PCDD/F and PCB water column partitioning examination using natural organic matter and black carbon partition coefficient models

Howell, Nathan L.; Rifai, H. S.

PCDD/F and PCB water column partitioning examination using natural organic matter and black carbon partition coefficient models

2016

Howell, Nathan L. | Rifai, H. S.

A 9-year water dataset from the Houston Ship Channel (HSC) was analyzed to understand partitioning in polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans (PCDD/Fs). Total PCBs had more mass as dissolved (74 %) whereas total PCDD/Fs did not (11 %). Generally, the limited number of PCDD/Fs (only 2378 substituted) explained these differences though differences in chemical behavior beyond log K ₒw also likely influence partitioning. The particular fractionation seen in the HSC also seemed related to a wide variation in particulate organic carbon (POC)/dissolved organic carbon (DOC) ratio (0.42–180 %). Published and unaltered linear free energy and linear solvation energy relationships for DOC, POC, and particulate black carbon (BC) resulted in predictions that were at best 27 % (PCB) and 25 % root-mean-square error (RMSE) (PCDD/F) partition fraction compared to observed (using estimated BC/POC fractions of 10 and 25 %, respectively). These results show, at least in light of the uncertainties in this data (e.g., precise fraction of BC), that a 25 % accuracy in model prediction of operationally dissolved or suspended fraction for any one PCB or PCDD/F congener is the best prediction that may be expected. It is therefore recommended that site-specific data be used to calibrate most any water column-partitioning model if it is to be expected to describe what actually occurs in field conditions.

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