Source control is the first principle for remediating contaminated sediment. Source control is critical because unaddressed sources will quickly reverse a thoughtfully developed and executed remedy.
While historical sources at a manufacturing facility are often addressed early on in a project's life cycle, an important component of sediment site assessment and remediation is understanding to what degree other sources may be present. For sediment sites, this includes the role of eroding creek banks as potential sources. In some cases, the normal erosion of creek banks may provide clean sediments to assist with natural recovery. In cases where chemicals remain in the creek banks, the eroding banks may represent an ongoing source to the aquatic system. This presentation will provide the results of an evaluation conducted for the Anniston PCB Site (Site) to assess the role of creek bank erosion as a potential PCB source to the aquatic portions of the Site. The Site is in north-central Alabama and includes over 40 miles of creeks and 7,000 acres of adjoining floodplains with PCB-containing soils, sediment, and biota.
Approach/Activities. The technical approach included surveying creek bank conditions, modeling creek bank erosion, collecting PCB data for floodplain soils/creek banks and surface water flux, and then estimating long-term sediment recovery for a downstream portion of the Site. We conducted creek bank surveys by applying qualitative erosion criteria to over 60 miles of creek banks (30 miles of creek) based on visual observations. To maintain rating quality consistency, we used the same group of subject matter experts to grade the entire system. For erosion modeling, we used a model developed by the USDA that included variable creek geometry, different soils horizons, water table effects, bank protection, and confining pressures. The model allowed us to include the erosion of creek bank soils with varying PCB concentrations. Outputs from the model were used to develop annual estimates of PCB flux from different creek locations and erosion ratings (e.g., banks in the upstream portion of the creek with moderate erosion). These results helped identify where targeted actions would have the largest reductions in annual PCB flux. We used erosion model outputs and surface water PCB flux measurements, including an upstream tributary, as inputs to a sediment recovery model. The recovery model calculated reductions in sediment PCB concentrations for a downstream portion of the Site that is accessible for public fishing.
Results/Lessons Learned. The creek bank evaluation results indicate that targeting the banks along the upper 10 miles of the creek with erosion rankings of severe, moderate, and minor could have a measurable effect in reducing annual PCB flux to the downstream portions of the Site. The calculated reduction in PCB flux associated with these targeted creek banks areas combined with remedial actions conducted for the upstream portions of the Site is projected to result in meaningful reductions in fish tissue PCB concentrations, which will likely be a significant remedial driver for the Site.