Assessment of Hydrologic Conditions in the Three Bays Watershed in Support of Nutrient Management Activities, Cape Cod, Massachusetts

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      In 2019 the USGS began a partnership with the U.S. Environmental Protection Agency (EPA) Office of Research and Development (ORD), EPA Region 1 Southeast New England Program for Coastal Watershed Restoration (SNEP), Barnstable Clean Water Coalition (BCWC), and other stakeholders to conduct hydrologic monitoring and assessment in support of multifaceted nutrient-management activities in the Three Bays watershed on Cape Cod. Hydrologic monitoring will be used to evaluate the effectiveness of non-traditional technologies such as innovative and alternative (I/A) septic systems and permeable reactive barriers (PRBs) for reducing groundwater nitrogen concentrations and loads.

Three Bays watershed, Barnstable, MA

Figure1. Three Bays watershed area and locations of four sites for preliminary hydrogeologic site characterization, Barnstable, Massachusetts. Click on image to enlarge. (Public domain.)

      The Three Bays watershed (fig. 1) has been selected by EPA ORD as a location to promote the development and implementation of innovative nutrient management solutions in a southern New England coastal setting. Similar to other areas on Cape Cod, Three Bays is a groundwater dominated watershed in which substantial fractions of the total nitrogen load to surface-water bodies are delivered by groundwater. Consequently, understanding the groundwater-flow system (for example, flow directions and rates, depth to groundwater, subsurface geologic conditions, and water quality) is important. Although the groundwater-flow system in the watershed is generally understood from regional modeling studies, assessments of local groundwater conditions will be needed for most of the technology demonstration projects.

      Current (2020) work is focused on identifying sites in the watershed that are potentially favorable for demonstrations of I/A septic systems at the neighborhood scale. More information about I/A systems is available at the Massachusetts Alternative Septic System Test Center (MASSTC) web site (https://www.masstc.org/). The effectiveness of new I/A system designs will be evaluated for individual systems installed in the watershed (for example, seasonal measurement of influent and effluent nitrogen concentrations for individual systems) and a group of 30-50 systems clustered in a selected neighborhood. The goals of the neighborhood-scale demonstration are to determine cumulative changes in groundwater quality, primarily concentrations of nitrate, and quantify changes in groundwater nitrogen loading to downgradient surface-water receptors in response to installation of clustered I/A systems.   

Results from USGS/EPA drilling and sampling, Barnstable, Massachusetts

Figure2. Results from USGS/EPA drilling and sampling at four locations in Barnstable, Massachusetts, November-December 2019. Click on image to enlarge. (Public domain.)

      To select a neighborhood for the demonstration, USGS conducted an analysis of hydrologic and land-use characteristics of the watershed in the fall of 2019 and recommended four sites for preliminary site characterization (fig. 1). In November 2019, USGS conducted the preliminary site-characterization field work in collaboration with EPA ORD. Vertical profiles of generalized geologic conditions and groundwater quality from well clusters installed at the four sites are shown in Figure 2. Based on the results of this work, a neighborhood adjacent to Shubael Pond in the northeastern part of the watershed (fig. 1) was selected for the I/A system demonstration. 

      Planned future work in the Shubael Pond neighborhood will include additional focused site characterization to better define local hydrologic and water-quality conditions followed by installation of a larger groundwater and surface-water monitoring network. The monitoring network will be sampled periodically to determine (1) baseline (pre-intervention) water-quality prior to I/A system installation, and (2) cumulative effects of installed I/A systems on groundwater-quality and nitrogen loading over time. Knowledge of local hydrologic conditions developed from the site characterization work will be used to inform the locations and orientation of the clustered I/A systems and monitoring network to optimize detection of changes in groundwater quality. Periodic sampling of the monitoring network likely will continue for several years after installation to account for the relatively slow movement of groundwater and associated response to I/A system installation.   

      In addition to the I/A system demonstration, USGS may provide hydrologic support for other EPA-led nutrient management activities in the watershed. Other potential nitrogen-reduction projects include installation of permeable reactive barriers, pond dredging, and restoration of cranberry bogs and wetlands. Baseline information needed to support these activities generally includes streamflow and pond-level monitoring; water-quality data for surface waters (rivers, ponds, embayments) and groundwater; and local-scale hydrogeologic data consisting of groundwater levels and flow directions and subsurface geology.

Results from USGS/EPA drilling and sampling at four locations in Barnstable, MA, November-December 2019.

Click here for the pdf version of the figure 2.