Long-term field study of nitrate and ammonium remediation using a permeable reactive barrier at a livestock feeding operation.

Protecting groundwater is a global challenge in modern agriculture. Nutrients from livestock detritus and manure have caused adverse effects on ecosystems and posed health risks associated with use of contaminated groundwater. Such occurred at a concentrated animal feeding operation (CAFO) where failed manure containment and buried carcasses led to toxic concentrations of ammonium and nitrate in groundwater. Here we evaluate a two-step approach to remediate ammonium using pump-and-treat technology, and nitrate using a permeable reactive barrier (PRB) composed of locally sourced hay as a carbon source to drive denitrification. Long-term monitoring (10-year dataset) revealed that effective mitigation of total nitrogen was accomplished through several mechanisms. Ammonium initially developed in the PRB from mineralization of protein in the hay; however, groundwater conditions permitted the possibility of dissimilatory nitrate reduction. Total nitrogen mitigation is attributed to anaerobic ammonium oxidation and denitrification. The reducing environment induced by the PRB caused reduction of iron oxyhydroxides as evidenced by increased dissolved iron and manganese in groundwater. Increased total phosphorus and arsenic mobilization was also locally observed. Some monitoring wells contained high levels of ammonium released from buried detritus. While the PRB effectively removed nitrate, elevated total phosphorus in stream water exceeded recommended limits and placed surface water at continued risk for eutrophication even ten years after installation. Locally sourced carbon sources deployed in a PRB can effectively mitigate nitrogen contamination in groundwater; however, future applications of organic carbon systems should consider the possible mobilization of secondary contaminants including phosphorus, arsenic, iron, and manganese.