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 forests distance from streams and nutrientsources.
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 sourcearea.
