CONCEPTUAL MODELING FRAMEWORK FOR THE PCB TMDL IN THE DELAWARE RIVER ESTUARY

The Delaware River Estuary was listed as impaired under Section 303(d) of the Clean Water Act by three bordering states due to the levels of PCBs in fish tissue. Court mandates and administrative agreements required development of a total maximum daily load (TMDL) for PCBs in December 2003. Several factors complicated the TMDL process including the complexity of the system, the limited available data on loadings and ambient concentrations, and an ambitious schedule.The transport and fate of toxic chemicals, especially hydrophobic organic chemicals (HOCs), is strongly influenced by sorption to organic carbon and interactions between the water column and bedded sediments. The conventional modeling approach is to represent total solids concentrations, often in terms of several different size classes, and assign constant fractions of organic carbon to water column and sediment solids. A disadvantage of this approach is that much effort can be expended on determination of external solids loadings and sediment transport dynamics, however, it is primarily fine-grain (silt and clay) solids and not coarse-grain (sand and gravel) solids that are the important sorbents for HOCs. Another disadvantage is that a large proportion of the organic carbon in aquatic systems can be produced internally by algal primary production, which is not represented in conventional models for total solids.We propose a simplified modeling approach that avoids the complexities of explicit representation of sediment transport, primary production or sediment diagenesis. This approach accounts for the principal organic carbon sorbents in the water column, and for net burial of solids and ultimate fate of organic carbon in the sediments. Mass balances are conducted only for biotic carbon (BIC) in the water column and particulate detrital carbon (PDC) in the water column and sediments. Simplifications in the water column include external specification of dissolved organic carbon (DOC) concentrations and BIC loadings from primary production. Simplifications in the sediment include external specification of DOC concentrations, temporally constant values for solids porosity and fraction organic carbon, and a first-order decay rate for PDC. This approach accounts for net solids burial and losses of organic carbon to diagenesis, and constrains the relationship between these parameters to be consistent with observed sediment properties and sediment oxygen demand.This conceptual model was implemented using a modified version of WASP5/TOXI5 and applied to penta-PCBs in the Delaware River Estuary. The model provided reasonable representations of spatial and temporal distributions of organic carbon and penta-PCB concentrations, and was judged acceptable for use in establishing the Stage 1 PCB TMDL. As part of an adaptive management strategy, this modeling approach can be especially useful for TMDLs in complex systems that have limited available data and/or demanding schedules. This approach is also sufficiently generic for application to other HOCs and heavy metals, and is transferable to other physical systems including streams, rivers, lake or estuaries.