Combining Leaching and Passive Sampling To Measure the Mobility and Distribution between Porewater, DOC, and Colloids of Native Oxy-PAHs, N-PACs, and PAHs in Historically Contaminated Soil

Different methods to quantify soil porewater concentrations of contaminants will provide different types of information. Passive sampling measurements give freely dissolved porewater concentrations (Cpw,free), while leaching tests provide information on the mobile concentration (Cpw,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 Cpw,leach was up to 5 orders of magnitude higher than Cpw,free; implying large biases when Cpw,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 KOW > 4. Average DOC/water-partitioning coefficients (KDOC) correlated well with KOW (log KDOC = 0.89 × log KOW +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 Cpw,free correctly or not using historically contaminated soils. POC/water-partitioning coefficients (KPOC) were orders of magnitude larger than corresponding KDOC, suggesting sorption to mobile particles/colloids is the dominant mechanism for PAC mobility.