Introduction to Water Contamination

There is evidence of widespread contamination of water resources in many areas of our country. The Environmental Protection Agency's National Water Quality Inventory of 1994 has identified agriculture, urban runoff/storm water, and municipal point sources as the largest pollutant sources for rivers, lakes, and estuaries(5).  Contaminants from these sources include pesticides, metals, nitrates, solvents, and other wastes. More than 200 chemical constituents have been documented in groundwater alone (2). The health effects of long term exposure to many of the contaminants entering our water supply are unknown. But it should be noted that not all contamination events pose a threat to our health. The National Water Quality Inventory 1994 Report to Congress states that 40% of flowing river and stream miles can be used for drinking water after conventional water treatment, and 37% of lake and reservoir acres meet the designated use criteria for drinking water.

Pollutant concentrations become diluted when they enter water sources and are further reduced by biological degradation, filtration, and adsorption to soil. Some chemicals, such as the man-made chlorinated hydrocarbons, are very stable in the environment. Some of these compounds accumulate in living organisms and are not readily metabolized and excreted.

The impacts of contamination events to lakes and reservoirs are more severe and persistent than streams and rivers because there is not a natural flushing process as is characterized by the flow in streams and rivers.Contamination is even more persistent in groundwater due to lack of biological degradation. The most biologically active bacteria live within the soil above groundwater supplies.

In contrast to surface water there is not a lot of mixing when a contaminant enters groundwater. When a contaminant first enters the soil it will travel down vertically with gravity until contact with groundwater. At this point it will begin to flow primarily in a horizontal direction (3). The contaminant will then spread out three-dimensionally like smoke from a chimney and is called a plume. Groundwater does not exhibit turbulent flows as found in surface water. The flow is defined by gravity, pressure and friction. It is much more constant than surface water. An aquifer can flow at a fraction of an inch per day up to a few feet per day (2).

The most effective way of reducing contaminants in drinking water is by controlling it at the source. Preventative measures such as modification of tilling methods and run-off control in agriculture, the use of lined catchment ponds for treatment of wastes, and double lined underground storage tanks can greatly reduce contamination of drinking water sources. As consumers we can educate ourselves about the source of our tap water, contamination, and the options for treatment.


1. A Citizen's Handbook on Groundwater Protection. 1984. Wendy Gordon, Natural Resources Defense Council, Inc. New York, NY. pp.1

2. EPA Handbook. Ground Water Volume 1: Ground Water and Contamination. September 1990. USEPA, Office of Research and Development, Washington DC.

3. EPA. Index of Watershed Indicators. Rev. September 8, 1997. URL:

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This page was prepared by T.L. Pedersen, June 1997, UCD EXTOXNET FAQ Team.
Revised by B.T. Johnson, November 1997.