Implementation of a Permeable Reactive Barrier (PRB) Pilot Study
Town of Eastham, MA
ES&M conducted the pilot PRB project to evaluate PRBs as an option for reducing the nitrogen load from Eastham watersheds impacted by nitrogen from septic systems and other sources before the groundwater discharges to the estuary. The PRB that was installed during the pilot study directly intercepts and removes nitrogen from groundwater moving through the watershed. The performance of the PRB and the feasibility of implementing full-scale PRBs are being evaluated with data collected during this project.
The PRB study area is within the Salt Pond and Nauset Estuary watershed and is located downgradient of a relatively dense neighborhood with mainly single-family residential homes using septic systems for wastewater disposal. These septic systems release nitrates to groundwater that migrate downgradient with groundwater flow through the pilot location. During transport, nitrates from septic systems are combined with additional nitrates from infiltrating precipitation containing nitrates from atmospheric sources and lawn fertilizer.
ES&M secured a Special Use Permit with the National Park Service (NPS) for installation of the demonstration PRB on their property. We also secured a Massachusetts Department of Environmental Protection (MassDEP) Resource Protection (BRP) WS06 Permit for the installation of Underground Injection Control (UIC) Class V aquifer remediation wells. The remediation “wells” were the temporary injection points used for adding the Emulsified Vegetable Oil (EVO) organic substrate to groundwater.
ES&M conducted Baseline Groundwater Assessment for PRB Design and Implementation. ES&M studied the hydrogeologic conditions around the Eastham Landfill for many years (between 2012 and 2017) relative to 1,4 dioxane in groundwater that had impacted private drinking water wells. A comprehensive study was completed in 2018 and 2019 to assess hydrogeologic conditions across an expanded study area, including Salt Pond, and to refine understanding of groundwater flow direction and gradients in the pilot area. Focused groundwater studies were also completed to define baseline groundwater chemistry in the vicinity of the pilot PRB. Additional baseline groundwater data was collected through depth to groundwater gauging and sampling before PRB injection. The data gathered was used to evaluate groundwater chemistry, flow direction, and velocity to develop a Conceptual Site Model (CSM) of the pilot area to provide a framework of information to help understand and communicate what is known about an environmental condition, visualize available information, identify gaps in data, and prioritize needed actions.
Installation of the demonstration PRB and included:
- Installation of 21 injection points along a single linear treatment line for substrate injection set at 10-foot intervals placed to form a PRB approximately 200 feet long and up to 12 feet wide (with a radius of influence of approximately six-plus feet at individual injection points).
- A vertical treatment (injection) interval of 43 feet along the 200-foot line, with injection points 1-10 on the western side of the PRB injected to 21 to 64 feet bgs, and injection points 11-21 on the eastern side of the PRB injected to 19 to 62 feet bgs. The different depths are based on ground surface at a higher elevation on the western end of the PRB. Substrate was injected in 5-foot lifts (injection screen interval) from the bottom up.
- Approximately 31,000 gallons of substrate was injected into 21 injection points (~1,500 gallons of dilute solution per injection point). The substrate consists of 6,600 gallons of 60% emulsified soybean oil diluted to 14% with water. Sodium lactate (264 gallons of 60% sodium lactate) was added evenly into the diluted EVO solution on-site prior to injection. A total of 550 pounds of sodium bicarbonate and 550 pounds of calcium carbonate were evenly mixed into the injection solutions as a pH buffer to maintain circumneutral groundwater pH conditions.
The injection points were driven to the desired depth and the injection screen was exposed to start the injection process. A specified volume of substrate was delivered, and the well screen was then pulled up to the next interval. This process was repeated until the required volume was injected into each point evenly across the target interval. The drive points were temporary and no drilling equipment or materials remained following the removal of the injection rods. All injection points were backfilled and sealed to the ground surface with bentonite chips. The PRB installation was completed between March 31 and April 8, 2020.
The system for preparation, mixing, and injection of substrate solutions consisted of mixing tanks, mixers, pumps, piping, meters, valves, and fittings set up in a secondary containment area constructed in the SPVC parking lot. Injection batches of 750 gallons were prepared in plastic tanks by adding appropriate quantities of EVO, sodium lactate, buffer, and water to achieve the selected dilution concentration. Mobile above-ground pumps and hoses were used to convey mixed EVO solution directly to the injection points. Flow totalizers, pressure gauges, and shut-off valves were used to monitor injection pressure, flow rates, and total volume added to each injection interval at each injection point.
Monitoring wells in the vicinity of the PRB were monitored for changes in groundwater elevation during injection. Generally, a radius of influence of 6 to 8 feet is expected; however, the radius of influence may extend further in highly permeable strata. Once the injection is completed, the EVO is expected to adhere to soil grains and does not continue to migrate, thus establishing the permeable reactive barrier. The PRB provides a flow-through biological treatment zone in groundwater where denitrifying bacteria remove nitrate by converting it to inert nitrogen gas.
ES&M is currently implementing the groundwater monitoring program that was developed for the Pilot PRB demonstration area. The monitoring program includes water level gauging, measurement of field parameters, and collection of groundwater samples for laboratory analysis. The resulting data provides a base for consideration of the PRB alternative technology as Eastham moves forward with implementation of watershed management plans leading to restoration of water quality in the estuaries.