Permeable Reactive Barriers for Removal of Nitrate from Groundwater Through Injection of Emulsified Vegetable Oil—A Sustainable Solution

Nitrate levels in groundwater and surface water are elevated in many areas of the world because of anthropogenic sources of nitrogen to groundwater, notably discharge from septic systems and nitrogen‐containing fertilizers from agriculture. Discharge of nitrogen‐enriched groundwaters into coastal embayments can lead to eutrophication, a process that negatively impacts water quality, ecology, and human health, while also adversely affecting the local economy through reductions in tourism and commercial fishing, as well as decreased property values. Identifying economical and sustainable methods for reducing nitrogen entering surface water bodies is an objective for communities across the United States and globally. One such method utilizes permeable reactive barriers(s) (PRBs) to remove and/or treat contaminants in groundwater. PRBs can address multiple sources of nitrogen, treating the groundwater in‐place before the nitrate discharges to surface water. PRBs have been widely used for the removal and/or treatment of a range of contaminants. One method for constructing PRBs uses direct injection of carbon substrate electron donors, specifically emulsified vegetable oil (EVO). This study provides best practices developed in the groundwater treatment industry, based on key learnings and results from four in situ demonstration tests of biological PRBs for denitrification, completed between 2016 and 2023 on Cape Cod and Martha's Vineyard, Massachusetts. These demonstrations ranged from 110 to 200 feet in length (perpendicular to the groundwater flow direction), 10–22 feet in width (parallel to the groundwater flow direction), and from 18 to 38 feet of vertical thickness. The calculated groundwater flow rate was moderate, ranging from 0.13 to 0.6 ft/day at the four locations. The first pilot demonstration exhibited persistent dissolved organic carbon (DOC) levels above 4 mg/L for over 5 years with evidence of denitrification at distances of up to 50 feet downgradient of the PRB. The second demonstration exhibited consistent DOC concentrations above 5 mg/L for over 2 years in wells up to 25 feet downgradient across the injection zone. The third PRB demonstration was designed to test the longevity of the PRB and exhibited extensive denitrification in the wells 125 feet downgradient with a lower loading of substrate and 64 feet downgradient in a PRB with a higher substrate loading for 22 months. In the fourth demonstration, the PRB was situated only 80 feet from a water body and exhibited no impact to the water body while 91% of the nitrogen (1.3 pounds) was removed per foot of PRB per year. Nitrate removal PRBs provide a key tool in the “nitrate reduction toolbox,” offering a viable, cost‐effective, and sustainable strategy to achieve almost immediate reductions in nitrate levels in impacted watersheds.