TRACING THE SOURCES OF SUSPENDED SEDIMENT INPUTS TO FALLS LAKE RESERVOIR, NEUSE RIVER, NORTH CAROLINA

Falls Lake, a 50 km² reservoir on the Neuse River, is the primary source of drinking water for nearly 500,000 Wake County, N.C. residents. Many of the major tributaries to the lake are on North Carolina’s 303(d) list for impaired streams, and in 2008 the lake was added to that list because of high values of turbidity (total suspended sediment-TSS) and chlorophyll-a concentrations. Pollutant sources such as urban and agricultural runoff, existing and new development, and point source dischargers are typically blamed for these water quality issues; however upland soil erosion resulting from post-European settlement agricultural practices, which led to large volumes of aggraded “legacy” sediment along regional valley bottoms, including behind historic milldams, may be a significant non-point source of current TSS. We hypothesize that the modern erosion of these legacy sediments now exposed along stream banks is the primary source of TSS in Falls Lake.

Suspended sediment samples were collected during high-flow events from six tributary streams that flow into Falls Lake. Composite sediment samples representing potential sources were collected from various locations (stream banks, forested areas, croplands, construction sites, dirt roads, pastures) within each of the tributary catchments. Sediment samples were analyzed for stable isotopes, radioisotopes and trace elements in order to identify tracers that are able to distinguish between potential sediment sources. Suitable tracers are being used to develop mixing models that can explain the relative source contributions. Preliminary results support the null hypothesis and suggest that erosion ensuing from the clearing of forested land to accommodate urban expansion for the City of Durham is the primary source of TSS in at least one of Falls Lake’s tributary catchments.