Investigations on the sediment chronology and trace metal accumulation in Sabine-Neches estuary, Beaumont, Texas

Abstract

The accumulation rates of sediments and trace metals (Co, Cr, Cu, Ni, Pb, Zn) were measured along with the concentrations of Al, Fe, Mn and organic carbon in four sediment cores from Sabine-Neches estuary, near Beaumont, Texas. A reliable geochronology of sediments and reconstruction of the history of trace metal inputs into this shallow estuarine environment was possible because the 239,240pu profiles closely tracked the bomb fallout history into the environment. The sedimentation rate was estimated to be about 4-5 mm/yr. Due to the very low and variable activities of excess 21OPb in the sediments, the 21OPb dating method did not prove to be very useful in the study area. One difficulty had to do with the large variability of grain size parameters in the sediments. The amount of fines varied from <20% to >90% within a single core. The activities of excess 21OPb and the concentrations of Al, Fe, organic carbon, and trace metals varied as a function of the amount of fine particles. 21OPb at the bottom of the sediment cores was in secular equilibrium with 226Ra, 23OTh, and 234U in some cores, while in others, this was not the case. The reasons for disagreement between 21OPb and 226Ra concentrations at depth were investigated. The mixing rates of surface sediments were low and was about 0.16-0.40 cm2yr-1. Down core variations of aluminum normalized enrichment factors for trace metals demonstrated that, since 1860, the sediments of this estuary have remained relatively "pristine" with respect to trace metal concentrations. While the concentrations of Pb and Zn in some sections of the sediment column were slightly enriched, Co, Cr, Cu, and Ni were depleted in all sediment cores analyzed. No significant enrichment of light rare earth elements was observed. Enrichment might have been expected from inputs of cracking catalysts used in refineries. Therefore REEs could not be used as non-steady tracers. The lack of strong enrichment of trace metals, light rare earth elements, and low inventories of radioisotopes could be a result of the short residence time of the estuarine water, long removal residence times of trace metals and radioactive elements in the water column, low salinity conditions, and possibly, complexation of these metals with dissolved organic matter.