme of this overview. Another aspect of reliability is the physical proximity of stored water to users of that water.

In southern California and many other urbanized areas, there is a heavy dependence on aqueducts hundreds of miles long to maintain water supplies. Aqueducts and their support facilities are subject to damage and potentially extended periods of service interruptions by natural hazards such as earthquakes, landslides, and even floods. They are also potential terrorist targets. The extensive use of ASR in urban areas can mitigate the effects of interrupted water import capacity by increasing the volume of water stored near users.

In addition to intensively managed artificial recharge programs there are a number of land use practices that can increase water recharge.

Enhanced recharge through vegetation management: One of the primary mechanisms that transports water from soils to the atmosphere is plant use, or transpiration. Replacement of deep-rooted vegetation, like trees, with plants with shallow root systems can increase recharge rates. There may well be unintended consequences such as habitat destruction, increased surface water temperatures, and sedimentation of steams and reservoirs.

Induced recharge: The creation of water gradients to induce water movement from streams to adjacent ground-water systems is a common result of ground-water pumping. This may be a deliberate management technique or an unintended consequence of pumping. It is sometimes used to “pretreat” water as it moves through stream bank and channel bottom sediments before recovery and treatment to use in public water supplies.

Incidental recharge: Surface-water management may result in additional recharged water when recharge was not an original objective. Urbanization, with land covered with impermeable surfaces, produces more runoff and has less evapotranspiration (the combined evaporation of water from the soil surface and transpiration from plants) than comparable un-urbanized areas. Urban runoff can be collected and stored in holding ponds for flood control to help meet Total Maximum Daily Load (TMDL) requirements in streams. There are inherent conflicts in the management of storm runoff water. For some managers there is a need to retain “first flush” waters with relatively high contaminant levels to meet water quality standards in receiving streams. Others want to have the “first flush” discharged to allow the capture of subsequent cleaner water for artificial recharge operations. Resolution of competing objectives is an ongoing process. Other activities contributing to incidental recharge include deep percolation of irrigation water (to prevent salt accumulation in the root zone), and wastewater discharge from septic systems.

CONCLUSIONS
Aquifer Storage and Recovery (ASR), Artificial Recharge (AR) and related water management practices are evolving rapidly to help meet present and future demands for high quality water. There is great potential for ASR, used in conjunction with other water management techniques, to make more efficient use of existing water resources and to reuse more water now discarded after a single use.

To be effective, increasingly intensive management of water resources requires a greater knowledge and understanding of the hydrologic and geologic characteristics of formations used for water storage. Much of the water used in ASR operations will be used for public water supply. Meeting drinking-water standards and the aesthetic expectations of water users requires that water managers evaluate both the quality of recharge waters and the contaminant conditions of the receiving aquifers.

By: Walter Swain