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Leaching Process
When rain or irrigation water is absorbed into soils, it percolates down through multiple layers of topsoil, subsoil and bedrock. Percolating water dissolves or picks up soil pollutants, also called contaminants or leachates, carrying them to deeper soil layers. Where soils sit above impermeable bedrock, these waters and pollutants eventually filter into streams and other drainage systems. If polluted soil sits above an aquifer, percolating water delivers the leachates to the water table. Acid rain can also cause serious changes in soil chemistry and underlying water tables to the point that they become unsuitable for drinking or irrigation.
Soil Types
Loose, sandy soils are more porous and permeable to rain or irrigation water than are dense clays, which block rapid percolation. Organic soils, rich in humus, also tend to filter leachates better than infertile soils. Pollutants in looser soils tend to reach groundwater tables much more quickly and in larger quantities. Soil depth also plays a role. Deeper soils hold a greater volume of material, and can filter and break down pollutants more effectively. Shallow soils, generally those less than 36 inches deep, do not possess enough volume to adequately filter dissolved pollutants, especially when they rest on permeable bedrock and shallow water tables.
Types of Pollutants
Some of the main aquifer pollutants are nutrients and pesticides that reach groundwater from agricultural areas. Nitrogen is a main component of many fertilizers. It dissolves easily in water and when fertilizers are applied to agricultural fields in excess, they can eventually reach and contaminate water tables. Pesticides, especially those that are more water-soluble, follow the same route. Other contaminants include leaks or spills of chemicals and fuels from industrial areas. Landfills can also pollute water tables when pollutants are leached through wastes from soil surfaces and carried down to underground aquifers.
Aquifer Impacts
Water tables recharge and drain very slowly by natural processes over time. This means that once pollutants reach an aquifer, they can remain there for long periods. The Ogallala aquifer is a massive water table that underlies much of the American high plains. Large portions of the region rely on the Ogallala for drinking water and crop irrigation. Because of the area's geography, the aquifer is sensitive to degradation from soil pollution. The U.S. Environmental Protection Agency has determined that portions of the Ogallala aquifer have become so contaminated in recent years that the water is unsafe for drinking.