ABSTRACT

Bottom sediments of the Pontchartrain Estuary consist predominantly of silty clays derived either from erosion of the St. Bernard lobe of the Mississippi delta, or directly from the Mississippi River via the Bonnet Carre' spillway. The clay mineralogy of the sediments is dominated by montmorillonite and kaolinite with lesser amounts of illite also present. The bottom sediments of the lakes, in terms of clay mineralogy and texture, reflect the influence of the Mississippi River.

Concentrations of copper, chromium, nickel, iron, zinc, vanadium, cobalt, lead, and barium were determined for bottom sediment samples collected during 1987. The areal distribution of metals in the estuary is controlled mainly by sediment texture. As the clay content of the sediment increases, so does metal content. This observation holds for most estuaries along the northern Gulf of Mexico. The concentrations of all metals except lead are found to be positively correlated with the iron content of sediments. Comparison of average metal contents with data from other estuaries along the Gulf Coast indicates that the estuary has not been heavily impacted by anthropogenic input of metals.

The Pontchartrain Estuary is the focus of an intense debate concerning hydraulic shell dredging and its effects. It has been alleged by the opponents of shell dredging that the mining process releases harmful concentrations of heavy metals into the water column. Because dredging disrupts redox conditions in the sediment, the potential release of metals into the water column is a concern. Bottom sediment heavy metal data combined with ion-site partitioning analyses provide a basis for estimating heavy metal release during dredging. Partitioning analyses of bottom sediments indicate that most of the heavy metals in the bottom sediments are held in relatively stable phases. Heavy metals in the exchangeable and organic phases, which are most easily affected by redox changes, are in low concentrations. The rapidity of hydraulic shell mining, the positive Eh of the water column, the fine-grained nature of suspended sediments in the plume, and the high abundance of hydrous iron oxides in the sediments result in a minor impact on water quality. Released metals are rapidly scavenged from the water column by adsorption and coprecipitation reactions.