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Artificial Recharge Enhances Aquifer Capacity
In many areas of the United States, artificial recharge techniques are emerging as effective solutions for maintaining or enhancing the water-bearing capacity of an aquifer.
Under natural conditions, aquifers are primarily recharged by the infiltration of precipitation and, to a certain extent, through surface water bodies and adjoining aquifers. However, population growth and the accompanying land development in many parts of the country have decreased the amount of open area available for the natural infiltration process, and diverted surface run-off to areas that are "out of reach" of the aquifer. In some regions, seasonal fluctuations in water supply and demand compound the problem.
Eventually, pumping reaches the point where it is, in essence, "mining" water. This not only affects groundwater supplies, but can also affect its quality. With less fresh water coming in, there is less dilution potential. In some cases, wells may have to be drilled deeper into the aquifer to make up for this loss of recharge. Contact can then occur with natural or man-made contamination sources.
Under the right conditions, artificial recharge can be used in both unconsolidated aquifers as well as bedrock aquifers. There is a variety of artificial recharge techniques used, including surface applications, such as retention basins and off-stream surface water impoundments; modification of surface water routes; and injection wells.
Methods that depend on the infiltration of surface water into an aquifer generally work best with shallow aquifers. However, these techniques require large amounts of land. The acreage necessary depends on the aquifer's permeability and configuration as well as the volume of water targeted for recharge. These surface applications do have the advantage of requiring very little maintenance.
Two Systems Side-by-Side
In northern New Jersey, the consulting firm of Leggette, Brashears & Graham, Inc. (LBG) helped a client take advantage of the artificial recharge capacity of its wastewater disposal system by evaluating the potential increase in the associated yield for its on-site community water supply well. At this site, four acres of land are used to dispose of approximately 0.2 million gallons per day (mgd) of tertiary treated wastewater (i.e., drinking water quality) that infiltrates into the aquifer system. Approximately 1,000 feet away from these beds are two production wells, each of which produces about 400 gallons per minute (gpm) of potable water from the same aquifer, recharged in part by the nearby wastewater disposal system.
This combination system is in an area of the state that has received much scrutiny over the past few years due to declines in groundwater levels and an overstressing of its aquifer. It is an example of how two systems can operate side-by-side and provide a reliable supply of water.
Decades ago, the firm also completed a study of Long Island that demonstrated how overdevelopment and surface water disposal of storm water and wastewater (which would have normally recharged the area's aquifers) was resulting in significantly declining groundwater levels. As a result, artificial recharge through the construction of a system of stormwater retention basins was implemented.
A recent follow-up study in the same area has shown that the retention basin system is working very well at keeping up with the demand for groundwater. There is actually a rebound and groundwater levels in portions of Long Island are rising.
Aquifer Storage and Recovery
A specific application of artificial recharge, known as aquifer storage and recovery (ASR), is an innovative water management approach especially appropriate for those areas of the country that experience seasonal variations in water supply and demand. Florida, for example, which experiences most of its 54-in. average annual rainfall from June through September, is an ideal setting both for artificial recharge and ASR.
This technique involves the injection of water directly into the aquifer, via one or more wells, for storage and later use. Typically, the same wells will be used both for injection and recovery. One of two methods is used: capture of surface water run-off during the rainy season, with injection and storage until the dry season; or the injection of treated wastewater for re-use in irrigation.
Use of treated wastewater has several advantages. It is easier and less expensive to capture than surface water run-off, ASR provides a method for its disposal, and its reuse cuts down on the use of potable water for irrigation.
Although ASR using treated wastewater is more difficult to permit than using potable water, it is feasible if the treated wastewater can be stored in a non-potable aquifer (e.g., in a coastal aquifer that contains brackish water).
In Florida, there is little topographic relief, making it extremely difficult to build new surface reservoirs. This limits use of ASR and artificial recharge in this manner. However, ASR and surface methods of recharge are feasible if there are existing natural or man-made reservoirs. For example, a pilot study is now underway for the City of St. Petersburg to pump collected surface water/stormwater run-off and treated wastewater to the city's well field, and from there discharge it to surrounding wetlands and lakes. This method will recharge the aquifer by rehydrating these wetland and lakes, which are low due to recent drought, changes in drainage from development, and pumpage. The city of Tampa is also looking at several projects where surface water and treated wastewater will be used with ASR and aquifer recharge.
In Reno, Nevada, an ASR project was recently designed for a municipal water authority involving injection and storage of treated surface water during the winter for use during the peak summer period. The population of 175,000 is provided for by 75p;80 percent surface water and 20p;25 percent groundwater.
ASR was used to supplement the ground water source using three existing production wells as injection wells. In a water-rights state such as Nevada, only the maximum permitted amount can be pumped. A partial alternative might have been to drill other wells, but that would have involved considerable expense in siting and permitting. ASR is viewed as an efficient use of the water supply.
Artificial recharge is clearly becoming an important water supply management tool. As the need increases to capture more of the natural recharge, or to reuse treated wastewater in a beneficial manner, improved recharge technologies and a growing number of innovative aquifer storage and recovery installations undoubtedly will become common in the future.
About the Authors:
Frank Getchell is a senior associate and Dave Wiley is an associate with Leggette, Brashears & Graham, Inc., a consulting engineering firm based in Trumbull, Connecticut.