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ABSTRACT: Heavy Metal Geochemistry of the Pontchartrain-Maurepas Estuarine Complex

George C. Flowers, Wayne C. Isphording

The Pontchartrain-Maurepas estuarine complex has become the focus of an intense debate concerning hydraulic shell dredging and its effects. Besides disrupting the benthic community, it has been alleged by the opponents of shell dredging that the mining process releases harmful concentrations of heavy metals into the water column. Bottom sediment heavy metal data combined with ion-site partitioning analyses provide a basis for estimating heavy metal release during dredging.

The concentrations of Cu, Cr, Ni, Fe, Zn, V, Co, Pb, and Ba were determined for bottom sediment samples collected during 1987. The areal distribution of metals in the complex is controlled largely by sediment texture. As the clay content of the sediment increases, so does metal content. This observation seems to hold for most estuaries along the northern Gulf of Mexico. The concentrations of all metals except Pb were found to be positively correlated with Fe-content. Comparison of average metal contents with data from other estuaries along the Gulf Coast indicates that the complex has not been heavily impacted by anthropogenic input of metals.

Because dredging disrupts redox conditions in the sediment, the potential release of metals into the water column is a real concern. However, partitioning analyses indicate that the majority of metal present in the bottom sediments is in relatively stable phases. The metal content of the exchangeable and organic phases, which are most easily affected by redox changes, was found to be minor. The rapid process of hydraulic shell mining, the positive Eh of the lake water, and the fine-grained suspended sediments result in a minor impact of dredging on water quality. Released metals are rapidly scavenged by adsorption and coprecipitation reactions.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990