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 parti tioning (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 (( K-oc)(obs)) of polychlorinated biphenyls, polychlorinated benzenes, polychl orinated dibenzo-dioxins and -furans, and p,p'-dichlorodiphenyltrichloretha ne (DDT) with metabolites were selected from the literature and compared wi th their respective OMP model estimates. For all compound classes and in a majority of the investigated cases, (K-oc)(obs) values were significantly l arger than predicted. This translated into factors of overestimated dissolv ed exposures ranging from 1 to 1,000. Various reasons are discussed for the discrepancies between predictions and actual observations, such as the eff ect of the diagenetic state and other properties of the particulate organic matter. The greater enhancement in (K-oc)(obs) of planar over nonplanar co mpounds suggests in certain cases that efficient interactions with aromatic soot phases may be significant. For an improved predictability of (K-oc)(o bs) and dissolved exposures of HOCs in the real environment, the inclusion of Boot and possibly other distinct subfractions of bulk organic carbon int o an extended solid-water partitioning model may be considered.