SEDIMENT BUDGET-BASED ESTIMATES OF TRACE-METAL INPUTS TO A CHESAPEAKEESTUARY

This article evaluates whether a sediment budget for the South River, Maryland, can be coupled with metals data from sediment cores to ident ify and quantify sources of historic metal inputs to marsh and subtida l sediments along the estuary. Metal inputs to estuarine marsh sedimen ts come from fluvial runoff and atmospheric deposition. Metal inputs t o subtidal sediments come from atmospheric deposition, fluvial runoff, coastal erosion, and estuarine waters. The metals budget for the estu ary indicates that metal inputs from coastal erosion have remained rel atively constant since 1840. Historical variations in metal contents o f marsh sediments have probably resulted primarily from increasing atm ospheric deposition in this century. but prior to 1900 may reflect cha nging fluvial sources, atmospheric inputs, or factors not quantified b y the budget. Residual Pb, Cu, and Zn in the marsh sediments not accou nted for by fluvial inputs was low to moderate in 1840, decreased to n ear zero circa 1910, and by 1987 had increased to levels that were one to ten times greater than those of 1840. Sources of variability in su btidal cores could not be clearly discerned because of geochemical flu xes, turbulent mixing, and bioturbation within the cores. The sediment -metal budgeting approach appears to be a viable method for delineatin g metal sources in small, relatively simple estuarine systems like the South River and in systems where recent deposition (for example, prog rading marshes) prevents use of deep core analysis to identify ''backg round'' levels of metal. In larger systems or systems with more variab le sources of sediment and metal input, however, assumptions and measu rement errors in the metal budgeting approach suggest that deep core a nalysis and normalization techniques are probably preferable for ident ifying anthropogenic impacts.