Combining Leaching and Passive Sampling To Measure the Mobility and Distribution between Porewater, DOC, and Colloids...

Different methods to quantify soil porewater concentrations of contaminants will provide different types of information. Passive sampling measurements give freely dissolved porewater concentrations (C_pw,free), while leaching tests provide information on the mobile concentration (C_pw,leach), including contaminants associated with dissolved organic carbon (DOC) and particles/colloids in the porewater. This study presents a novel combination of these two measurements, to study the sorption and mobility of polycyclic aromatic compounds (PACs) to DOC and particulate organic carbon (POC) in 10 historically contaminated soils. The PACs investigated were polycyclic aromatic hydrocarbons (PAHs), oxygenated-PAHs, and nitrogen containing heterocyclic PACs. Observed C_pw,leach was up to 5 orders of magnitude higher than C_pw,free; implying large biases when C_pw,leach is used to assess bioavailability or soil partitioning. Sorption of PACs to DOC and POC was important for the mobility of compounds with log K_OW > 4. Average DOC/water-partitioning coefficients (K_DOC) correlated well with K_OW (log K_DOC = 0.89 × log K_OW +1.03 (r² = 0.89)). This relationship is likely more accurate for historically contaminated soils than previously published data, which suffer from artifacts caused by problems in measuring C_pw,free correctly or not using historically contaminated soils. POC/water-partitioning coefficients (K_POC) were orders of magnitude larger than corresponding K_DOC, suggesting sorption to mobile particles/colloids is the dominant mechanism for PAC mobility.