Permeable Reactive Barrier at Salt Pond 

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

Eastham PRB

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.

PRB Installation

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. Please find a video of the installation here.  

What is a Layer Cake Septic System

A "layer cake" treatment area is basically a leach field that is placed in layers, using materials that allow for the successive nitrification and denitrification of septic tank effluent as it percolates through the layers. In this excavation 18 inches of a sand-sawdust mix is first placed over a two-inch layer of washed pea stone followed by 18 inches of "Title 5" sand. Atop the sand layer, the distribution system will be placed (shallow pressurized drain field product, drip dispersal).

Layer Cake Pilot Program

A property on State Highway received piloting approval for a "layer cake" septic system. The property is upgradient of Town Cove's Sentinal monitoring station. At the property, Samples will be taken monthly for two years. Samples will be taken from the pump chamber as well as from a series of pan lysimeters under the system. In addition, water use and pump-run counters will be checked during each sampling event. Following the period of the grant, the homeowner will be responsible for causing an annual inspection of the system and any monitoring required by the Commonwealth's DEP. We anticipate that annual monitoring will be required and annual inspection and adjustment of the low-pressure distribution system will be needed.

Read more about Innovative/Alternative Septic Systems.

Visit the Barnstable County Alternative Septic System page.

Targeted Wastewater Management Plan¹ 

Eastham has been developing a Targeted Wastewater Management Plan (TWMP) over the course of several years.  A TWMP is needed to identify the impaired areas and to provide a framework for nitrogen mitigation approaches.  The area of focus for the TWMP is the Nauset Estuary with Salt Pond as an area of priority.  

The TWMP identifies waterbodies that do not meet state standards and a preferred approach to Total Maximum Daily Load (TMDL) compliance for nitrogen. TMDLs calculate the maximum amount of a pollutant (nitrogen in this case) that a waterbody can assimilate and still support a healthy ecosystem – meet surface water quality standards and support public health. Nitrogen acts as a fertilizer in estuarine waters.  Wastewater is the largest contributor of nitrogen loads.    

The TWMP provides a summary of:  

• Previous wastewater and nitrogen planning evaluations and reports 
• Nitrogen and wastewater flows and loads 
• Nitrogen management alternatives and additional evaluations, including:  
  • PRB Pilot • Shellfish Pilot Program (on hold) 
  •  Advanced I/A septic systems 
  • Stormwater improvement projects 
  • Freshwater pond remediation projects 
  • Wastewater collection, treatment, and recharge

Watch the November 21, 2022 update to the Select Board here.

¹ PRB & Wastewater Planning for Nauset Estuary - Update presentation by GHD