Hydrologic Site Assessment for Passive Treatment of Groundwater Nitrogen with Permeable Reactive Barriers, Cape Cod, Massachusetts

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In 2019 USGS completed a study designed to develop and evaluate a phased site-assessment approach for determining the hydrologic suitability of sites being considered for permeable reactive barrier installation on Cape Cod. The approach provides a template for town officials and other stakeholders to follow when considering PRBs for passive treatment of nitrogen in groundwater on Cape Cod and elsewhere.

    Wastewater disposal associated with rapid population growth and development on Cape Cod, Massachusetts during the past several decades has resulted in widespread contamination of groundwater with nitrogen. As a result, water quality in many of the streams, lakes, and coastal embayments on Cape Cod is impaired by excess nitrogen. To reduce nitrogen loads to these impaired water bodies, watershed-based planning is currently (2019) underway following a regional strategy, the Section 208 Areawide Water Quality Management Plan Update, which is an update to the 1978 Section 208 Plan for Cape Cod. In the updated plan, both traditional (sewering) and alternative wastewater management options are under consideration for restoring water quality in impaired surface-water bodies. Permeable reactive barriers (PRBs), which are reactive zones emplaced below the water table for passive treatment of groundwater contaminants, are one of the alternative options being considered by Cape Cod towns as a potentially cost-effective technology for the removal of nitrogen from groundwater. However, the effectiveness of PRBs is dependent on local conditions and site-specific hydrologic and water-quality data are needed to inform the decision to install a PRB in a given location. These data are not available in most locations on Cape Cod, and, consequently, site assessments are needed prior to selecting this treatment option. 

Map of potential permeable reactive barriers sites on Cape Cod, MA

Locations of the sites investigated to determine hydrologic suitability for installation of permeable reactive barriers to treat groundwater nitrogen, Cape Cod, Massachusetts.

    To address this need, the U.S. Environmental Protection Agency, U.S. Geological Survey, and Cape Cod Commission formed a technical team in 2015 to develop and evaluate a hydrologic site-assessment approach for PRB installation. The approach developed by the technical team includes a preliminary regional assessment followed by a phased on-site investigation. The approach was intended to provide the hydrologic data needed to make informed decisions on the site suitability and to support installation and monitoring should the site be deemed appropriate for a PRB. The factors that were evaluated to characterize local hydrologic conditions and inform site selection included groundwater flow directions and rates; depth to the water table; hydraulic conductivity and degree of heterogeneity of the aquifer; spatial distribution and concentration of nitrate and redox-sensitive constituents; thickness and depth of the treatment zone; distance to downgradient water bodies, and access for drilling and PRB installation. The approach was demonstrated on Cape Cod by conducting a preliminary assessment of 27 sites from which five sites were selected for on-site investigations. Results indicated that the site-assessment approach was successful for screening sites and characterizing the geologic, hydrologic, and water-quality conditions at the sites selected for on-site investigations.

    The study was conducted in cooperation with the U.S. Environmental Protection Agency, Region 1, Southeast New England Coastal Watershed Restoration Program.