Cape Cod Permeable Reactive Barrier Pilot Projects

Treatment of Nitrate in Groundwater

PROBLEM | Cape Cod is a national treasure which maintains a resident population of around 200,000 with a summertime population of approximately a half million people. Indeed tourism, and its attendant hospitality services, is the primary industry on the Cape—an industry kept vital through maintaining the beauty and sustainability of the area’s natural resources. One growing risk to water resources is excessive nutrients. Inorganic nitrogen is a threat to groundwater and to sensitive ecosystems, adversely affecting aquatic flora and fauna and drinking water quality. Cape communities received funding through the EPA Southern New England Project (SNEP) to begin the process of developing a site investigation approach to pilot passive nitrate removal methods.

SOLUTION & METHOD| Truslow Resource Consulting LLC, through the WaterVision LLC joint venture, managed the contract from the EPA to analyze six pilot sites on the Cape for their suitability to install permeable reactive barriers (PRBs) for removal of inorganic nitrogen from groundwater. PRBs are a passive technology that create a barrier to nitrate in groundwater by introducing a carbon source, such as emulsified vegetable oil or wood chips, which promotes anaerobic denitrification given the proper subsurface conditions. These pilot studies were vital in order to:

• Implement and refine a scope of work to assess sites for PRB implementation.
• Select one or more candidate sites for pilot PRB design and installation.
• Ultimately help to evaluate the feasibility of more widespread PRB use on Cape Cod.

Investigations began in late 2015 and final reporting was complete in June 2017. A synthesis paper is also being developed by project partners to share the insights gained during the project.

Initial hydrogeologic characterization was completed at sites in Barnstable, Dennis, Orleans, Falmouth, and Mashpee, MA. as well as a second site in Falmouth to supplement a hydrogeologic study for a town-initiated, PRB study. We installed wells and piezometer clusters, collected sediment cores, and conducted comprehensive water quality analyses and water level measurements. We found that subsurface sediments were largely outwash sands and gravels on the western Cape with finer sediments encountered at depth in the mid and eastern Cape with wide variations in geochemistry and nitrate concentrations between sites. Mass flux of dissolved nitrate-nitrogen was assessed at each site using the initial characterization data.

A second phase was initiated in late 2016 in Dennis, Falmouth and Barnstable to include additional well installation and sampling, hydraulic conductivity testing, monitoring of tidal influence at several wells, and near shore pore water sampling. Also included was an analysis of stable nitrogen isotope concentrations at selected wells to identify in-situ denitrification characteristics.