WATER QUALITY MODEL DEVELOPMENT FOR AN INTERSTATE PCB IMPAIRMENT

Several segments of the Shenandoah River were identified on Virginia's and West Virginia's 1998 Section 303(d) list as impaired due to polychlorinated biphenyl (PCB) contamination (U.S. EPA, 2001). Because the impaired river crosses the Virginia-West Virginia border, the U.S. Environmental Protection Agency (EPA) Region 3, Virginia Department of Environmental Quality (VADEQ), and West Virginia Department of Environmental Protection (WVDEP), with the support of Tetra Tech, Inc., coordinated to develop Total Maximum Daily Loads (TMDLs) using a consistent methodology that considered all sources and evaluated all relevant water quality standards. To identify contributing PCB sources in the Shenandoah watershed, a focused sampling project was undertaken. This included collection of water column, sediment, and tissue samples at locations upstream, downstream, and within the impaired river segments to better ascertain the spatial distribution of PCBs in the aquatic environment. Data collected during the sampling project filled-in unknowns related to correlations between water column, sediment, and tissue PCB levels and contributed to developing an analytical model of the system.In order to establish a TMDL for the Shenandoah River, it was necessary to represent the linkage between source contributions and in-stream response for the river. This linkage was developed through construction of a site-specific simplified mass balance model. The model considers potential sources, critical in-stream processes and interactions, and a suite of relevant water quality targets, including fish tissue concentrations and water column standards based on risk assessment methods. Technically, it operates as a series of plug-flow reactors with source inputs at different longitudinal locations. The reactors represent PCBs in sediment and water phases and consider mechanisms such as burial, resuspension, and diffusion. Water column and sediment PCB concentration profiles are determined with respect to distance along the river. Data from the sampling project were used to define boundary conditions and inputs to the model. To identify allowable source-based load inputs to the system while satisfying both water column and tissue criteria, the model was run under critical conditions.