Artificial Recharge

Artificial recharge is the technique of capturing water that might otherwise go to waste, such as flood runoff or treated sewage effluent, and using it to replenish groundwater supplies by allowing it to infiltrate the soil or forcing it underground with recharge wells. Not only can this technique help to conserve drinking water resources, but it can also correct problems caused by excessive pumping, such as seawater invasion and land subsidence.

Natural Recharge

Groundwater is one of the world’s most vital resources. More than 20 percent of all freshwater used in the United States comes from groundwater sources, and demand has been steadily increasing over the years. Groundwater typically comes from rainfall and snowmelt, which infiltrates the soil and percolates slowly downward through a region of soil and rock known as the unsaturated or vadose zone. At some level below the surface, the small spaces in and between the soil and rock particles become completely filled with water. This area is the saturated zone, and the water it contains is by definition groundwater. The upper surface of the saturated zone is called the water table and is equivalent to the level of water in a well that might intersect it. When a water well is pumped, water is withdrawn from the soil and rock pores, and the level of the water table is lowered correspondingly.

Recharge of the groundwater occurs when more water from precipitation infiltrates the unsaturated zone. When this infiltrating water reaches the water table, the water table level will rise again. Natural recharge can also occur when a stream flows over permeable sands and gravels and water seeps downward through the stream bed. Streams that leak water into the ground in this way are called losing streams, and, for the most part, are found in arid climates. Many streams in humid climates actually gain water from the ground, and are accordingly referred to as gaining streams.

In many regions of the United States, the amount of rainfall is small enough that natural groundwater recharge occurs very slowly, even though groundwater may be abundant. In addition, humans often modify the land in such a way as to reduce natural recharge. For example, impervious coverings of the soil surface such as parking lots, streets, roof tops, and airports all prevent rainfall from infiltrating the ground. Small losing streams that might also have previously recharged the groundwater are frequently diverted into sewer systems and hastened out of an area. For a combination of these reasons, many cities that pump groundwater have experienced a severe and continuous drop in the level of the water table, which has not only threatened the water supply but also resulted in other undesirable side effects, such as subsidence (sinking) of the land surface or invasion of saltwater from the ocean into the fresh groundwater supply.

Advantages of Artificial Recharge

Artificial recharge is a technique used to increase the amount of surface water moving into the ground. Surface water is manipulated by some method of construction such as building pits or basins, the use of permeable materials for ground covering, spreading it on bare ground surface, or injecting it directly into the ground via recharge wells. In this manner, natural recharge is supplemented, and water that might otherwise go to waste (such as floodwater) can be stored underground for later use. The advantages are numerous. Storing water inside the earth rather than in traditional surface reservoirs means that little or no water will be wasted as a result of evaporation. There is also no wastage of land because of the flooding of a reservoir and no expensive dam to build that later may threaten to break catastrophically. Underground reservoirs do not eventually fill with sediment as do surface reservoirs, and they are less vulnerable to contamination.

Artificially recharged groundwater can be used not only to replenish water supplies and raise the water table but also to increase the pressure of the underground water enough to prevent seawater from migrating farther into the ground or even to flush the intruding seawater out of an area where pumping wells are located. In some areas, severe lowering of the water table (or the underground water pressure) through overpumping has caused the soil structure to compress irreversibly, resulting in a lowering, or subsidence, of the actual ground surface. This subsidence is usually gradual, but in some areas with limestone bedrock, sinkholes can collapse suddenly. In either case, ground subsidence can result in substantial damage to buildings or even flooding of the area. Artificially recharged water does not and cannot restore the ground surface to its former levels, but it can substantially reduce or even halt further subsidence. Another use of artificial recharge is for the storage of energy. The demand for heating or air-conditioning is seasonal. During the summer, surplus hot water can be stored underground through recharge wells. This water has a different viscosity (flow behavior) from that of the cold regional groundwater and thus mixes with it surprisingly little. Even after a storage period of up to three months underground, as much as three-fourths of the stored heat can be recovered.

