Aquifer Storage and Recovery

INTRODUCTION

Aquifer storage and recovery (ASR) is a water resources management technique for actively storing water underground during wet periods for recovery when needed, usually during dry periods. The timeframe can range from months to decades. Intentional aquifer storage, with the intent of using the water later, has been used for hundreds of years, but is being further developed and refined as demand for fresh water threatens to exceed supply in California and many other parts of the world. "Conjunctive use" and "artificial recharge" are closely related water resource management practices, and the terms are sometimes used interchangeably. Conjunctive use is a combination of management practices intended to make the best use of surface water during wet periods and ground water during dry periods, but does not necessarily imply the active water storage activities used in ASR. Artificial recharge (AR) is focused on actively moving water from the surface into ground water systems. AR can be seen at as the storage part of aquifer storage and recovery. More than 100 ASR facilities are in operation worldwide. Many states (Arizona, California, Florida, Nevada, and Texas to name a few) have ASR sites ranging from pilot projects to full operations.

BACKGROUND FOR EVALUATING ASR

As California's population continues to grow, so will demands on California's water resources. Used in combination with other practices such as more efficient irrigation technologies, urban conservation, water recycling, and desalination, many water managers expect ASR to become an increasingly important tool for meeting future water demand.

Additional on-stream sites for large dams are scarce, and construction of new dams is increasingly controversial, primarily for environmental and economic reasons. Dammed surface water reservoirs have some problems that are generally not shared with underground water storage. These include: high construction costs, environmental effects, potential for catastrophic failure, evaporative loss of water, reservoir induced earthquakes, water eutrophication, reservoir sediment accumulation and downstream scouring, and conflicts among competing reservoir uses (water storage, flood control, recreation and hydroelectric power production). Siting and construction of off-stream surface reservoirs may be less controversial from some environmental perspectives because rivers may not be directly affected, but other issues remain. Potential problems with ASR are generally centered on the existence and availability of suitable aquifers and water quality. These are discussed below.

As population centers grow, some of the water resources historically used for irrigated agriculture shifts to urban uses. Changes in water use locations suggest that additional storage in and near urban areas may be needed. With limited space in urban settings, underground water storage through artificial recharge is an increasingly attractive option.

In California, ground water provides approximately 40 percent of the fresh water supply. Long term pumping rates in excess of recharge have adverse effects, such as reducing aquifer water pressures, lowering water tables, causing land subsidence and infrastructure damage, impairment of water quality and significantly increasing pumping costs. In some areas of the State (Mojave Desert, parts of the Central Valley, and many deeper aquifers) the ground water is old, dating back tens of thousands of years to the Pleistocene. Under present-day climatic conditions these aquifers usually are not recharging at appreciable rates. Pumping this water is similar to mining a non-renewable resource, a practice called "overdrafting." To control or even reverse the adverse effects of overdrafting, artificial recharge can be employed.

Although AR has been used for a long time, the development of ASR facilities with California's complex water management demands and practices requires comprehensive information on the physical and chemical characteristics of the recharged geologic formations and the quality of recharge water from multiple sources. In addition, ASR facilities must be integrated with local and regional water distribution systems to allow optimal use of available water resources, legal control of stored and recovered water needs to be established, and potential off-site effects should be identified and evaluated to avoid unintended consequences.

ASR SCIENTIFIC AND TECHNICAL ISSUES
Historically and currently, spreading basins are the primary technique used for artificial recharge. Ideally, basins are located in or adjacent to natural streams, have sand or gravel beds, and good hydrologic connection to a well-defined, high storage capacity aquifer. Ideal conditions are rare. Techniques continue to develop and evolve, enabling water managers to recharge water at higher rates in areas with geologic materials that do inhibit relatively rapid recharge. At the opposite end of the AR spectrum from spreading basins are aquifer injection wells that are designed to place recharge water directly into an aquifer. The same wells may be used for recovery. 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 meet drinking water standards prior to storage. 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 concentrations of THMs that exceed drinking water standards.

One of the most common water quality problems associated with AR 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 and mobilization of trace elements. If the potential for mineral precipitation is identified it can be sometimes 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 ASR processes. 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 water intercepted the nitrates, in some cases causing nitrate levels to exceed the EPA's maximum contaminant level. Knowledge of the presence and distribution of anthropogenic and natural contaminants in an AR project area is needed to avoid mobilization of contaminants.

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:
Physical, biological, and chemical clogging of infiltrating surfaces and injection wells with the resulting reduction in infiltration rates is perhaps the most obvious problem in artificial recharge systems. However, the unseen subsurface hydrogeologic features of the recharged geologic formations have considerably more influence on the ultimate success or failure of a project.

Surface infiltration systems require permeable soils and unsaturated zones to get water into the ground and to the 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, or worse.

Many coastal aquifers, in California and around the word, have been overdrafted for decades. One of the results has been a reversal of ground water flow, causing seawater to be drawn inland through the aquifer, making water in affected aquifers unsuitable for most uses.

In the Los Angeles Basin for example, AR is used to create a barrier to seawater intrusion. Because of the basin's shape, a deep basin with a low permeability and a relatively impermeable shallow structure (or "lip") facing toward the ocean, injection wells have been placed over the lip and very large amounts of reclaimed water are injected into the ground. The result is a freshwater mound that acts as a barrier between the ground-water basin and intruding seawater. Some of this water moves to the sea, but much is recovered within the basin. Without the effective placement of the injection wells, ground water in the basin would be contaminated by seawater.

A potential hazard that can occur from ASR/AR is liquefaction, caused by creating a very shallow water table in poorly consolidated geologic materials that is subsequently shaken by an earthquake of sufficient magnitude. San Francisco's Marina District was a well publicized example of liquefaction immediately following the 1989 Loma Prieta Earthquake, where structures were shaken off their foundations. 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 with loss of many lives. In California, earthquakes are an everyday occurrence and this is a significant risk.

OTHER AR ISSUES
Water for artificial recharge comes from many sources, including: perennial and intermittent streams, water imported through aqueducts and pipelines, storm runoff from urban, suburban and agricultural areas, irrigation districts, and drinking water 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 looses its identity and becomes aesthetically more acceptable to the general public.

A primary issue of importance for water managers 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, but recharge was not an original objective. Urbanization, with land covered with impermeable surfaces, produces more runoff and has less evapotranspiration than comparable un-urbanized areas. Urban runoff can be collected and stored in holding ponds for flood control or, increasingly, 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 these kinds 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 tanks.

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 more 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.

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