The E. coli project in the Accotink Creek watershed pinpoints the sources of bacteria in surface water. Land use affects water quality. In the Potomac River Basin, about half of the land is covered by forests and other vegetation, about 35 percent of the land is used for agriculture, eight percent is urbanized, and five percent is covered by water. In forested areas of the Potomac River Basin, rain and snow infiltrate into the soil to become ground water. The cleansing action of water percolating through soil is one of the most important water purifiers.
Agriculture is the second largest use of land in the Potomac River watershed.
These areas receive the highest amounts of the nutrients nitrogen and phosphorus, from fertilizer applications, manure, and discharge from septic systems. Although nitrogen and phosphorus are essential for growth of plants and animals, in excess quantities they can adversely affect human health and the environment. In urbanized areas, much rain water is channelled into storm sewers and released into streams rather infiltrating soils and being cleaned.