Heavy metal inputs to Mississippi Delta sediments

Abstract

Heavy metal concentrations were determined in suspended particulates, filtered water and sediment collected in the Mississippi River and from its marine delta. More than 90% of the metal load of the river is associated with particulate matter, which is relatively constant in chemical composition with time and place. The Mississippi River suspended material is similar to average crystal rocks in Fe, Al, V, Cr, Cu, Co, Mn, and Ni concentration but is generally enriched in Zn, Cd and Pb. Sediment cores dated by the Pb 210 method show that the Cd and Pb enrichments are recent phenomenon and are most likely due to the activities of man. About 6000 tonne of Pb and 300 tonne of Cd are being added to the delta sediments by man each year, more than 30 times the amount added to the Southern California Bight. River particulate matter is essentially identical to deltaic sediments in Al, Fe, Cr, V, Cd and Pb concentration, but the sediments are depleted in Co, Cu, Mn, Ni and Zn by 20 to 40%. Chemical leaching of the solids show the metal losses to be primarily from the oxide phase, suggesting diagenetic reduction and mobilization as a mechanism. Trace metal concentrations in filtered Mississippi River water were below the limits for safe drinking water and were similar to world average river values. The abundant river suspended matter and high pH combine to keep dissolved trace metal concentrations low.