REMEDIATION OF GROUNDWATER NITRATE FROM A LARGE ONSITE WASTEWATER SYSTEM USING A PERMEABLE REACTIVE BARRIER

A permeable reactive barrier (PRB) was installed down-gradient from a large onsite wastewater system (OWS) to mitigate elevated NO₃^--N concentrations in groundwater. Groundwater samples collected from a monitoring well 8 m from the OWS had shown elevated and increasing concentrations of NO₃^--N (> 20 mg/L for 2 years). The barrier was constructed using an excavator and had dimensions of 6 m (length), 1.2 m (width), and 8 m (depth). Woodchips of various sizes were placed in the open trench and then the trench was backfilled using excavated soil. Groundwater samples were collected from the down-gradient monitoring well five times over a four month period after the PRB was installed. Groundwater NO₃^--N concentrations dropped by an average of 5 mg/L, while dissolved organic carbon (DOC) concentrations increased by 11 mg/L, and Cl concentrations stayed relatively stable (< 2 mg/L increase). The data suggest that denitrification may be contributing to the lower NO₃^--N concentrations. Monitoring will continue at the site to determine the longer-term water quality trends. More work should be initiated to determine the applicability and feasibility of PRBs as a best management practice for non-point sources of pollution in nutrient sensitive areas.