Transport and Fate Model for PCB Total Maximum Daily Loads (TMDLs) in the Potomac River Estuary

The Potomac River estuary is listed as impaired under Section 303(d) of the Clean Water Act due to the levels of polychlorinated biphenyls (PCBs) in resident fish species, thus requiring development of a Total Maximum Daily Load (TMDL). The impaired region includes the tidal freshwater portions of the Anacostia and Potomac rivers, and extends approximately 118 miles downstream to the mouth of the Potomac Estuary at Chesapeake Bay. There are 28 PCBimpaired waterbody segments in the Potomac that represent three different jurisdictions (DC, Maryland and Virginia), each with different water quality standards.PCBs are hydrophobic organic chemicals whose transport and fate are strongly influenced by sorption to organic carbon and interactions between the water column and bedded sediments. To determine the TMDL an integrated mass balance model for hydrodynamics, organic carbon dynamics, and PCB transport and fate was developed. External loading sources included tributaries, direct drainage, wastewater treatment plants, combined sewer overflows, contaminated sites, and atmospheric wet and dry deposition. Autochthonous organic carbon loadings were specified using output from the Chesapeake Bay Water Quality Model. The PCB model was applied to a 257-segment spatial grid representing the Potomac and Anacostia rivers, and calibrated to available data for 2002–2005.Evaluation of model skill was a weight-of-evidence approach that used multiple quantitative metrics and best professional judgment. The metrics included time series plots at fixed locations, spatial profiles at fixed points in time, bivariate plots with lines of 1:1 correspondence, regression statistics, cumulative frequency distributions, comparisons of seasonal median values, and comparisons of computed first-order PCB loss rates with those from available historical data for PCB body burdens in benthic feeding fish.The primary drivers for contemporary PCB concentrations in the water column are mass loads from the Potomac at the upstream boundary, legacy contamination in the sediments, and sediment-water mass transfer rates. Most of the PCB mass enters the system from the upstream boundary and is either transported downstream to Chesapeake Bay or lost to gas phase flux (volatilization). There is attenuation of surface sediment PCB concentrations due to solids cycling and bio-enhanced diffusion across the sediment-water interface, and net solids burial to deeper sediment layers.The TMDL design conditions correspond to quasi-steady state, dynamic equilibrium among external PCB mass loads, and concentrations in the water column and sediments. Under these conditions there is no net flux of PCB across the air-water interface, and both the surface and deep sediment layers are net sinks for PCB, not sources. Diagnostic simulations were conducted with the calibrated model to evaluate the impact of legacy contamination in the sediments on present PCB concentrations in the water column, and the time required to achieve dynamic equilibrium. Results from these simulations indicated that approximately 50 years or more is required to reach the TMDL design conditions of quasi-steady state, dynamic equilibrium.Given the model assumptions and the available data for model inputs and ambient water quality conditions, results from the calibrated model are a reasonable representation of seasonal magnitudes and spatial distributions for water surface elevation, salinity, organic carbon sorbents, and PCBs in the tidal Potomac and Anacostia Rivers. In consideration of the overall weight-of-evidence, the Potomac PCB TMDL Steering Committee judged that the model was scientifically credible and acceptable for use in developing the PCB TMDL. The final PCB TMDL was approved by U.S. EPA Region 3 on October 31, 2007.