The severity of an occurrence is also related to the ability of the soil and groundwater system to degrade or dilute the contaminants and the degree to which the contamination will interfere with uses of the water. Contamination is usually more serious in a drinking water supply than in water for other uses. Except where contaminated water is injected directly into an aquifer, essentially all groundwater pollutants enter the aquifer through recharge water from the land surface. These are considered harmful if ingested with drinking water and may be carried into surface water bodies by groundwater. 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. Protecting our groundwater from contamination will require thoughtful management and cooperation on the part of citizens and the various levels of government. In many cases, land-use planning is the best instrument available for protecting aquifers still containing good quality water.