Effects of sediment depositional environment and ground-water flow on the quality and geochemistry of water in aquifers in sediments of Cretaceous age in the Coastal Plain of South Carolina

Abstract

The quality and geochemistry of ground water are significantly affected by the depositional environment of aquifer sediments. Cretaceous sediments in the Coastal Plain of South Carolina have been deposited in fluvial, deltaplain, marginal-marine, and marine environments. Depositional environments of sediments within a single aquifer may grade from nonmarine, fluvial, or upper delta plain near the updip limit of the aquifer to transitional, lower delta plain and to marine toward the coast. In nonmarine sediments the major source of inorganic carbon in the water is the decomposition of organic material. The major aqueous geochemical processes are the dissolution and alteration of silicate minerals. Silica makes up a major part of the dissolved constituents in water from these sediments. In transitional and marine sediments the major aqueous geochemical processes are (1) the dissolution of calcium carbonate by hydrolysis and by carbonic acid derived from the decomposition of organic material and (2) the exchange of calcium in solution for sodium on the marine-clay minerals. The clay minerals may also serve as buffers by neutralizing the hydroxyl ion produced by hydrolysis. The effects of incompletely flushed dilute saltwater on water quality increase toward the coast and toward the northeast.

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

“…Despite this decrease, the transmissivity of the Middendorf aquifer near the coast is as great as or greater than that of adjacent aquifers. Conclusions about transmissivity are based on lithologic data, aquifer-test data (Aucott and Newcome, 1986), and the fresher quality of the water (Speiran and Aucott, 1991), which indicates more complete flushing and thus a more active flow system in the Middendorf aquifer than in overlying or underlying aquifers.…”

“…One major process is an increase in the concentration of inorganic carbon in the form of bicarbonate ions. The most important source of inorganic carbon is the dissolution of carbonate minerals (shell material and calcite) by carbonic acid and hydrolysis (Aucott and Speiran, 1986;Speiran and Aucott, 1991). Dissolution commonly occurs in parts of the aquifers containing calcareous sediments and results in greater buffering of water in these parts of the aquifers.…”

Groundwater Protection Management Program

“…Sodium is clearly the dominant cation further down-dip in the Midville Aquifer (Shorn and Kaback 1992). Cation exchange processes have led to water compositions downgradient from SRS " comparable to those formed by exchange processes observed in other regions of the South Carolina Coastal Plain (Speiran and Aucott 1991).…”

Groundwater Protection Management Program

“…Cation exchange processes have led to water compositions downgradient from SRS " comparable to those formed by exchange processes observed in other regions of the South Carolina Coastal Plain (Speiran and Aucott 1991).…”