Recharge zone

The area in which water enters an aquifer . In a recharge zone surface water or precipitation percolate through relatively porous, unconsolidated, or fractured materials, such as sand, moraine deposits, or cracked basalt, that lie over a water bearing, or aquifer, formation. In some cases recharge occurs where the water bearing formation itself encounters the ground surface and precipitation or surface water seeps directly into the aquifer. Recharge zones most often lie in topographically elevated areas where the water table lies at some depth. Aquifer recharge can also occur locally where streams or lakes, especially temporary ponds, are fed by precipitation and lie above an aquifer. Karst sinkholes also frequently serve as recharge conduits. A recharge zone can extend hundreds of square miles, or it can occupy only a small area, depending upon geology, rainfall, and surface topography over the aquifer. Recharge rates in an aquifer depend upon the amount of local precipitation, the ability of surface deposits to allow water to filter through, and the rate at which water moves through the aquifer. Water moves through the porous rock of an aquifer sometimes a few centimeters a day and sometimes, as in karst limestone regions, many kilometers in a day. Surface water can enter an aquifer only as fast as water within the aquifer moves away from the recharge zone.

Because recharge zones are the water intake for extensive underground reservoirs, they can easily be a source of groundwater contamination. Agricultural pesticides and fertilizers are especially common groundwater pollutants. Applied year after year and washed downward by rainfall and irrigation water, agricultural chemicals frequently percolate into aquifers and then spread through the local groundwater system. Equally serious are contaminants leaching from solid waste dumps. Rainwater percolating through household waste picks up dozens of different organic and inorganic compounds, and contamination appears to continue a long time: pollutants have recently been found leaching from waste dumps left by the Romans almost 2,000 years ago. Perhaps most serious are petroleum products, including automobile oil, which Americans dump or bury in their back yards at the rate of 240 million gal (910 million l) per year, or 4.5 million gal (18 million l) each week. On a more industrial scale, inadequately sealed toxic waste and radioactive materials contaminate extensive areas of groundwater when they are deposited near recharge zones. Because recharge occurs in a vast range of geologic conditions, all these contamination sources present real threats to groundwater quality.

See also Hazardous waste; Radioactive waste

[Mary Ann Cunningham Ph.D. ]

RESOURCES

BOOKS

Fetter, C. W. Applied Hydrology. Columbus, OH: Charles E. Merrill, 1980.

Freeze, R. A., and J. A. Cherry. Groundwater. Englewood Cliffs, NJ: Prentice-Hall, 1979.