Non-equilibrium conditions during organic contaminant mobilization, transport and degradation in the subsurface

Wehrer, Markus GND

Processes determining the fate of organic contaminants in the subsurface are often characterized by non-equilibrium conditions. This habilitation treatise explores the kinetic characteristics of the processes influencing the fate of organic contaminants in the subsurface employing computational-, laboratory- and field experiments. In the first part, release and transport processes of hydrophobic contaminants from multicomponent non-aqueous phase liquids (NAPLs) are examined in 5 individual studies. These comprise a review of the literature on the fate of contaminants at manufactured gas plants, laboratory experiments and modeling of the release kinetics of polycyclic aromatic hydrocarbons (PAHs) from aged and fresh tar phases, and a field study of a crude oil contaminated aquifer employing geophysical, hydrogeochemical and microbiological methods. The second part comprises three studies, which explore transport and degradation kinetics of hydrophilic compounds, which are used as deicing chemicals at airports. In this part, a laboratory percolation experiment was evaluated with a numerical model and the degradation of the deicing chemicals propylene glycol and formate under field conditions was characterized in lysimeters, employing, among other methods, electrical resistivity tomography (ERT). The studies show that multiple interdependencies exist between contaminant transport time-scales and time-scales of mobilization and degradation. Solubilization of PAHs from tar oils was diffusionally highly restricted with extremely long equilibration times. Hydrophobic compounds are often subject to mobilization and transport as colloids, which was observed in all experimental studies with aged NAPLs. Degradation kinetics of deicing chemicals were found to be dependent on a dynamic reaction of the biomass in column experiments and did not agree with first-order kinetics. The field lysimeter experiments showed that the ratio of degradation time-scale to transport time-scale determines the fate of propylene glycol. As a consequence, non-equilibrium transport conditions resulted in rapid discharge of large amounts of propylene glycol from the topsoil. The results have consequences for experimental procedures aiming for process identification and parameter quantification of mobilization, transport and degradation of organic contaminants. For a realistic estimation of risk of exposure, natural attenuation and biological remediation, experimental and numerical evaluation including transport conditions is inevitable.



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