Preliminary Evaluation of a Permeable Reactive Barrier for Reducing Groundwater Nitrate Transport from a Large Onsite Wastewater System

Abstract

A permeable reactive barrier was installed between a large Onsite Wastewater System (OWS) and a monitoring well located down-gradient from the OWS. Groundwater samples from the well had shown elevated and increasing concentrations of NO₃^--N (>20 mg L^-1 for 2 years). The barrier was constructed using woodchips from various tree species that were placed in a trench excavated to approximately the same depth as the well which was experiencing elevated NO₃^--N concentrations. Groundwater samples (5) were collected from the well between two weeks and four months after the barrier was installed. Groundwater NO₃^--N concentrations dropped by an average of 5 mg L^-1, while Dissolved Organic Carbon (DOC) concentrations increased by 11 mg L^-1 and Cl concentrations stayed relatively stable (<2 mg L^-1 increase). The data suggest that denitrification fueled by the DOC from the barrier may be contributing to the lower NO₃^--N concentrations. Monitoring will continue at the site to determine the longer-term water quality trends. More research should be conducted to evaluate the applicability of permeable barriers as a best management practice for non-point sources of pollution in nutrient sensitive areas.