Will It Work Here? Site Assessment to Determine Suitability for Passive Treatment of Nitrogen

Nitrogen loading to embayments and the resulting eutrophication of estuaries are among the most pressing environmental challenges faced by communities on Cape Cod. Currently, local and regional governments are engaged in a planning process to address nutrient pollution in embayments, the largest controllable source of which is septic systems. In the hydrogeologic setting of Cape Cod, high densities of septic systems distributed across the landscape produce large areas of elevated inorganic nitrogen in the aquifer. While nutrient reduction via sewering is one option communities are implementing, communities are also exploring approaches that provide alternatives to conventional sewering. One such alternative approach is the use of permeable reactive barriers (PRBs) using carbon based media for remediation of nitrate in groundwater.

In 2015, the U.S. Environmental Protection Agency, with assistance from the U.S. Geological Survey and the Cape Cod Commission, developed an assessment protocol to identify hydrologic site conditions appropriate for use of PRBs under typical Cape groundwater and nitrogen source circumstances. The protocol was applied at five potential PRB sites in the towns of Falmouth, Barnstable, Orleans, Mashpee, and Dennis. Sites were selected following an earlier regional-scale evaluation of candidate sites that utilized available data such as expected nitrogen loading, modeled groundwater elevation and flow direction, and distance to receiving surface waters. At each of the sites, a series of monitoring wells was installed to measure hydraulic gradients, monitor water quality and characterize aquifer materials. The protocol developed for site assessments helps define the level of effort needed to determine the suitability of a site for PRB treatment. The results from using this protocol underscore the importance of collecting site-specific hydrologic data before concluding that PRB treatment is an appropriate alternative approach to remediate nitrogen, even in settings where elevated groundwater nitrogen may impact large areas of an aquifer.