Evaluation of a Permeable Reactive Barrier for Reducing Groundwater Nitrogen Transport
Grant
Overview
abstract
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Rodgers Elementary School in Bear Grass, NC uses an onsite wastewater treatment system (OWS), which is also known as septic system, to treat wastewater generated at school. When the OWS was installed, three monitoring wells were also installed around the perimeter of the OWS for groundwater sample collection and analysis. Over the past several years the NO3-N (form of nitrogen) concentrations increased in one of the wells that is down-gradient from the onsite system, and is now consistently higher than the standard of 10 mg/L. The overarching goal of the study is to reduce septic system derived nitrogen transport by implementing a best management practice that is called a Permeable Reactive Barrier (PBR). A PRB is a trench that is excavated down-gradient from the sources of NO3, and at the depth of the NO3 plume. The trench is backfilled with a carbon source such as woodchips and native soil. When groundwater with NO3 flows through the woodchips, microorganisms can convert the NO3 to a gas (N2) that escapes into the atmosphere. This process reduces groundwater NO3 concentrations by transforming dissolved NO3 into a gas. A similar process happens when groundwater with NO3 flows through wetland and riparian areas near streams and lakes. The barrier helps reduce NO3 concentrations but allows groundwater to flow through the barrier. Prior studies have shown that barrier can reduce groundwater NO3 concentrations by more than 70%. We hope the PRB that will be installed at Rodgers Elementary will also be effective at reducing groundwater NO3 transport.
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