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.
Highlights
Onsite Wastewater Treatment Systems and NitrogenOnsite Wastewater treatment Systems (OWS) are used in locations where centralized sewer service is not available
There was a moderate (r = 0.685) but significant (p = 0.029) correlation between the NO3--N concentrations and years after sampling was initiated. These data indicate that groundwater downgradient from the Onsite Wastewater System (OWS) was becoming more enriched with NO3--N over time, especially near Well 2
Groundwater NO3--N concentrations steadily increased by 1.8 mg L−1 each year since 2006 near Well
Summary
Onsite Wastewater treatment Systems (OWS) are used in locations where centralized sewer service is not available. Wastewater Total Dissolved Nitrogen (TDN) entering the septic tank is mostly Organic Nitrogen (ON) and ammonium nitrogen (NH4+-N), with very low concentrations of nitrate nitrogen (NO3--N) (Lowe et al, 2007). The organic nitrogen is typically mineralized to NH4+-N in the septic tank, septic tank effluent is typically 75% or more NH4+-N (Cardona, 2006). Nitrate is an anion and is susceptible to leaching through soils, resulting in elevated groundwater NO3-N concentrations beneath and down-gradient from OWS. NO3-N concentrations exceeding 20 mg L−1 in groundwater beneath and/or adjacent to septic systems have been reported for the Coastal Plain of North
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