Groundwater is critically important in supplying water to streams and wetlands, and in providing water for irrigation , manufacturing, and other uses. In the United States, 80 to 90 percent of available fresh water comes from groundwater. Dependence on this resource will continue to grow, particularly in areas where surface water is limited or has already been fully allocated for existing uses, including instream requirements (and particularly ecosystem needs).
Groundwater once was thought to be of unlimited quantity and naturally protected by the soils above it. It is, however, significantly vulnerable to overuse and the improper use and disposal of chemicals at the land surface. The proper use and protection of this resource requires an understanding of how the groundwater system works. In particular, water managers need to know what groundwater is, where it comes from, how it occurs in the subsurface, and how it moves below the ground.