Once the nature of well contamination is understood, the protective measures required are more easily understood. Concentrated sources of pollution can overload the natural filtering capacity of the soil. Soil is nature's protection against groundwater contamination Most of the well contaminants that commonly cause concern originate above ground, often as the result of human activities. Disease causing bacteria and viruses, pesticides, fuels, and industrial chemicals are examples of pollutants that can contaminate a groundwater supply. Spills and careless storage or use of chemicals, and poor treatment and disposal of waste materials are often to blame.
Soil overlaying the water table provides the primary protection against groundwater pollution.
Bacteria, sediment, and other . Some chemicals are adsorbed or react chemically with various soil constituents, thereby preventing or slowing the migration of these pollutants into the groundwater. In addition, plants and soil microorganisms use some potential pollutants, such as nitrogen, as nutrients for growth thereby depleting the amount that reaches the groundwater. Just as any manufactured filtering device can be overloaded, so can the natural filtering capacity of soil. Figure 1 shows how large amounts of potential pollutants concentrated in a small area can cause localized groundwater contamination, depending on the depth and type of soil above the water table.