Also, the depletion of deeper aquifers may be permanent where the weight of overlying sediments causes the aquifer to compress as the water is pumped out. The aquifer would therefore never again be fully recharged even if pumping ceased, because its capacity to store water has been reduced. This occurs in areas of high water demand such as large municipalities and agricultural operations that require heavy irrigation. Sometimes groundwater contamination occurs naturally, but serious contamination is usually the result of human activities on the land surface. Each human activity has a particular impact on groundwater.
Some agricultural activities add nitrate nitrogen and pesticides to groundwater.
Residential areas with septic systems usually add nitrate nitrogen, bacteria, viruses, and synthetic organics used in household cleaning products and septic tank cleaners. Industrial activities tend to add organic chemicals and metals, though in widely varying amounts. Roadways contribute petroleum pollutants leaked from vehicles and metals from exhaust fumes. The most concentrated impact comes from older sanitary landfills, whose leachate may contain many different chemicals at relatively high concentrations. Except where contaminated water is injected directly into an aquifer, essentially all groundwater pollutants enter the aquifer through recharge water from the land surface.