Sewage Effluent

The water that is used to recharge groundwater artificially normally comes from excess storm runoff collected by various means. One of the most attractive aspects of artificial recharge, however, is that treated sewage effluent can be used as the water source. The effluent can be disposed of in this way, and if the operation is carefully designed and monitored, the wastewater becomes cleansed and purified during the recharge process as a result of the very nature of groundwater movement: The slow percolation of water through the tiny soil and rock pores allows the earth itself to act as a filter for the water. Movement of sewage effluent through the unsaturated zone can remove all of the bacteria and viruses, along with the solid matter and a large proportion of any undesirable chemical contaminants. Artificial recharge of sewage effluent must be carefully planned and carried out so that the water does not percolate too rapidly or reach the water table before most of the contaminants have filtered out; otherwise, the groundwater will become polluted.

Artificial recharge of groundwater has been widely practiced for more than two hundred years throughout the world for a variety of purposes, but there are some difficulties with the method. One of the biggest problems in the United States is that there are often separate laws dealing with surface water and groundwater, as well as separate governing bodies or “owners.” That makes the legal and economic aspects of the conjunctive use of surface water and groundwater extremely complicated. Artificial recharge may not be possible in some areas, or it may be too expensive in others. If it is not carried out properly, it can lead to groundwater contamination or other problems. In spite of these drawbacks, artificial recharge has been highly successful in restoring groundwater levels and reducing seawater intrusion and subsidence.

Recharge Pits

The most common technique used for artificial recharge involves the excavation of pits or basins to collect local storm runoff or diverted stream flow. Economically, old gravel pits can also be used. The pits or basins must intersect soils or layers of rock that have a high permeability, such as sands and gravels, and be located well above the water table. When a storm occurs, floodwater is diverted to a basin, or series of basins, paralleling a naturally losing stream channel. This water then fills the basin and, over a few days or weeks, gradually infiltrates through the permeable sand or gravel at the bottom of the basin and works its way downward to the water table. The water table then rises directly below the basin, forming a mound shape, which grows upward and spreads outward as recharge of the groundwater occurs. This “mound” formation is exactly analogous to the formation of a zone of depression in the water table at a location such as a well when water is removed. Both reflect the differential between the rate at which water is added to or removed from the water table and the rate at which water flows within the water table.

Recharge pits and basins require continual maintenance, as the infiltration rate decreases sharply with time. Fine-grained material (clay and silt) suspended in the floodwater settles to the bottom and works downward into the uppermost soil pores, clogging them, as do subsequent algae and bacteria growth. To reopen the soil pores, it is necessary to allow the basin to dry out from time to time. This causes the clays to dry and crack and allows the organic matter to decompose. Sometimes, it may even be necessary to till and scrape the basin floor. Clogging of the pit floor may not be problematic, however, if it is possible to construct the pit so that it is steep-sided and deep, as long as the pit’s walls are sufficiently permeable.

Since 1935, Long Island, New York, has used recharge basins to divert stormwater, which would otherwise run out to sea via sewers, to the groundwater. As of the twenty-first century, more than three thousand recharge basins that dispose of approximately 230,000 cubic meters (8,122,373 cubic feet) of water per day. These basins are unlined open pits about 3 meters (10 feet) deep and open to the underlying gravels. Their infiltration rate is high enough that almost all of the basins are dry within five days after a 2-centimeter (3/4 inch) rainfall.

Water Spreading

Another common artificial recharge technique is known as water spreading. Small losing tributary streams in a relatively flat area are modified in such a way that floodwaters are diverted and spread as a thin sheet over the land surface instead of racing down the stream bed. The floodwaters then infiltrate permeable soils and recharge the groundwater over an extensive area. The changes to the stream bed are inexpensive and generally involve the construction of low check dams bulldozed from river-bottom material and perhaps reinforced by vegetation, wire, or rocks. An added benefit of this technique is that by trapping floodwaters harmlessly in the upstream areas, urban areas that are located downstream are also better protected from flood damage. Some drawbacks to the technique are the large amounts of land required, ice buildup in winter, and the fact that the small dams are easily washed out by larger floods and must constantly be rebuilt. If the check dams were to be constructed of permanent materials, however, they might create a flood hazard upstream. Water spreading recharge techniques can be adapted to steeper terrains by constructing a series of shallow, flat-bottomed, closely spaced ditches or furrows near the losing stream channel. The exact configuration of the ditches (contoured, tree pattern, or trellis) can be adjusted to the local terrain, but the lowest ditches should flow back into the mainstream channel to avoid any overflow flood hazard. Such ditches are somewhat more costly to construct than are check dams. They also require a lot of land area and, like recharge basins, are subject to clogging.

