ASSESSING THE IMPACT OF NONEQUILIBRIUM DENUDATION ON THE FLUX OF TRACE METALS TO THE CHESAPEAKE BAY

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. The question that remains is, what portion of the toxic metal flux to the bay is the result of natural processes like chemical weathering and the rate of denudation? The urbanization of large areas of the Chesapeake Bay Watershed along with continued agricultural activities serve to potentially enhance erosion rates therefore, creating nonequilibrium denudation in the region. These conditions, coupled with the potential to enrich soils in trace metals during the chemical weathering of bedrock may provide a significant non-point source for metals being transported to the bay. An investigation of a tributary system with in the Jones Falls Watershed, a subordinate of the Chesapeake Bay Watershed, indicates that chemical weathering and high rates of denudation enhance the flux of toxic trace metals.

A geochemical investigation of soils forming on the Baltimore Gneiss, and related stream sediments in a tributary system within the Jones Falls Watershed indicates the presence of nonequilibrium denudation conditions. Based on a general relationship between the Chemical Index of Alteration (CIA) and sediment yield (t/km²/yr) for the major rivers of the world, the soils and stream sediments are under weathered for a watershed the size of Jones Falls. One possible cause for this discrepancy is the highly impacted nature of the tributaries with in the watershed. The Jones Falls Watershed is located within suburban Baltimore County and the City of Baltimore. The urbanization of this watershed has significantly reduced infiltration and enhanced erosion. Enhanced erosion has reduced the residence time of soils with in the soil profile and resultantly reduced the CIA values expected for the region.

Despite the reduced weathering residence time, trace metals such as Cr, Ni, Cu, As and Zn are enriched up to 18 times the associated concentrations in the bedrock. This enrichment coupled with the enhanced rate of erosion with in the watershed creates a flux of trace metals from the watershed to the Chesapeake Bay that is up to 40 times greater than expected.