Ubiquitous observations of enhanced solid affinities for aromatic organochlorines in field situations: are in situ dissolved exposures overestimated by existing partitioning models?

This paper investigates the ability of the traditional organic matter partitioning (OMP) model to predict the solid-water distribution, and hence the dissolved exposures, of hydrophobic organic compounds (HOCs) in real field situations. Observed organic-carbon-normalized partitioning coefficients (Koc)obs) of polychlorinated biphenyls, polychlorinated benzenes, polychlorinated dibenzo-dioxins and -furans, and p,p'-dichlorodiphenyltrichlorethane (DDT) with metabolites were selected from the literature and compared with their respective OMP model estimates. For all compound classes and in a majority of the investigated cases, (Koc)obs values were significantly larger than predicted. This translated into factors of overestimated dissolved exposures ranging from 1 to 1,000. Various reasons are discussed for the discrepancies between predictions and actual observations, such as the effect of the diagenetic state and other properties of the particulate organic matter. The greater enhancement in (Koc)obs of planar over nonplanar compounds suggests in certain cases that efficient interactions with aromatic soot phases may be significant. For an improved predictability of (Koc)obs and dissolved exposures of HOCs in the real environment, the inclusion of soot and possibly other distinct subfractions of bulk organic carbon into an extended solid-water partitioning model may be considered.