Utilizing gamma isotope tracers to determine sediment source at reef sites near the Charleston Ocean Dredged Material Disposal Site

Abstract

The Charleston, South Carolina Ocean Dredged Material Disposal Site (ODMDS) has been heavily utilized as a disposal site for dredged material resulting from maintenance and channel deepening in the Charleston Harbor. Continuous monitoring by the South Carolina Department of Natural Resources at the ODMDS has indicated the presence of fine-grained sediment within the monitoring zones. However, since the Charleston Harbor is formed by the conjunction of three rivers, it has been suggested that some of the fine-grained sediment surrounding the ODMDS could be due to river transport rather than solely by disposal activities. In order to trace the outflow of sediment from the harbor, natural and man-made isotopes were utilized. 7Be (natural cosmogenic isotope) and 137Cs (man-made isotope) are often associated with estuarine sediments. Both isotopes were used as tracers in an attempt to determine the extent of density driven sediment flow from the Charleston Harbor. 7Be was detected in many of the offshore sampling stations indicating a direct correlation to the harbor. 137Cs was only found in one sediment trap sample offshore, but none the less indicated some transport from the harbor. Further study for utilizing isotopic tracers in determining offshore sediment transport is still being conducted at the disposal site. It is anticipated that further 7Be and 137Cs isotopic monitoring offshore Charleston will aid in determining the role that tidal and density driven sediments play in the sediment budgets at the hard bottom reef sites.