Sediment-Hosted Contaminants and Distribution Patterns in the Mississippi and Atchafalaya River Deltas

By

FLOCKS, JAMES G., KINDINGER, JACK L., and SWARZENSKI, PETER W.

US Geological Survey, Center for Coastal Marine Regional Studies, St. Petersburg, FL

The Mississippi and Atchafalaya Rivers transport very large amounts of bedload and suspended sediments to the deltaic and coastal environments of the northern Gulf of Mexico. Absorbed onto these sediments are contaminants that may be detrimental to the environment. To adequately assess the impact of these contaminants it is first necessary to develop an understanding of sediment distribution patterns in these deltaic systems. Once these patterns are identified, the natural and industrial contaminant inventories and their depositional histories can be reconstructed. Deposition in deltaic systems is controlled by discharge, as well as channel and receiving-basin dimensions. Current energy controls the sediment distribution pattern. This results in a coarse grained or sandy framework, in-filled with finer grained material occupying the overbank, interdistributary bays, wetlands and abandoned channels. It has been shown that these fine-grained sediments can carry contaminants through absorption and intern them in the sediment column or redistribute them depending on progradation or degradation of the delta deposit.

Sediment distribution patterns in delta complexes can be determined through high-resolution geophysical surveys, groundtruthed with direct sampling. In the Atchafalaya and Mississippi deltas, remote sensing using High Resolution Single Channel Seismic Profiling (HRSP) and Sidescan Sonar was correlated to 20 foot vibracores to develop a near-surface geologic framework that identifies variability in recent sediment distribution patterns. The surveys identified bedload sandwaves, abandoned-channel back-fill and distributary mouth bars within the most recently active portions of the deltas. These depositional features respond to changes in deltaic processes and through their response may intern or transport absorbed contaminants. Characterizing these features provides insight into the fate of sediment-hosted contaminants.