Modeling of polychlorinated biphenyls (PCBs) in the Baltimore Harbor

Abstract

A three-dimensional polychlorinated biphenyls (PCBs) model is developed to explore the fate and transport processes of PCBs in the Baltimore Harbor (BH), a coastal embayment located in a highly urbanized watershed of the Chesapeake Bay. The PCB model is an organic carbon-based model coupled with an eutrophication model and is driven by a three-dimensional hydrodynamic model. The modeling framework links nonpoint source loadings, estuarine circulation, bottom sediment dynamics, and air–water exchange, and is capable of assessing estuarine restoration. High-resolution, curvilinear-orthogonal grids are used to represent the complex geometry of the BH for simulating its unique three-layer estuarine circulation induced by transport processes. Model sensitivity tests are conducted to evaluate model uncertainties with respect to model parameters, loadings, and boundary conditions. Analyses of measurements indicate that tetra-, penta-, and hexa-PCBs are the dominant homologs in the BH and the sum of these homologs accounts for 60.1% and 61.7% of the total PCB (t-PCB) concentration in the water column and the bottom sediment, respectively. These three homologs are simulated independently by the model and the results are used to derive the t-PCB concentration based on the statistical relations derived from data analyses. Model simulations indicate that PCB loadings from the bottom sediment, nonpoint sources, atmospheric deposition, and point sources account for 63.4%, 24.6%, 9.5%, and 2.6% of the t-PCB input to the BH, respectively. The sediment flux dominates the PCB sources to the water column. A major pathway for PCB loss is volatilization (384ng/m2/day), which is approximately 8.6 times the atmospheric deposition through the surface of the BH. The PCB exchanges between the BH and the Chesapeake Bay resulted in a net transport of PCBs out of the BH, but the loss of PCBs due to such net transport is much less than that due to volatilization. The results indicate that the model is reliable for simulating PCBs in estuaries.