Hydrogeology, Hydrochemistry, and Computer Simulation of Ground-Water Flow in Southeastern Coastal Plain Aquifer System

Robert A. Renken, Rene A. Barker, Roger W. Lee

A study of the southeastern coastal plain aquifer system has provided a comprehensive assessment of the regional geology, hydrology, and geochemistry of clastic Tertiary and Cretaceous strata in parts of South Carolina, Georgia, Alabama, and Mississippi. Clay, shale, and chalk confining units separate four regionally extensive sand and gravel aquifer units that have a highly variable hydraulic conductivity as they extend from outcrop to the deep subsurface. The configuration and extent of aquifer and confining units are primarily a function of gradational changes in permeability. As a result, aquifer and confining unit boundaries do not correspond everywhere to traditional geologic contacts.

Dissolved solids range from less than 50 mg/L near recharge areas to greater than 100,000 mg/L in downgradient areas. Cretaceous aquifers usually contain water dominated by sodium and bicarbonate with dissolved solids less than 1,000 mg/L. Most Tertiary strata are calcareous and contain water dominated by calcium and bicarbonate with dissolved solids less than 1,000 mg/L. Dissolved solids concentrations greater than 1,000 mg/L may result from mixing of ground water with sodium chloride brines present in the deep parts of the system.

A finite-difference model was calibrated to simulate pre-development conditions in the regional flow system. The simulated recharge to the confined system is about 1 in./year, of which about 75% discharges to major streams. The remaining 25% flows downgradient toward a freshwater-saltwater interface and discharges into overlying units as diffuse upward leakage. The simulated water budget quantifies the contrasts between the shallow, unconfined flow regime and the deeper confined parts of the system.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.