The Role of Forests in Water Purification

Two-thirds of U.S. clean water supply comes from precipitation that is filtered through forests and ends up in streams. Forests help prevent impurities from entering streams, lakes, and ground water in a number of ways. Root systems of trees and other plants keep soils porous and allow water to filter through various layers of soil before entering ground water. Through this process, toxins, nutrients, sediment, and other substances can be filtered from the water. Leaves and other debris on the forest floor play a role, too. Through the process of denitrification, for example, bacteria in wet forest soils convert nitrates—a nutrient that can lead to harmful algal blooms if too much of it enters bodies of water—into nitrogen gas, releasing it into the air instead of into local streams. How well a forest keeps nutrients out of water bodies is a function of several factors including the forest’s distance from streams and nutrient sources.

The water purification benefits of forests are economically valuable. Analysis conducted by the American Water Works Association and the Trust for Public Land concluded that drinking water treatment costs decrease as the amount of forest cover in the relevant watershed increases. They found that 50-55 percent of the variation in operating treatment costs could be explained by percentage of forest cover in the water source area.

According to the U.S. Environmental Protection Agency, NPS pollution from agriculture and urban and suburban development accounts for more than 60% of impairment in U.S. waterways, including many drinking water sources.