Run Off

Surface water that fails to seep into the ground. runoff occurs when natural absorbers of water, such as tree and shrub roots have been removed from a given location. runoff in suburban areas contains non-point source pollutants (gasoline, crank case oil, herbicides, pesticides, fertilizers, for example) that find their way into riparian systems. Often they drain into impoundments that are used for drinking purposes, raising serious issues of long-term health risks for humans, as well as for all the other living things that depend upon clean water.

Links
  http://wwwga.usgs.gov/edu/runoff.html
http://www.bafg.de/grdc.htm