Partitioning of polynuclear aromatic hydrocarbons between sediments from San Francisco Bay and their porewaters

The in situ sediment-porewater partition coefficients (K-oc's) of a suite of 3-6 ring polynuclear aromatic hydrocarbons (PAHs) increased with the organic carbon content and with the amount of fines in surface sediments collected from a mudflat in San Francisco Bay along an intertidal gradient during the dry and wet seasons in 1993-1994. The K-oc's were an order of magnitude or more higher during the wet period of high surface runoff than during the dry season; the in situ measurements bracketed the corresponding values of K-ow. Moreover, K-oc' decreased from the high- to the low-intertidal zone during the dry season, indicating heterogeneity in partitioning behavior along this spatial gradient. A plausible explanation for these trends in the partitioning behavior is a heterogeneity of the sediment organic carbon matrices, principally the content of highly aromatic soot particles; an increase in K-oc' might be anticipated in association with the lower activity coefficients of the aromatic PAHs in a highly aromatic solid phase matrix. If correct, this interpretation would qualify the utility of equilibrium partitioning models in formulating sediment quality criteria for PAHs.