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

Human population growth and subsequent human development is closely linked to contemporary increases in sediment and associated contaminant fluxes to fluvial systems, lakes, reservoirs and coastal zones worldwide. In more urbanized basins the abundance of contamination sources often results in contaminant loadings and water quality declines in neighboring aquatic ecosystems. 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 reaches of the basin in the heavily populated suburban zone south of Washington, D.C. Sediment cores from the two lakes were used in ²¹⁰Pb based sediment accumulation rate analysis. The temporal and spatial distributions of Al, Fe, Cd, As, Se, Pb, Cu, and Zn in the sediments were also examined in sediment cores and grab samples from the two lakes. Watershed GIS-based models were also used in evaluating sediment fluxes and basin soil erosion rates.

Results of ²¹⁰Pb sediment accumulation rate estimates in Lake Occoquan range from 0.126 g/cm²/yr in the upper reaches to 0.135 g/cm²/yr in the lower reaches with temporal values showing progressive modern increases from 0.125 to 0.154 g/cm²/yr. Lake Manassas had a comparable value of 0.140 g/cm²/yr and a temporal range of 0.120 to 0.157 g/cm²/yr. Watershed modeling results show sediment flux estimates of 0.11ton acre²/yr in the upper mixed land-use areas of the watershed and 0.03 ton acre²/yr in the lower more urban zones of the watershed. Historic watershed modeling resulted in a decrease in estimated erosion over time as a factor of impervious surfaces improved runoff management. Sediment core trace metal data show a correlation between contemporary urban expansion and spikes in trace metal input. Correlations between historical land use and temporal and spatial trace metal distribution will also be used to evaluate the history of trace metal loading and probable sources in the basin.