SEDIMENT ACCUMULATION RATES AND TRACE METAL INPUT HISTORY IN LAKE MANASSAS AND THE OCCOQUAN RESERVOIR, VIRGINIA, USA

Contemporary increases in anthropogenic land use and cover alterations in fluvial basins has led to significant increases in sedimentation in lakes and reservoirs worldwide. The sediment fluxes and associated contaminant inputs, i.e. nutrients, trace metals, and organic matter, have led to significant environmental problems such as loss of capacity and progressive decline in water quality. This study analyzes the environmental evolution of two reservoirs within the Occoquan basin, a sub-watershed of the Chesapeake Bay. Lake Manassas is located in the upper reaches of the watershed, characterized by mixed land use and cover of mostly forest, residential, and agriculture, whereas Occoquan Reservoir is located in the urbanized lower reach of the basin in the heavily populated suburban zone south of Washington, D.C. Three sediment cores from each lake were used in ²¹⁰Pb based sediment accumulation rate and organic matter content analysis. Analysis of the distributions of Al, Fe, Cd, As, Se, Pb, Cu, and Zn in the sediments was also completed on the sediment cores and on grab samples from the two lakes. Hydroacoustic geophysical survey and watershed GIS-based models were also used to further evaluate sediment fluxes and basin soil erosion rates.

The preliminary results of ²¹⁰Pb based sedimentation rate estimates ranges from 0.126 g cm^-2 yr^-1 in the upper reaches to 0.099 g cm^-2 yr^-1 in the lower reaches of Occoquan Reservoir, with temporal values showing progressive modern increases from 0.066 to 0.154 g cm^-2 yr^-1. Lake Manassas, located in the upper basin, had comparable average sedimentation rates of 0.130 g cm^-2 yr^-1 and a temporal range of 0.071 to 0.157 g cm^-2 yr^-1. Further analysis will provide insight into the role of urbanization-related increase in impervious surfaces in exacerbating modern sediment production via degradation of lower reaches floodplains, as well as contribution from upstream mixed land use sediment source zones. Correlations between historical land use, and temporal and spatial trace metals distribution will also help evaluate the history of trace metals loading and probable sources in the basin.