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. Sinkholes are a special case as they can provide a direct connection to the aquifer and allow contaminants in surface runoff to move straight to the groundwater. Since groundwater moves slowly, many years may pass before a pollutant released on the land surface above the aquifer is detected in water taken from the aquifer some distance away. Unfortunately, this means that contamination is often widespread before being detected.