Predictions of long-term performance of granular iron permeable reactive barriers: Field-scale evaluation

Long-term performance is a key consideration for the granular iron permeable reactive barrier (PRB) technology because the economic benefit relies on sustainable operation for substantial periods of time. However, predictions on the long-term performance have been limited mainly because of the lack of reliable modeling tools. This study evaluated the predictive capability of a recently-developed reactive transport model at two field-scale PRBs, both having relatively high concentrations of dissolved carbonate in the native groundwater. The first site, with 8 years of available monitoring data, was a funnel-and-gate installation, with a low groundwater velocity through the gate (about 0.12 m d^-1). The loss in iron reactivity caused by secondary mineral precipitation was small, maintaining relatively high removal rates for chlorinated organics. The simulated concentrations for most constituents in the groundwater were within the range of the monitoring data. The second site, with monitoring data available for 5 years, was a continuous wall PRB, designed for a groundwater velocity of 0.9 m d^-1. A comparison of measured and simulated aqueous concentrations suggested that the average groundwater velocity through the PRB could be lower than the design value by a factor of two or more. The distribution and amounts of carbonate minerals measured in core samples supported the decreased groundwater velocity used in the simulation. The generally good agreement between the simulated and measured aqueous and solid-phase data suggest that the model could be an effective tool for predicting long-term performance of granular iron PRBs, particularly in groundwater with high concentrations of carbonate.