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Developing Total Maximum Daily Loads (TMDLs) for a complex pollutant in a complex estuarine ecosystem with numerous point and non-point sources is an enormous task requiring substantial levels of effort, funding and time. Stage 1 TMDLs for polychlorinated biphenyls (PCBs) in four sections (Zones 2-5) of the tidal portion of the Delaware River Estuary were developed using an innovative approach. Pentachlorobiphenyls (penta-PCBs) were used as a surrogate for the total PCBs to develop the Stage 1 TMDLs.

The fate of PCBs is closely linked to carbon dynamics and sediment-water interactions because of their hydrophobic chemical property. Even though the TMDLs are targeted to the compliance with water column concentrations of total PCBs, TMDLs must be developed under the equilibrium conditions because PCBs concentrations in water column are affected not only by external PCB loads but also by internal loads from the sediments until the system reached to the equilibrium condition. Hydrologic and hydrodynamic conditions of representative single year were extracted and cycled for decadal scale model simulations to establish the equilibrium conditions. The net flux exchanges between the air-shed and water column, and sediment layer and water column will be zero when all the systems within the estuary are reached to equilibrium condition when the TMDLs are achieved. This allows us to ignore the interactions of water column with air shed and sediment layers in the TMDL allocation simulations since no net flux exchanges are expected. Dramatic time and efforts were saved by applying this equilibrium concept in TMDLs development. In addition, the finding of a critical location for the compliance of the applicable water quality criteria enabled the use of a simplified TMDL development approach that focused on the determination of assimilative capacity at the critical location, and the determination of the contributions of the loadings to each zone to the critical location. These contributions, relationships between loadings and response or contribution factors (CFs), were developed for each management zone. Because of the linearity of the CFs, allowable loading of penta-PCBs for each management zone could be back calculated while maintaining the allowable assimilative capacity at the critical location in Zone 5.

The approach of using contribution factors can be applied in other water bodies where water quality standards have a sharp spatial transition. The approach may be especially attractive for persistent bioaccumulative pollutants, like PCBs, which often require model forecasts that extend for several decades due to their sorption properties and in water bodies where water quality standards dramatically change due to the presence of state or international boundaries or to changes in the controlling water quality standards. Selective approaches or the entire concept can be applied to the development of TMDLs in other water bodies.
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Document Type: Research Article

Publication date: 01 January 2004

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