Assessment of Nitrate Concentrations in Groundwater Near an Onsite Wastewater System Before and After Installation of a Permeable Reactive Barrier

Abstract

There are approximately 2 million onsite wastewater systems (OWS) in North Carolina that provide soil-based treatment of wastewater. Wastewater contains elevated concentrations of environmental pollutants including nitrogen. Prior studies have shown that many OWS are efficient at oxidizing nitrogen, but groundwater plumes enriched with nitrate (NO3-) may extend to surface waters and/or wells creating various environmental and public health risks. Groundwater monitoring near the OWS drain field of a school in Eastern North Carolina showed NO3- concentrations routinely exceeded the 10 mg/L groundwater standard. In 2014, a permeable reactive barrier (PRB) was installed between the OWS and a monitoring well (with elevated NO3-) to enhance nitrogen removal via denitrification. The PRB was constructed by excavating a trench with the approximate dimensions of 1.2 m wide x 6 m long x 8 m deep. The bottom of the trench was dug below the water table. Woodchips were used to fill the bottom 2 to 3 m of trench, and the rest of the trench was filled with the excavated soil. The woodchips were used as a carbon source for denitrifying microorganisms. Groundwater samples were collected from the well and analyzed for NO3- three times each year (2005- 2023) following the installation of the PRB. Groundwater NO3- concentrations were lower post (most < 8 mg/L) relative to pre (mean = 13.7 mg/L) PRB installation. Results show that PRBs may be effective practices for reducing the groundwater transport of NO3- for many years with little to no maintenance