Geochemistry of groundwater in tertiary and cretaceous sediments of the southeastern Coastal Plain in eastern Georgia, South Carolina, and southeastern North Carolina

Abstract

Geochemical samples of groundwater taken along hydrologic flow paths in eastern Georgia, South Carolina, and southeastern North Carolina, from noncalcareous sand aquifers, largely of Cretaceous age, are dominated by sodium and bicarbonate ions. Calcareous sand aquifers, largely of Tertiary age, contain water whose chemistry is dominated by calcium and bicarbonate ions, but may evolve downgradient to sodium and bicarbonate dominance. Water chemistry in both types of aquifer evolves to sodium chloride dominance as a result of fresh water mixing with subsurface brines or seawater present in the deeper downgradient parts of the aquifers. Principal aqueous chemical reactions appear to occur in five reaction zones in the aquifers and include feldspar hydrolysis to kaolinite, calcite dissolution, calcium‐for‐sodium cation exchange, and neoformation of sodium smectite in the downgradient parts of the aquifers. Redox reactions produce dissolved iron concentrations greater than 1 mg/L near the recharge areas. Organic matter in the aquifers is oxidized to CO2 by iron reduction and sulfate reduction processes. Production of CO2 by a methanogenic process may also occur. Geochemical mass‐transfer models simulating the observed chemistry in western Alabama and eastern Mississippi have been extended to account for higher concentrations of sodium and bicarbonate observed in the South Carolina part of the aquifers.

14C Groundwater Age and the Importance of Chemical Fluxes Across Aquifer Boundaries in Confined Cretaceous Aquifers of North Carolina, USA

“…The dominant dissolved constituents in groundwater of the Black Creek and Upper Cape Fear aquifers are DIC and Na + ( Table 2). The increase in concentration of DIC and Na + with distance from recharge area ( Figure 3) is consistent with past work (Lee and Strickland 1988;Chapelle and McMahon 1991;McMahon and Chapelle 1991b). It suggests that calcite dissolution followed by Ca 2+ /Na + ion exchange is an important process affecting groundwater chemistry of the aquifers and that groundwater age increases coastward.…”

The influence of microbial activity and sedimentary organic carbon on the isotope geochemistry of the Middendorf Aquifer

“…The sediments at well C-10 in this study were in unique contrast to Middendoff sediments from the other boreholes in that they were deposited under a lower delta plain environment and contain fine to medium quartzitic sands with thick silt and clay beds [Sargent and Flierroans, 1989]. Sodium concentrations in C-10 may be a direct result of ion exchange with layer silicates [e.g., Lee, 1985;Lee and Strickland, 1988] The iron concentrations are elevated in groundwaters from wells P-29, P-28, and P-24 (Table 1). Although the groundwater samples were filtered through a 0.2-/am filter in the field, subsequent filtering in the laboratory through a 0.1-/am filter substantially reduced the concentration of iron, suggesting that much of the iron measured may be in the colloidal state.…”

An interactive code (NETPATH) for modeling NET geochemical reactions along a flow PATH

“…It is commonly observed in Coastal Plain sand aquifers, in proceeding from oxygenated recharge downgradient, that as the O2 begins to vanish, there is an abrupt increase in dissolved ferrous ion. Lee and Strickland (1988) noted dissolved iron contents of 2 mg/L in Coastal Plain sediments of Cretaceous age in Georgia, and South Carolina. These values are large compared with background values of tens of micrograms per liter dissolved Fe in the oxygenated zone.…”