Permeable Reactive Barrier at Salt Pond
Permeable Reactive Barrier Process
A permeable reactive barrier (PRB) is a permeable treatment area that runs perpendicular to groundwater flow. The treatment zone intercepts and removes contaminants before they can travel further down-gradient. The Eastham PRB is designed to remove nitrate from groundwater flowing into Salt Pond.
Depending on the reactive material, contaminants are removed through different processes:
- Contaminants sorb (stick) to the surface of the reactive material. For example, carbon particles have a surface onto which contaminants, such as petroleum products, sorb as groundwater passes through.
- Metals dissolved in groundwater precipitate, which means they settle out of the groundwater by forming solid particles that get trapped in the wall. For example, limestone and shell fragments can cause dissolved lead and copper to precipitate in a PRB.
- Contaminants react with the reactive material to form less harmful ones. For example, reactions between iron particles and certain industrial cleaning solvents can convert the solvents to less toxic or even harmless chemicals.
- Contaminants are biodegraded by microbes in the PRB. Microbes are very small organisms that live in soil and groundwater and eat certain contaminants. When microbes digest the contaminants, they change them into water and gases, such as carbon dioxide.
The Eastham PRB was constructed by injecting a food source (emulsified vegetable oil) into the groundwater for particular types of naturally-occurring microbes to eat. When these microbes process the food, they create conditions favorable for a different type of denitrifying bacteria. These bacteria are what ultimately intercept nitrate in the groundwater and make it inert, all without changing the natural groundwater flow. PRBs are considered to be sustainable, unobtrusive, low energy and low maintenance nitrogen removal methods.
Before installing the PRB, a conceptual site model was developed of the Salt Pond sub-embayment to characterize its hydrogeologic and environmental conditions. A multi-organization project team collected exhaustive data before selecting the most effective PRB location. After three years of site characterization, the PRB was finally installed in April 2020 at the Salt Pond Visitor Center.