Sources of groundwater pollution. Because it is mixed and circulated over a large area, groundwater is relatively clean, but the increased population and industrialization of the twentieth century has led to serious groundwater contamination problems in many parts of the country. Farming contaminants include pesticides, herbicides, animal waste, and manure. A variety of contaminants from city and county dumps such as heavy metals (mercury, lead, chromium, copper, cadmium, arsenic) and other industrial compounds enter the groundwater from rainwater that has percolated through the landfill. Wastes from septic tanks, sewage plants, and slaughterhouses may also contribute dangerous bacteria and parasites to the groundwater. Industries frequently use radioactive compounds, cyanide, polychlorinated biphenyls, and a degreaser called trichloroethylene that are being found in increasingly greater amounts in groundwater. Gasoline and other fuel derivatives such as xylene and benzene are carcinogens that frequently enter the groundwater from leaking storage tanks. Old mining sites contribute mercury, cyanide, and heavy metals to the groundwater; smoke from old smelters contaminated soils for hundreds of square kilometers with metals such as lead, arsenic, and cadmium, which also migrated into the groundwater.
Contamination identification and cleanup. Most compounds form a contamination plume in the groundwater that grows wider as it spreads outward from the point of contamination, called the point source. If the plume is flowing through sand, a portion of the contaminants are naturally filtered from the groundwater. Even though the plume widens downgradient, the concentration of the contaminants tends to decrease through filtering, dilution, or the natural breakdown of substances over time and distance called natural attenuation.
A contamination plume is identified by drilling monitoring wells and routinely sampling the water for contaminants. A series of monitoring wells studied over time reveals details about the direction of groundwater flow and the level of contamination. Once the point source is identified, cleanup work includes removing contaminated material and soil at the surface and treating the groundwater. Groundwater is typically pumped out of the ground through a system of wells, cleansed, and pumped back into the aquifer. This procedure can last for thirty years or longer.