STORM WATER CONTROLLED TRACE METAL LOADING IN THE CHESAPEAKE BAY WATERSHED

Natural weathering processes will tend to enrich the trace metal content of soils relative to the upper continental crust (UCC). Resultantly, the erosion of these soils will increase the loading of toxic trace metals to specific watersheds. The trace metal problems of the Chesapeake Bay have been well documented and it is clear that since the industrialization of the United States the flux of anthropogenic contaminants to the bay has steadily increased. However, recent work in the Patuxent River Basin suggests that much of the dissolved metal load is the result of desorption from suspended and bedload sediments and that current estimates of the trace element inputs from the sediment load are inadequate to evaluate their ultimate source. The questions that remain are, what portion of the toxic metal load being transported to the bay is the result of natural processes like chemical weathering and erosion and what portion of soil derived metals are available to aquatic biota?

An investigation in the Patapsco River Watershed, a subordinate drainage system with in the Chesapeake Bay Watershed, has determined that the ultimate source of most trace metals being transported to Baltimore’s Inner Harbor are soils that are being eroded within the watershed. Furthermore, the flux of suspended sediments and associated trace metals are dominated by storm water discharge events. The geochemical characteristics of the suspended sediments being transported to the Inner Harbor are consistent with material derived from the chemically weathered UCC and as much as 50% of the total trace metal flux associated with suspended sediments is potentially bio-available.

The sediment yield of the Patapsco River Watershed, which in some cases is >30,000 t/km2/yr, is closely correlated with impermeable ground cover. The ground cover conditions within the Patapsco River Watershed have created non-equilibrium denudation conditions leading to an unusually high sediment flux and corresponding trace metal transport into Baltimore’s Inner Harbor. Resultantly, any effort to limit and/or control the accumulation of trace metals in the Harbor must include storm water management and erosion control measures through out the watershed.