Trace metals in the waters of a partially mixed estuary

Abstract

The distributions of the trace metals iron, manganese, cobalt, nickel, copper, zinc and cadmium in the upper St. Lawrence estuary have been determined. The most important influence upon the distribution and behaviour of the metals, apart from the character of the mixing between fresh and saline source waters, is the distribution and transport of suspended particulate matter. The estuarine turbidity maximum, which contains suspended particles enriched in cobalt relative to particles within both marine and river source waters, modifies the distribution of metals in the estuary and influences the character of the mixed outflowing water of salinity up to 20%‰. The influence of the turbidity maximum, which appears to be formed by particle trapping and the periodic settling and resuspension of sedimentary material is, however, confined to the estuary. Although the incoming and outgoing fluxes of suspended particulates in water are balanced, significant proportions of the influxes of iron (12%), manganese (19%), copper (24%), cobalt (27%) and zinc (44%) are removed within the system. The losses of iron, manganese and cobalt occur predominantly at the expense of their particulate fractions. Suspended matter leaving the estuary in mixed surface water is depleted in manganese and cobalt suggesting that the losses of these metals within the estuary occur primarily through the removal of exchangeable metal from riverborne particles within the turbidity maximum.