Physiochemical weathering

Steady-state modeling calculations were performed to examine how congener-specific properties (such as sediment-water partition coefficients, Henry s law constants, and molecular diffusion rates) affect the transport and fate of PCBs. A basic description of the model, along with modeling results, is presented here to further explain the importance of physiochemical weathering processes in controlling the fate and distribution of PCB congeners in Twelve Mile Creek and the upper portion of Lake Hartwell. 

Overall, the steady-state model for physiochemical weathering provided a good description of observed variations in congener distributions for sur-ficial sediments in the Twelve Mile Creek-Lake Hartwell system. In general,... 

Steady-state model calculations were performed to further examine physiochemical weathering behavior. Results were consistent with congener distributions in surficial sediments. In general, they showed that a preferential depletion of the lower chlorinated congeners occurred in the upper portion of Twelve Mile Creek, where volatilization was the only removal mechanism. In the lower portion of Twelve Mile Creek and the upper portion of Lake Hartwell, burial of PCBs in deeper sediments played a more important role. A preferential depletion of higher chlorinated congeners occurs when burial is the dominant process by physiochemical weathering. 

Differences in the hydrologic characteristics for the upper and lower portions of Twelve Mile Creek and for Lake Hartwell are believed to have a significant effect on physiochemical and biochemical weathering processes. The upper portion of Twelve Mile Creek (0-32 km) is characterized by relatively shallow waters and high water velocities. In the lower portion (32-39 km), the creek deepens and widens because of the impoundment of Lake Hartwell. Water velocities are much slower in this portion of the creek, and a significant portion of the solids load is deposited in the sediments. Flow from Twelve Mile Creek eventually enters the top of Lake Hartwell, where it mixes with uncontaminated waters from the Keowee River. The waters then move slowly down this 50-km-long reservoir toward the Hartwell Dam. 

Examination of the literature regarding bioavailability of TCDD from environmental materials (Table I) reveals several important points. (1). Bioavailability of TCDD appears to be quite variable, depending on the site and material tested, and the species used as an assay system. (2). Laboratory contaminated samples yield different reactions than environmentally contaminated samples. It appears that TCDD added to soil in the laboratory is much more bioavailable, at least initially, than TCDD on materials contaminated in the environment. (3). Apparently the amount of time TCDD has been in contact with the substrate greatly affects its bioavailability and chemical extractablility. This effect may be a physiochemical ageing or a weathering phenomena. 

Hablot, E., et al. (2014). Effect of simulate weathering on physiochemical properties ad inherent biodegradation of PLA/PHA nonwoven mulches. Journal of Polymers the Environment, 22(4), 417-429. 

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