Hydrologic conditions affect the quality of streams and ground water in the Potomac River Basin. Fertilizers, pesticides, and other chemicals that are applied to the land surface may infiltrate to ground water or run off to streams during rainfall depending on the season, soil properties, and the intensity and duration of precipitation. During dry periods, flow in most streams of the Potomac River Basin is sustained by discharge from ground water (fig. 7) and streamwater quality is similar to that of ground water. When flows are elevated during storms, streams of the basin typically become concentrated in chemicals that are washed from the land (fig. 8), although streams in some areas may become diluted.