Sometimes, it is possible to avoid the construction costs of ditches and check dams entirely by using preexisting irrigation canals for water spreading. In this case, cropland is irrigated by floodwaters during the dormant or winter season as well as the growing season. Care must be taken with this method to ensure that the constant leaching (removal by downward percolation) of salts and nutrients from the topsoil does not adversely affect the quality of the groundwater or the crops during the growing season. If treated sewage effluent is used as the recharge water in this technique, although agricultural land is usually spray-irrigated from a surface water source, nutrients are actually added to the soil. This not only benefits the crops but also cleanses the recharged effluent. The problem with this technique is that irrigation can overwhelm the natural filtering characteristics of the unsaturated zone and allow the groundwater to become contaminated.

Recharge Wells

The artificial recharge techniques already discussed are practical only in areas where permeable soils on rock beds allow direct infiltration from the surface to the water table. In many areas, the layers of the earth bearing the groundwater (aquifers) are deeply buried below impermeable geological materials, and there is no direct access to the surface. It is still possible, however, to recharge these layers artificially using recharge wells (sometimes called injection wells). Unlike pumping wells, in which water is pumped from the groundwater to the surface, recharge wells allow surface water to be forced under pressure into the ground and underlying permeable rocks. Large volumes of water can be stored in this way and later pumped out as the need arises. The recharged freshwater can also be used to force seawater, which may have invaded the ground and contaminated drinking water supplies, back out to sea. An additional benefit of this particular technique is that, unlike recharge basins or water spreading, it does not require much land area, so it is practical to use in urban areas.

Recharge wells are not without their drawbacks. Compared with other artificial recharge techniques, they are expensive to construct and require continual maintenance. Their main problem, similar to recharge basins, is well clogging. Any fine-grained sediment in the recharge water will enter the rock pores and clog them. Dissolved air, also very abundant in the recharge water, will enter the pore spaces as well and prevent water infiltration. Bacteria growing in the water will coat the well sides and rock pores with clogging growths, and chemical reactions between the recharge and groundwater will also cause clogging substances, such as rust and carbonate salts, to grow. For these reasons, the recharge water must be carefully treated before it is injected, and the clogged wells must be pumped frequently to restore some of their original permeability. Finally, recharge wells allow direct access of the surface water to the ground, bypassing the filtering characteristics of the unsaturated zone. That means that if any contaminant finds its way into the recharge water, it may rapidly enter the ground and contaminate drinking water supplies.

Recharge wells have been used successfully in Los Angeles, California, to prevent contamination of drinking water supplies by invading seawater. More than ninety wells have been placed in a line about 15 kilometers (9 miles) long and parallel to the coast. Filtered, chlorinated wastewater is injected into four underlying, water-bearing layers at an average rate of 1,500 cubic meters (52,972 cubic feet) per day per well. The wells create a ridge of pressurized water along the coast that separates the invading seawater from pumping wells farther inland. Water levels in the pumping wells can even be drawn below sea level with no danger of saltwater contamination.

Principal Terms

groundwater: the water contained in soil and rock pores or fractures below the water table

groundwater recharge: the water that infiltrates from the ground surface downward through soil and rock pores to the water table, causing its level to rise

losing stream: a stream that is located above the elevation of the water table and that loses water to the ground via infiltration through the stream bed; the opposite of a gaining stream

recharge well: a well created to pump surface water underground in order to recharge the groundwater; sometimes called an injection well

unsaturated zone: the area of the soil or rock between the land surface and the water table, in which voids between the soil and rock particles contain both air and moisture; also called the vadose zone

water spreading: an artificial recharge technique in which floodwaters are diverted from the stream channel and spread in a thin sheet over a flat land surface, allowing the water to infiltrate the ground

water table: the upper surface of the saturated zone or groundwater, below which all soil and rock pores are filled with water

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