Hydrology - SCWRC Report 139

The Ground-Water Resources of Charleston, Berkeley, and Dorchester Counties, South Carolina

By
A. Drennan Park
1985

ABSTRACT

Ground water of good quality exists in most of Charleston, Berkeley, and Dorchester Counties. This ground water is obtained from aquifers in the Middendorf, Black Creek, Peedee, and Black Mingo Formations, the Santee Limestone, and shallow sand and shell beds of Miocene to Pleistocene age. The Late Cretaceous Middendorf, Black Creek, and Peedee Formations contain the thickest and most extensive water-bearing units. Wells screened in the Middendorf Formation typically produce more than 1,000 gallons per minute, whereas wells screened in the overlying Black Creek or Peedee Formations usually produce less than 1,000 gallons per minute.

Sand beds of the Paleocene-Eocene Black Mingo Formation and permeable zones within the Santee Limestone are the most widely developed artesian aquifers. Aquifers in the two formations are tapped by open-hole wells that yield as much as 500 gallons per minute, the same well commonly penetrating, and thus to a limited degree connecting, the formations.

Pleistocene terrace deposits supply sufficient water for domestic purposes and are relied on by a number of public water utilities on the coastal islands. The shallow aquifers are thickest and most productive near the coast, thinnest and least productive in central Dorchester and Berkeley Counties.

The chemical quality of ground water varies considerably with locality and depth. The Cretaceous formations contain a soft, alkaline, sodium bicarbonate type of water, and fluoride concentrations typically exceed the 1.6 mg/L maximum recommended by the U.S. Environmental Protection Agency.

Water from the Black Mingo Formation is also of the sodium bicarbonate type and is characteristically high in fluoride and dissolved silica. Water from the Santee Limestone is hard to moderately hard and of a calcium bicarbonate type. Open-hole wells that tap both the Black Mingo Formation and Santee Limestone generally produce water with a chemistry reflecting that of the lowermost aquifer penetrated, indicating the higher productivity of the Black Mingo aquifers.

Shallow aquifers contain water that is low in dissolved solids but locally hard and high in iron.

Substantial water-level declines have occurred in the Black Creek aquifers at Mt. Pleasant and in the Santee Limestone and Black Mingo aquifers in the central part of the study area and at Charleston. Although a partial water-level recovery has occurred in the Black Mingo Formation and Santee Limestone at Charleston, the declines are expected to continue as the area’s population and economy expand.

Saltwater intrusion may be occurring in the Black Creek Formation at Mt. Pleasant, and increased chloride concentrations have been recorded in the Santee Limestone and Black Mingo Formation in the vicinity of Charleston. Contamination has also occurred in southern Charleston County, where open-hole wells have permitted interchange between saltwater and freshwater aquifers of the Santee Limestone and Black Mingo Formation.


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