Hydrology of the southeastern Coastal Plain aquifer system in South Carolina and parts of Georgia and North Carolina

Abstract

The wedge of sediments present beneath the Coastal Plain of South Carolina and adjacent parts of Georgia and North Carolina consists of sand, silt, clay, and limestone. These strata have been subdivided into six regional aquifers: the surficial aquifer, the Floridan aquifer system, the Tertiary sand aquifer, the Black Creek aquifer, the Middendorf aquifer, and the Cape Fear aquifer. Intervening confining units separate the aquifers, except for the Floridan aquifer system and the Tertiary sand aquifer, which together function as a single hydrologic unit. The quality of ground water from the Coastal Plain aquifers of South Carolina generally is acceptable for most uses in most areas. The water in most aquifers under most of the Coastal Plain contains low concentrations of dissolved solids (less than 500 milligrams per liter) and no dominant constituents in the recharge areas. Downgradient, the water is a calcium bicarbonate or sodium bicarbonate type throughout most of the Coastal Plain. Sodium-chloride-type water is present still farther downgradient, near the coast. A quasi-three-dimensional, finite-difference digital ground-water flow model was constructed to simulate flow in the Coastal Plain aquifers prior to development. The model also was used to evaluate the hydraulic responses to pumping that have occurred up to November 1982. The model consisted of five layers and a 48 by 63 node grid with a uniform square grid cell of 4 miles on a side. The Coastal Plain aquifers are recharged primarily by precipitation in their outcrop areas. Discharge is primarily as base flow to upper Coastal Plain rivers, to overlying aquifers by leakage through confining units, and to wells. Total simulated flow in the deep ground-water system was 967 cubic feet per second at the end of the transient simulation (1982). Recharge to the deep flow system simulated by the model was 793 cubic feet per second in the study area in 1982. Simulated aquifer discharge to large rivers was 660 cubic feet per second. Discharge to smaller rivers was not simulated because of the scale of the model. Changes resulting from ground-water pumping were significant as of 1982. The simulated water budget indicates that in 1982, 249 cubic feet per second were discharged from the aquifer system by wells. This pumping was balanced by the following changes from predevelopment conditions: 110 cubic feet per second derived from storage, 67 cubic feet per second decrease in aquifer-to-river discharge, 44 cubic feet per second increase in net inflow from source-sinks, and a net increase in inflow of 28 cubic feet per second across boundaries. Head declines in the Black Creek and Middendorf aquifers have occurred throughout much of the eastern part of the Coastal Plain of South Carolina as a result of pumping in the Myrtle Beach and Florence areas. Simulation indicates that the dominant sources of water for upper Coastal Plain pumping centers such as the city of Florence are decrease in flow to rivers in the upper Coastal Plain and water derived from storage. The dominant sources of water for pumping centers in the Myrtle Beach area are water derived from storage, leakage from overlying aquifers, and net increases in inflow across boundaries. Transmissivity values used in the flow simulation range from less than 1,000 feet squared per day near the updip limit of most aquifers to about 30,000 feet squared per day in the Middendorf aquifer in the Savannah River Plant area. Vertical hydraulic conductivity values used in simulation of confining units range from about 6x10 -7 feet per day for the confining unit between the Middendorf and Black Creek aquifers in coastal areas to 3x10 -2 feet per day for most of the confining units near their updip limits. Storage coefficients used in transient simulations were 0.15 where unconfined conditions exist and 0.0005 where confined conditions exist.