The Vertical Profile of PCBs at the Estuarine Turbidity Maximum Zone in a Coastal Plain River and the Influence of Salinity-Induced Flocculation on PCB Concentrations in Particles
Date
2010-01-27T19:50:32Z
Authors
Housman, Kathleen J.
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Abstract
Polychlorinated biphenyls (PCBs) are ubiquitous, persistent organic pollutants. It is known that this class of chemicals has a high affinity towards suspended particles in the water column based on their hydrophobic properties and relatively high octanol-water partition coefficients (K(ow)). An estuarine turbidity maximum (ETM) is a known region of enhanced concentrations of sediment in suspension. The dynamics of the ETM are driven by multiple variables including tidal resuspension, estuarine stratification, and electrical double-layer influenced flocculation. Because PCBs bind readily to sediments, the ETM is an important zone to study fate, distribution, and transport of contaminants in coastal rivers. The ETM is especially important in the Potomac River because it is located downstream of two major PCB hotspots, including the Anacostia River and Quantico Bay, VA. This region of the Potomac River has not been previously studied for PCB fate, and remains an important missing piece of the puzzle in understanding the downstream transport of PCBs from regions of severe contamination to the Chesapeake Bay and Atlantic Ocean. Environmental sampling of the Potomac River ETM took place on three occasions during the spring of 2008 and 2009. Potomac River water was sampled along a vertical transect at ~2 m intervals through the halocline of the ETM to 30 m depths using a submersible pump. Both suspended sediment and bed sediments were collected, extracted, and analyzed for PCBs. Water geochemistry parameters such as total suspended matter (TSM) and salinity along with sediment organic matter (OM) were also determined. PCB-bound suspended sediments were found at elevated levels as compared to bed sediments (238 ± 126 compared to 28 ± 13 ng/g), indicating that the ETM is not a likely depositional zone for PCBs in the Potomac River. It is likely that PCB-bound suspended materials are transported farther downstream and are widely dispersed in coastal waters. Because ETM sampling is extremely variable, a controlled laboratory study was performed by flocculating resuspended sediment material that was highly contaminated with PCBs. The goal was to study the influence of salinity-induced flocculation on the concentrations of PCBs in river particles, thus simulating the Potomac River ETM. The salinity-induced flocculation studies were performed on a 20-L scale by first mixing PCB contaminated sediment and water. The large particles in resuspended sediment were allowed to settle, and the salinity of decanted overlying water containing colloid-sized particles was enhanced to promote flocculation. The floc material was filtered, extracted, and analyzed for PCB concentrations. An unexpected trend of decreasing PCB concentrations in floc with increasing salinity was found for both PCBs and OM, indicating that dilution in particles results during PCB flocculation. In addition, evidence was found for the natural PCB homologue fractionation of PCBs along the downstream transect of the Potomac River from the Anacostia River hotpot to the ETM. The higher molecular weight, heavily chlorinated PCBs were more abundant in Anacostia River sediments while less chlorinated congeners dominate ETM sediments. It is clear from the current work that the ETM is a dynamic region and must be more rigorously studied in order to fully understand the dynamics of contaminant transport from highly contaminated regions such as the Anacostia River and Quantico Bay, VA to the Chesapeake.
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Keywords
Polychlorinated biphenyl, PCBs, ETM, Potomac River, Flocculation, Estuarine turbidity maxima