What is groundwater?
Beneath the earth's surface, water is present in underground areas called aquifers. Surface water runoff from rain and snow melt replenishes (or recharges) groundwater by percolating through the soil to the aquifer. Surface water that is contaminated may contaminate groundwater.
Groundwater can emerge as springs on hillsides or seeps in the bottom of streams, lakes, wetlands, and oceans. Groundwater affects not only the quantity of surface water, but also its quality because polluted groundwater can contaminate surface waters.
A study by the United States Geological Survey found that groundwater supplies 52% of all stream flow in the country - that is, over half of all surface water is replenished by groundwater.