In general, a release of potentially hazardous material that can or has reached usable groundwater. A mill tailings pile is an example of a "groundwater discharge" because the liquid and solid wastes in the pile can release contaminants that can be transported into the underlying rocks that contain groundwater.
The removal of water from the saturated zone is called groundwater discharge. The discharge area is simply the geographic area in which discharge occurs.
Ground water entering coastal waters, which has been contaminated by land-fill leachate, deep well injection of hazardous wastes and septic tanks.