The transport of heavy metals by the Mississippi River and their fate in the Gulf of Mexico

Abstract

Significant differences in total and leachable concentration of some metals were observed between Mississippi River suspended matter and Gulf of Mexico sediments. These variations are related to diversity in Gulf sediment redox conditions, accumulation rates and grain-size distribution. Suspended particulates transport >90% of the Mississippi River metal load. Hydrous oxides and aluminosilicate lattices are the primary phases of particulate metal transport. In general, total and leachable metal concentrations in river suspended matter are seasonally and spatially uniform. During low river flow, an eight-fold increase in the organic content of the suspended matter brought about a decrease in particulate Fe and Al values and a marked increase in Mn, Zn, Cu and Cd concentrations. Average SIO2, Al2O3 and K2O concentrations in Mississippi River particulates are similar to crustal abundances. Particulate Na2O, CaO and MgO values are 60-80% below crustal levels; however a high river dissolved load of these elements offsets the low particulate levels. No significant change in the total or leachable heavy metal concentrations of river suspended matter was found across the freshwater-seawater interface.