APPLICATION OF RUSLE2 AND ISOTOPIC SEDIMENT CORE ANALYSES TO ESTIMATE WATERSHED SOIL LOSSES AND RESERVOIR SEDIMENT ACCUMULATION RATES FOR LAKE ANNA, VIRGINIA

Soil erosion and reservoir sedimentation are a threat to the sustainable use of surface water resources through loss of volume storage capacity and as conveyance of pollutants to receiving water bodies. These threats are present at Lake Anna in central Virginia where recently, three sections of the reservoir have been included in the 2004 US EPA 303(d) Report on Impaired Waters due to the presence of polychlorinated biphenyls (PCBs) in fish tissue at concentrations warranting a fish consumption use impairment. Currently, the source of the PCBs in Lake Anna is unknown, but sediments derived from upland soils could be a significant source. There have been several previous attempts to quantify sediment flux rates for the watershed which relied heavily on qualitative observations and the use of the Universal Soil Loss Equation (USLE), with no indication of what types of land are most susceptible to erosion. For our study the updated RUSLE2 empirical model and isotopic sediment core analyses were used to evaluate watershed erosion and sediment accumulation rates for Lake Anna. Our RUSLE2 analysis estimated a watershed sediment flux rate of 6.70 x 10¹⁰ g y^-1. Land cover was the dominant control for the watershed erosion rates, with forestry and agricultural activities contributing the most sediment; whereas stable forest and herbaceous lands were the least eroded. In addition to RUSLE2 data, two separate ²¹⁰Pb and ¹³⁷Cs based sediment accumulation rates were estimated for the reservoir. These analyses yielded a sedimentation rate of 0.19 g cm^-2 y^-1 within the lacustrine lower reaches and 1.57 g cm^-2 y^-1 in the deltaic upper reaches of the reservoir. Based on these results it is evident that the Lake Anna reservoir and its relatively pristine watershed are not presently experiencing siltation and excessive erosion problems but the predominance of highly erosive soils (soil erodibility factor > 0.30) within the watershed makes this system highly vulnerable to future anthropogenic stressors and possible contaminant influx.