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The Delaware River Estuary is listed as impaired due to elevated levels of PCBs in fish tissue. A Stage 1 Total Maximum Daily Load (TMDL) was developed using an integrated mass balance modeling approach for hydrodynamics, sorbent (organic carbon) dynamics and PCB transport and fate. The calibrated model was used to conduct diagnostic and sensitivity analyses to better understand PCB dynamics in the estuary, and forecast simulations to support development of the TMDL.

Under existing conditions, the principal external sources of PCB loadings are the following, in order of decreasing importance: non-point source runoff, point discharges, Delaware River at Trenton, Schuylkill River, runoff from contaminated sites, the sum of all other tributary inputs, CSOs, and atmospheric wet and dry deposition. Water column PCB concentrations are influenced by ongoing watershed sources as well as legacy contamination in the sediments. They are also influenced by exchanges across external boundaries with the C&D Canal and Atlantic Ocean, and the air-water interface. The surface sediments are a net source of PCBs to the water column in the upper estuary and a net sink in the lower estuary because of disequilibrium between water column and surface sediment PCB concentrations in these regions. For the estuary as a whole there is net PCB flux from the water column to the surficial sediments, and net burial of PCBs from the surficial sediments to the deep sediments. Net volatilization of PCBs occurs throughout the estuary.

Current water column concentrations of total PCBs in the estuary are several hundred times higher than the water quality criteria; consequently, substantial reductions will be required in external PCB loadings. TMDLs were determined for four water quality zones in the estuary under long-term, quasi-steady state, equilibrium conditions. Under these conditions there is no net flux of PCBs across the air-water interface, and both the surface and deep sediment layers are net sinks for PCBs throughout the estuary. Loading reductions will require implementation actions both within and outside of the Delaware River Basin. External PCB loadings from each individual source category are substantially higher than the TMDL for the entire estuary. PCB concentrations at the C&D Canal and Atlantic Ocean boundaries are approximately 50 to 300 times higher than the water quality criteria and are sufficient by themselves to cause violations throughout much of the estuary. Atmospheric gas phase PCB concentrations are two orders of magnitude higher than the gas phase PCB concentrations required to be in equilibrium with dissolved phase PCB concentrations at the water quality criteria.

Remediation of sediment PCBs is not necessary to achieve the water quality criteria because under the TMDL conditions the sediments are a net sink for PCBs not a source. The significance of sediment PCBs is their influence on the response time of the estuary to changes in external PCB loadings. Model results indicate that depending on location within the estuary, water column PCB concentrations could require 20–30 years to achieve the water quality criteria even if all external PCB loadings were completely eliminated. Although remediation of sediment PCBs is not necessary to achieve the water quality criteria under the TMDL design conditions, it could potentially shorten the time frame required for achievement.

Document Type: Research Article


Publication date: January 1, 2005

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