COI Funded Project: Radiocarbon Content of Ground Water and Submarine Ground Water Discharge in the South Atlantic Bight: An Initial Survey

Daniel McCorkle

Project Duration: 6/1/97-5/31/98
Key Words: South Atlantic Bight, groundwater flux, radiocarbon tracer

Proposal Abstract

Recent studies suggest that there is a substantial flow of ground water into the South Atlantic Bight (SAB) off Georgia and the Carolinas. There is clear evidence for at least two types of ground water input: (1) freshwater springs in coastal marshes and inner shelf (approx. 10 m isobath) sediments, which can lie as far as several kilometers offshore, and (2) salty or brackish discharges through the sediments of the middle shelf (10 to 30 meter isobath), at distances of up to tens of kilometer offshore. These ground water discharges have a dramatic impact on the radium budget of the SAB water column, and imply that the rate of submarine ground water discharges (SGWD) into the SAB may be as much as 40 percent of the river discharge rate in this region. If these flux estimates are correct, SGWD can have a substantial effect on geochemical balances in coastal and nearshore waters. SGWD may be an important mechanism for the transfer of dissolved substances from the continents to the oceans.

The possibility exists that the chemical signals of SGWD may get recorded in the shells and skeletons of mollusks and corals, which are receiving increased attention as high-resolution recorders of water chemistry (including , stable carbon and oxygen isotopes, and trace metals). This possibility offers an added incentive to understand the factors that control the chemistry of nearshore waters.

A major uncertainty in this story has to do with the actual rate of fresh ground water discharge into coastal waters. The SGWD radium flux is particularly high because radium is desorbed from aquifer sediments by salt water intrusion. This intrusion can occur due to natural processes (tidal pumping or natural changes in aquifer recharge), or due to anthropogenic effects (increased pumping of the aquifer as a consequence of population growth in coastal areas, or breaching of aquifer caps by channel dredging). Radium is thus a more sensitive indicator of salt intrusion than of actual land-sea ground water flow. To quantify the rate of freshwater discharge will require a chemical tracer of ground water that is not dominated by salinity effects.

To develop such a tracer, it will be necessary to determine the radiocarbon content of ground water samples and samples of fresh and brackish submarine discharge waters from the two SAB study areas: North Inlet (Long Bay), South Carolina, and Onslow Bay, North Carolina. RCRC-supported work is being conducted in parallel with that of Dr. W. Moore (USC) and Dr. A. Spivak (UNC-Wilmington). The data will make a valuable contribution to their studies; ground water dissolved inorganic carbon concentrations are expected to be high, and much of the added carbon is presumably derived from dissolution of the limestone aquifer formation, and will thus be -free. However, only by carrying out the preliminary measurements proposed here will it be possible to see whether this low- signal will be strong enough to provide a constraint on the rates of freshwater submarine ground water discharge (and thus on the land-sea ground water discharge (and thus on the land-sea ground water chemical fluxes), and whether SGWD is likely to lead to anomalous values in shelf or estuarine waters.