Mercer County is home to over 1,848 miles of streams which are tributary to the Ohio River. The county is drained by two tributary basins of the Ohio River. French Creek and Sandy Creek flow to the Allegheny River Basin. Wolf Creek and the Shenango River are part of the Beaver River Basin. There is a total of 121.75 miles of streams within the county listed by the PA Department of Environmental Protection as impaired, meaning that the water quality does not meet its designated use. This is due to historical mining activities, industry, and agriculture. The largest known cause of water quality impairment within the county is abandoned mine drainage.
Groundwater quality is generally good and an ample supply exists for domestic and stock use. The largest supplies are usually obtained by tapping the glacial outwash deposits or from bedrock along perennial stream valleys. Four main types of groundwater are found in the county. Shallow wells drilled into ground moraine generally yield a calcium sulfate water. Glacial till and shallow bedrock wells generally produce water with calcium bicarbonate. Sodium bicarbonate water is found in deep valley sediments and deep bedrock aquifers. Finally, underlying all freshwater aquifers in the county is sodium chloride or saltwater.