ABSTRACT

Bottom sediments of the upper Barataria Basin estuarine complex consist predominantly of silty clays and clayey silts derived from the erosion of the Lafourche and St. Bernard lobes of the Mississippi River delta and sediments associated with historic crevasses along the Mississippi River. The acid-leachable concentrations of 14 trace metals were determined for 136 bottom sediment samples and 40 core interval samples by plasma emission analysis. The areal distribution of the trace metals is controlled predominantly by the texture and mineralogy of the sediments. Near the centers of the Barataria Basin lakes, elevated trace-metal concentrations reflect the significant correlations that exist with smaller grain-size sediments.

Sediment accumulation rates were determined from five cores within the study area using ²¹⁰Pb geochronology. Rates varied from 0.21 to 0.37 cm/yr. Within the 2-cm core intervals, trace-metal concentrations of Ba, Cr, Cd, Pb, and Zn increased by 10 to 30% in sediments accumulated within the last 75 yr. Little variation in trace-metal content was noted from sediments accumulated during the 1800's with those of the last 25 yr.

Trace-metal contents were plotted against aluminum, iron, clay, and total carbon. The most significant correlations occur between trace-metal concentrations and the aluminum and iron content of the sediment samples. Of the 14 trace metals analyzed, Ba, Cd, Pb, and Zn are the least correlatable and show the greatest anthropogenic influence. Natural trace-metal variability was determined by the use of an aluminum-normalization model based on Florida and Louisiana estuarine sediments, basinwide and Gulf Coast trace-metal comparisons, sediment geochronology, and grain-size corrected data. Trace-metal concentrations from sediments of the study area have greater mean concentrations han Florida estuarine sediments due to their finer grain size, lack of CaCO₃, and higher clay and carbon content. Anthropogenic trace-metal inputs of As, Ba, Cd, Pb, V, and Zn were detected in sediments associated with oil and gas drilling and production, sandblasting and shipbuilding, dredging, and stormwater, municipal, and industrial discharges. Although evidence of trace-metal contributions within the study area exists, comparisons of average trace-metal concentrations with those of other Gulf Coast estuaries, and with the average shale concentration, reveal that only isolated industrialized areas within the upper Barataria Basin have been impacted.