Abstract

The fate of polycyclic aromatic hydrocarbons (PAH) is known to depend on the release and redistribution of dissolved organic carbon (DOC) and particles. We studied the release of PAH, DOC, and particles up to a size of 200 μm with column outflow experiments using gravelly soil material. The material was collected at an abandoned industrial tar-oil contaminated site. To detect rate-limited release, the experiments were performed at two different mean pore water velocities, while multiple flow-interrupts were imposed. Effluent was analyzed for DOC, pH, electrical conductivity, turbidity, and particles larger than <0.7 μm after filtration. The 16 Environmental Protection Agency PAHs were analyzed in the filtrate and in the particle fraction. Upon onset of flow large initial effluent concentrations were found for DOC, particles, turbidity, and particle-associated PAHs. This so-called first-flush export levelled off after a few pore volumes had been exchanged. The release of DOC and PAH in the filtrate was strongly rate limited. Measured PAH concentrations differed markedly from those calculated by Raoult's law. Equilibrium dissolution seems to be of minor importance for the studied materials. Particle release as well as the release of particle-associated PAHs was dependent on the flow velocity. However, effluent concentrations decreased significantly during no-flow conditions due to sedimentation of larger particles. At the lower flow velocity, 33% of the total PAHs were found in the retentate (66% in the filtrate), while at the higher flow velocity the amount of particle-associated PAH increased to 42%. The comparison of the PAH pattern in the filtrate and the retentate suggests that PAH transport takes place predominantly in the form of small NAPL droplets or fragments. The strong correlation of DOC with the PAH in the filtrate implies a marked influence of DOC on PAH transport. From these findings we conclude that PAH release and transport at contaminated sites is affected by three processes, i.e., (i) first-flush export, (ii) detachment of particle-associated PAHs due to hydraulic mobilization and (iii) the rate-limited release of particles and particle/colloid-associated PAHs.

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