Modeling Field Scale Performance of a FeS - Permeable Reactive Barrier for Arsenic Plume Remediation

Reactive transport modeling is used to assess the potential long-term performance of a FeS-coated sand based permeable reactive barrier (PRB) for remediation of arsenic contaminated groundwater. The laboratory-calibrated 2-D multi-component reactive transport model was developed using PHAST, a versatile groundwater flow and reactive solute transport simulator developed by the USGS.  Field simulation scenarios were based on the features of a real arsenic contaminated site in Maine. The model explicitly includes arsenic release from reductive dissolution of host iron oxides under different scenarios, as well as the treatment of the arsenic plumes by a FeS-coated sand PRB installed downstream in the aquifer. Arsenic retention in the PRB was modeled as a combination of precipitation as arsenic sulfide minerals and pH-dependent surface complexation onto the mineral assemblage on the FeS-coated sand. Model simulations reveal that, under field conditions, the composition of the arsenic plume can be very complex, consisting of multiple contaminants of concern that may affect the PRB performance in different ways. Results suggest that combining the FeS-coated sand PRB with an upstream bio-PRB is likely the best option to treat such a reductive dissolution induced arsenic plume. However, the risk of arsenic remobilization under changing chemical conditions in the PRB must be considered. Excavation and safe disposal of the exhausted PRB material and replenishment with new material should be conducted periodically, based on close monitoring of groundwater chemistry within the PRB and in the aquifer.