Long-term field study of a permeable reactive barrier for groundwater clean-up at an acid battery recycling site

Operations at a former battery recycler resulted in harmful concentrations of lead and other contaminants in groundwater. We report on a long-term monitoring study of a permeable reactive barrier (PRB) strategically deployed to intercept and treat groundwater using limestone and organic material (cattle manure and wood chip). The PRB neutralized groundwater acidity and created reducing conditions to drive sulfate reduction to sulfide by native bacteria. Increased pH and precipitation of low-solubility metal sulfides sequestered lead, cadmium, nickel, and zinc. In addition, the PRB effectively removed acidity via limestone dissolution and carbon mineralization over a 14-year period. No measurable evidence of PRB clogging or limestone armoring was observed over the course of 14 years of monitoring. Groundwater flow monitoring indicated a minor amount of bypass around the PRB. Analysis of core samples collected 14 years into operation revealed uptake of aluminum, cadmium, copper, iron, lead, and zinc within the PRB and downgradient soil. Scanning electron microscopy revealed co-precipitation of nickel with pyrite and precipitation as oxides with cubic and acicular morphologies. Lead was found in mixed precipitates, notably as sulfides. Reducing conditions precluded capture of arsenic and phosphorus which remained mobile in groundwater. While metal contaminants were safely sequestered as minerals in the PRB and in downgradient soil, risk for future re-mobilization may occur with potential return to acidic conditions.