PERMEABLE REACTIVE BARRIERS

Buried just beneath the sand at the beach below the bluff at the Waquoit Estuarine Reserve, a fifty foot long trench filled with wood chips runs along the water’s edge.  A few tubes protruding from the sand—used for sampling the groundwater that runs in and out of the trench—are the only evidence that this experimental ‘permeable reactive barrier’ (PRB) even  exists.   Installed in the summer of 2005, this experimental technology could be one alternative that helps Falmouth solve its wastewater problem.  The idea behind the barrier is fairly simple.

permeablereactivebarrier
On Cape Cod, most of our groundwater flows into estuaries in a very narrow band along our sandy bay shorelines. That’s because fresh ground water coming in from the watershed floats over the deeper salt water that naturally underlies the bays. A barrier dug into the beach line can intercept—and treat—this water.

Most nitrogen reaches Falmouth’s estuaries in the form of nitrate. Nitrate is formed as wastewater passes through properly‐functioning backyard septic systems and leaching fields. Nitrate passes easily through soils and travels long distances in Cape Cod’s groundwater where   oxygen is abundant.The wood chips in a permeable reactive barrier create anaerobic conditions that cause the  transformation of nitrate to nitrogen gas in a process called denitrification.

Studies of the two barriers installed in Falmouth (the second is in FWS former board member Joe Apicella’s back yard on Child’s River) indicate that the system generally works.   Both of these barriers are proprietary technology of Lombardo Associates, an environmental consulting firm in Newton, MA. Studies of Falmouth’s experimental barriers by Ken Foreman at the Marine Biological Laboratory show that water generally enters the barrier rich in nitrate and leaves with almost all the nitrate stripped out. And, as Foreman notes, “The service of putting the incoming nitrogen in the form of nitrate is done by the watershed for free.” The barrier has no moving parts and uses no electricity. Once it’s installed it works on its own.  Another advantage is that the barrier treats all the nitrate entering from the watershed in that specific area—regardless of whether it came from wastewater, lawn fertilizer or atmospheric deposition of acid rain.

  SOME POTENTIAL DRAWBACKS TO PRBS

  • They require trenches through countless back yards along bay shorelines—highly sensitive ecological zones.A properly‐ functioning permeable reactiive barrier turns inflowing groundwater anoxic. While the effect of an experimental fifty‐ foot section of shoreline is likely small, the effect on marine life such as clams and other bottom‐organisms of an entire bay ringed by barriers is not known.
  • They treat water arriving at the bay but do nothing to stop the flow of wastewater into the ground. To work well, barriers must intercept all of the groundwater. And we know that groundwater does not flow into bays at a uniform depth—so installation of each segment might require more detailed studies of groundwater flow to be most effective.
  • Barriers also don’t last forever. The wood chips eventually lose their effectiveness and must be dug up and replaced.
  • While barriers could be cheaper than conventional sewering, it’s very difficult to estimate the cost of their implementation on a large scale. For example, we don’t know whether the barriers need “recharging” after ten or fifty years.
  • The State of Massachusetts does not currently allow cities and towns to base their comprehensive wastewater management plans on permeable reactive barriers. At Falmouth’s April 2011 Spring Town Meeting, Falmouth voted to appropriate $250,000 for the study of PRBs as one alternative wastewater treatment option.

Permeable Reactive Barriers By Chris Neill, PhD 5/2011