Permeable Reactive Barrier (PRB)

Permeable Reactive Barriers (PRB) were described in Chapter 9 of the 2007 Alternatives Screening Analysis Report (attached in Appendix 2-6). At that time they were called Nitrate Barrier Walls and were identified as a possible method to treat the nitrogen in groundwater before it recharges to the marine waters. That nitrogen could be from any number of sources:
  • Atmospheric deposition
  • Fertilizers
  • Septic systems
  • Stormwater, etc.

Concept


The concept involved the construction of a "permeable wall" of reactive material that would allow the groundwater to flow through but would react with the nitrate in the groundwater and convert it to nitrogen gas. In 2007, the technology was experimental and the reactive material, was a patented media called NITREX. This material would reduce the dissolved oxygen in the groundwater and supply organic carbon to a level at which biological denitrification could occur. The barrier would need to be deep enough to prevent any nitrogen-laden groundwater from flowing under the wall. If the technology were used alone to help an estuary meet its TMDL, the barrier would need to be constructed the full length of the peninsulas bordering the estuary.
In 2007, the technology was being studied by a group of scientists led by Dr. Joseph Vallino and Dr. Kenneth Foreman of the Marine Biological Laboratory. Two pilot studies in the Waquoit watershed were under way: a 50 feet barrier on a beach, and a shorter barrier on a private lot bordering the estuary. Several issues had emerged:
  • Effects of salt water flooding of the NITREX media during storms due to proximity to the marine water.
  • Feasibility of installing the wall deep enough to prevent nitrogen-laden groundwater from flowing under the wall.
  • A full understanding of the nitrogen removal mechanism occurring in the barrier. Need for long-term monitoring both upstream and downstream of the barrier to measure results.
  • Permitting issues because of the proximity to lands subject to the Wetlands Protection Act
  • Problems of access to private property for either installation or monitoring.

Assessment


The assessment in the Alternatives Screening Analysis Report was that this method was not ready for full scale application due to technical issues and regulatory feasibility, and it was screened out. In 2010, the CWMP Review Committee re-examined the technical and regulatory issues, and the current monitoring data showing continued success in removing nitrate from the groundwater. The Committee noted that the patent on the reactive media had expired, that carbon sources such a woodchips could be tried, and that a demonstration project could be designed to address the issues that had been raised. The Board of Selectmen adopted this recommendation and PRBs were added back into the mix of possible technologies described in the Draft Report and Notice of Project Change. Town Meeting agreed in April 2011 to fund a possible demonstration project by passing Article 17, a bond issue, and the voters agreed.