[1] Heterogeneity of subsurface environments and insufficient site characterization are some of the reasons why decisions about groundwater exploitation and remediation have to be made under uncertainty. A typical decision maker chooses between several alternative remediation strategies by balancing their respective costs with the probability of their success or failure. We conduct a probabilistic risk assessment (PRA) to determine the likelihood of the success of a permeable reactive barrier, one of the leading approaches to groundwater remediation. While PRA is used extensively in many engineering fields, its applications in hydrogeology are scarce. This is because rigorous PRA requires one to quantify structural and parametric uncertainties inherent in predictions of subsurface flow and transport. We demonstrate how PRA can facilitate a comprehensive uncertainty quantification for complex subsurface phenomena by identifying key transport processes contributing to a barrier's failure, each of which is amenable to uncertainty analysis. Probability of failure of a remediation strategy is computed by combining independent and conditional probabilities of failure of each process. Individual probabilities can be evaluated either analytically or numerically or, barring both, can be inferred from expert opinion.

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Artificial recharge improves several water quality parameters, but has only minor effects on recalcitrant pollutants. To improve the removal of these pollutants, we added a reactive barrier at the bottom of an infiltration basin. This barrier contained aquifer sand, vegetable compost, and clay and was covered with iron oxide dust. The goal of the compost was to sorb neutral compounds and release dissolved organic carbon. The release of dissolved organic carbon should generate a broad range of redox conditions to promote the transformation of emerging trace organic contaminants (EOCs). Iron oxides and clay increase the range of sorption site types. In the present study, we examined the effectiveness of this barrier by analyzing the fate of nine EOCs. Water quality was monitored before and after constructing the reactive barrier. Installation of the reactive barrier led to nitrate-, iron-, and manganese-reducing conditions in the unsaturated zone below the basin and within the first few meters of the saturated zone. Thus, the behavior of most EOCs changed after installing the reactive barrier. The reactive barrier enhanced the removal of some EOCs, either markedly (sulfamethoxazole, caffeine, benzoylecgonine) or slightly (trimethoprim) and decreased the removal rates of compounds that are easily degradable under aerobic conditions (ibuprofen, paracetamol). The barrier had no remarkable effect on 1H-benzotriazole and tolyltriazole.