Difference in PAH release processes from tar-oil contaminated soil materials with similar contamination history

Tar-oil contaminated sites, for example, former manufactured gas- and tar-processing sites, pose a continuing threat to soil and groundwater in Europe. In this study, the release processes of polycyclic aromatic hydrocarbons (PAHs) of five soil materials from four locations in Germany are discussed. All materials were investigated by means of column outflow experiments. Variable flow conditions were applied to reveal possible rate-limited release processes. Flow conditions and high-resolution break through curves of the master variables dissolved organic carbon, turbidity, pH and electrical conductivity, are examined in consideration of their possible influence on PAH release. Although the history of the sites is similar with respect to age and type of contamination, different processes and time scales control the release and discharge of PAHs. For example, the release from a silt from Rositz and from a sandy reference material was governed by equilibrium and according to Raoult's Law, while a gravel from Munich and a clayey marlstone from the ‘Testfeld Süd’ released PAHs bound to particles and at non-equilibrium. Physico-chemical gradients as well as the nature of the residual tar phase are key parameters for the characteristics of release and transport. The results show that risk assessment at sites contaminated with tar-oil requires an experimental evaluation in order to estimate the PAH fluxes. Besides, it is crucial to consider the effect of mobile particles.