Influence of infrequent floods on the trace metal composition of estuarine sediments

By use of iron variation diagrams, it is shown that the concentrations of Mn, Ni, Cu and Zn in sediments of upper Chesapeake Bay are 50–75% lower than expected if they were simple mixtures of their apparent source materials: eroding Atlantic Coastal Plain deposits and material delivered by the Susquehanna River under ordinary discharge conditions. The Fe concentrations in the sediments, on the other hand, are consistent with derivation from these sources. Evidence is presented that the ratios of total Mn, Ni, Cu and Zn to Fe in the Susquehanna River decline during high discharge events. Because such events are responsible for removal of a major fraction of the total material carried out of the river basin, it is likely that the upper bay sediments simply reflect the long-term average composition of material from the basin. Averaging over a period much greater than one year is necessary to obtain a meaningful estimate of the trace element composition of material being removed from this river basin.