Organic Carbon−Water Concentration Quotients (Πsocs and πpocs): Measuring Apparent Chemical Disequilibria and Exploring the Impact of Black Carbon in Lake Michigan

Chemical concentration quotients measured between water and total organic carbon (TOC) in sediment (Πsoc) or suspended particulates (πpoc) in southern Lake Michigan reveal up to 2 orders of magnitude differences for polychlorinated biphenyl (PCB), dibenzo-p-dioxin (PCDD), dibenzofuran (PCDF), and polycyclic aromatic hydrocarbon (PAH) compounds with similar octanol–water partition coefficients (K ows). Apparent disequilibria for PAHs, PCDDs, and PCDFs, determined as measured Πsocs or πpocs divided by their organic carbon equilibrium partitioning values, are significantly greater than disequilibria of PCBs with similar K ows. Apparent disequilibria, when adjusted for black carbon content by using published black carbon nonlinear partition coefficients (K f,bcs) and a Freundlich exponent (n f) value = 0.7, still exceed equilibrium predictions for the PAHs, PCBs, and PCDDs but with the PCDF disequilibria uniquely below equilibrium. While Monte Carlo analysis of all the variables associated with the black carbon adjusted disequilibria provides wide confidence intervals for individual chemicals, the large class disequilibria differences between PAHs and PCDFs with respect to the PCBs and PCDDs are highly significant. Use of the PCDD K f,bcs for calculating both the PCDF and PCDD disequilibria eliminates their extreme divergence. On the basis of the complexity of carbonaceous geosorbent effects and the apparent variable degrees of chemical sequestration in particles, the disequilibria can be adjusted by chemical class to meet expected near equilibrium conditions between suspended particles and water in the hypolimnion. Although these adjustments to the disequilibria calculations produce consistent and plausible values, the complexities of variable carbonaceous geosorbent affinities for these chemicals in Lake Michigan presently favor use of measured, rather than a priori modeled, steady-state total organic carbon−water concentration quotients indexed to TOC as biogenic organic carbon.