Determination of effective release rates of polycyclic aromatic hydrocarbons and dissolved organic carbon by column outflow experiments

Risk assessment of groundwater pollution requires quantitative information on the release kinetics of pollutants and organic matter from contaminated soil. We applied a new experimental design for column outflow experiments to investigate the release of polycyclic aromatic hydrocarbons (PAHs) and dissolved organic matter under water-saturated flow conditions. We used materials originating from a soil contaminated with non-aqueous phase liquids. To distinguish between release at equilibrium and release limited by mass transfer, we used two flow velocities and multiple interruptions to the flow. We quantified release and transport parameters by inverse numerical simulation of the individual breakthrough curves, applying a model based on the advection–dispersion equation including non-equilibrium and non-linear sorption. Release of the dissolved organic C takes place in two steps. Initially, a large amount of readily available organic matter is released. This first flush is followed by an outflow with typical characteristics of rate-limited release: larger concentrations in slower flow and increased concentrations after interruptions. The breakthrough of PAHs responds neither to the different flow velocities nor to the interruptions. We hypothesize that release of PAHs from the contaminated material is governed by dissolution at equilibrium according to Raoult's law. The boundary conditions of the experimental design, i.e. the flow velocities and multiple interruptions, enable us to distinguish between release at equilibrium and that which is rate-limited. Also, the response of the breakthrough behaviour to the boundary conditions can be used to estimate inversely effective release parameters.