San Antonio, Texas, with 1.2 million residents, claims to be the largest city that depends on groundwater alone for domestic water use. Any product, consumed in large enough quantities and soluble in water, is capable of moving into the groundwater. It's found underground The idea of groundwater conjures up images of underground lakes and rivers. In fact, groundwater flows very differently than does surface water. Groundwater can move only a fraction of an inch to a few feet per day through zones called aquifers.
Aquifers act like giant filters, made up of sand, gravel and rock.
The rate at which groundwater travels depends on the medium through which it moves. Aquifers can cover a few miles or spread out over hundreds of thousands of square miles. There are 20 major aquifers in the U.S., each with its own unique geologic characteristics. Since groundwater moves so slowly, any contaminants that filter down also move along at a slow rate, creating a pollution plume. Hydrogeologists have learned to track groundwater pollution plumes using geology maps and monitoring wells.