Single Event–Driven Export of Polycyclic Aromatic Hydrocarbons and Suspended Matter from Coal Tar–Contaminated Soil

Abstract

Mobile colloidal and suspended matter is likely to affect the mobility of polycyclic aromatic hydrocarbons (PAHs) in the unsaturated soil zone at contaminated sites. We studied the release of mobile particles and dissolved organic matter as a function of variable climatic boundary conditions, and their effect on the export of PAHs at a coal tar–contaminated site using zero‐tension lysimeters. Seepage water samples were analyzed for dissolved organic carbon (DOC), pH, electrical conductivity, turbidity, and particles larger than 0.7 μm. The 16 Environmental Protection Agency PAHs were analyzed in the filtrate <0.7 μm and in the particle fraction. Our results show that extended no‐flow periods that are followed by high‐intensity rain events, such as thunderstorms, promote the mobilization of particles in the size 0.7 to 200 μm. Mobilization is enforced by extended drying during summer. High particle concentrations are also associated with freezing and thawing cycles followed by either rain or snowmelt events. The export of PAHs is strongly connected to the release of particles in the 0.7‐ to 200‐μm size fraction. During the 2‐yr monitoring period, up to 0.418 μg kg−1 PAHs were mobilized in the filtrate (<0.7 μm) while the eightfold mass, 3.36 μg kg−1, was exported with the retentate (0.7–200 μm). Equilibrium dissolution of PAHs and transport in the dissolved phase seem to be of minor importance for the materials studied. Extreme singular‐release events occurred in January 2003 and January 2004, when up to 55 μg L−1 PAHs per one single seepage event were observed within the retentate. Freezing and thawing cycles affect the PAH source materials, that is, the remnants of the nonaqueous phase liquid (NAPL). High mechanical strain during freezing results in the formation of particles. At the onset of the thawing and following rain or snowmelt events, PAHs associated with these particles are then exported from the lysimeter.