Abstract: Accumulation and Trace-Metal Variability of Estuarine Sediments, Barataria Basin, Louisiana

Kenneth Landrum

Prior to government regulation, little monitoring of metal discharges into the canals, bayous, and rivers that drain estuarine systems occurred. Discharges of heavy metals are now limited through the use of regulatory permit programs for industries and municipalities. Resource management of economically important estuarine systems has fostered increasing concern over the accumulation of heavy metal pollutants in water, sediments, and biota of these dynamic areas.

As a result of the low solubility of most metals, very small amounts are transported in solution. Most metals transported by rivers are tightly bound in the aluminosilicate phases associated with the suspended and bottom sediments. The bottom sediments of lakes and estuaries have served as sinks for the accumulation of heavy metals from natural weathering products, spills, effluents, runoff, and atmospheric sources. These accumulations have left a metal signature upon essentially every bay and estuary in the northern Gulf.

Bottom sediments of the upper Barataria basin estuarine complex have a significant potential for absorbing metals from the water column. They 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 metals (Al, As, Ba, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn) were determined for 136 bottom sediment samples and 40 core interval samples by plasma emission analysis. The lateral distribution of the metals is controlled predominantly by the texture and mineralogy of the sediments. Elevated concentrations in heavy metals near the centers of the Barataria basin lakes reflect the strong correlations between increased trace-metal concentrations and smaller grain-size sediments.

Sediment accumulation rates were determined from four cores within the study area using ²¹⁰Pb geochronology. Sedimentation rates range from 0.21 to 0.37 cm/yr. Heavy metal concentrations within the 2-cm core intervals indicate increases in Ba, Cr, Cd, Pb, and Zn (10-30%) within Barataria basin sediments accumulated in the last 75 yr.

Heavy metal concentrations were normalized against aluminum, iron, clay, and total carbon concentrations in an effort to determine any correlations. The strongest correlations exist between the heavy metal concentrations and the aluminum and iron content of the sediment samples. Of the 14 metals analyzed, barium, cadmium, lead, and zinc are the least correlated and represent those heavy metals with the greatest anthropogenic influence.

Although anthropogenic heavy metal contributions within Barataria basin exist, comparisons of average trace-metal concentrations from the study areas with those of other Gulf Coast estuaries and with the average shale indicate that the heavy metal contents of upper Barataria basin sediments have not reached alarming levels.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994