Groundwater is rainwater that has had a chance to be absorbed into the ground where it "percolates" through the ground layers. This natural cleansing process removes most of the pollutants before they enter streams. Porous soil layers, detention areas, and forested areas help absorb rainwater and greatly eliminates a lot of damaging stormwater runoff. When groundwater has a chance to percolate through the ground layers and eventually draining into the streams as a natural process, this provides a steady flow of "clean" water to streams.
Runoff is rainwater that runs over the landscape picking up any pollutants that may be on the ground surface and carries these pollutants directly into the nearest stream. Runoff occurs when (1) there is an increase in impervious cover (rooftops, driveways, parking lots, etc.), (2) when soil is compacted by heavy construction equipment, or (3) when land is cleared by removing plants and trees that would normally help absorb rainwater. When rainwater cannot be absorbed naturally into the ground layers, rainwater runoff enters the streams too quickly causing erosion and flooding conditions.