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The geochemical evolution of aqueous sodium in the Black Creek Aquifer, Horry and Georgetown counties, South Carolina

Water Supply Paper 2324

By:
,

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Abstract

The Black Creek aquifer contains dilute seawater near the North Carolina State line, probably the result of incomplete flushing of ancient seawater. Data do not indicate that the dilute seawater has migrated toward areas of fresh ground-water withdrawals. The concentration of chloride in ground-water samples ranges from 5 to 720 milligrams per liter and that of sodium from 160 to 690 milligrams per liter. Ion-exchange reactions (sodium for calcium and fluoride for hydroxyl) occur with the calcium carbonate dissolution reaction which produces calcium, bicarbonate, and hydroxyl ions. The reaction sequence and stoichiometry result in an aqueous solution in which the sum of bicarbonate and chloride equivalents per liter is equal to the equivalents per liter of sodium. Calcium ions are exchanged for sodium ions derived from sodium-rich clays upgradient of the dilute seawater. The cation-exchange reaction equilibrates at a sodium concentration of 280 milligrams per liter. Amounts of sodium greater than 280 milligrams per liter are contributed from dilute seawater. The cation-exchange reaction approaches an equilibrium which represents a mass-action limit in terms of the ratio of sodium to calcium in solution versus the ratio of exchangeable sodium to calcium on clay surfaces. Where the limit of calcium carbonate solubility is approached and dissolution ceases, some precipitation of calcite probably takes place. The dissolution of calcite exposes fossil shark teeth which release fluoride ions to the ground water through anion exchange with aqueous hydroxyl ions.

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
The geochemical evolution of aqueous sodium in the Black Creek Aquifer, Horry and Georgetown counties, South Carolina
Series title:
Water Supply Paper
Series number:
2324
Edition:
-
Year Published:
1988
Language:
ENGLISH
Publisher:
U.S. G.P.O.,
Description:
iv, 15 p. :ill., maps ;28 cm.