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December 31, 2021

Excess nitrogen from wastewater disposal has impaired groundwater quality and reduced the ecological health of rivers and coastal embayments across New England. The New England Water Science Center, in cooperation with multiple partners, collects data and conducts scientific assessments to support the management and restoration of groundwater and surface waters impaired by wastewater nitrogen.

Evaluating Changes in Groundwater Quality in Response to Sewering in a Densely Coastal Residential Neighborhood

In 2016, the USGS, in collaboration with the U.S. Environmental Protection Agency Southern New England Program, installed a groundwater monitoring network in a densely developed neighborhood on the Maravista Peninsula in Falmouth, Massachusetts, on Cape Cod to better understand changes in groundwater quality in response to installation of a municipal sewer system.

Site location map
Study area on the Maravista Peninsula in Falmouth, Massachusetts.
Vertical profiles showing changes in nitrate and dissolved oxygen between June 2016 (before sewering) and September 2021 (two years after sewering) at site MA-FSW 760 on the Maravista peninsula, East Falmouth, Massachusetts
Changes in nitrate and dissolved oxygen before (2016) and after (2021) sewering, Falmouth, Massachusetts.

 

Homes in this area historically were served by onsite septic systems and cesspools, which impaired groundwater quality and delivered high nitrogen loads to nearby coastal water bodies. The Little Pond Sewer Service Area project is intended to improve water quality in Little Pond by connecting about 1,400 properties on the Maravista Peninsula to the existing town sewer system.

Connections to the expanded sewer system were made available starting in the spring of 2017. As detailed in a new publication by McCobb and others (2021), scientists from the USGS New England Water Science Center monitored groundwater conditions before and during sewer connections to document changes to the underlying groundwater-flow system for the period 2016–19.

The USGS team found that prior to sewering, groundwater was strongly affected by wastewater disposal, as characterized by elevated specific conductance and nitrate concentrations (up to 26 milligrams per liter as nitrogen) and low pH and dissolved oxygen. By September of 2019, when sewer connections in the study area were largely completed, changes in groundwater quality and nitrate loading to Little Pond were modest, but observable water-quality improvements were beginning to occur. The team notes that the rate of improvement in groundwater quality and the associated decrease in loading to the adjacent coastal ponds primarily depends on groundwater travel times, which are on the order of 10 years in the study area.

Since 2019, USGS has continued to monitor groundwater quality and hydrologic conditions during the postsewering period (McCobb and others, 2019). During this 2-year period, groundwater quality, as indicated by decreasing nitrate and increasing dissolved-oxygen concentrations, has continued to improve in response to the removal of the local sources of wastewater input to the groundwater system.

 

 

Simulating Groundwater Flow and Nitrogen Transport in Watersheds on the North Shore of Long Island Sound

 

Aerial view of Long Island Sound and adjacent areas of New York, Connecticut, and Rhode Island, with the study area outlined in white
Aerial view of Long Island Sound with the study site outlined in white.

Aquatic systems in and around Long Island Sound (LIS) provide a variety of ecological and economic benefits such as flood and storm protection, water filtration, recreation, commercially and recreationally important fish and bird populations, and carbon sequestration. Reducing nitrogen loads to surface waters has been identified as a priority for the region. Although measurable improvements have been made through controls on major point sources such as wastewater treatment plants, reduction of nitrogen contamination from nonpoint sources in the LIS watershed that are smaller and more diffuse, such as septic systems, is an ongoing major challenge. Much of the nonpoint source nitrogen migrates through groundwater prior to discharge to surface waters. Loads of nitrogen that discharge from groundwater either directly to LIS or to streams that flow to LIS are difficult to estimate and relatively poorly understood.  

In 2018, the USGS New England Water Science Center, in collaboration with the Long Island Sound Study (LISS) and Connecticut Department of Energy and Environmental Protection (CTDEEP), began a study to develop a regional-scale groundwater-flow and nitrogen-transport model for watersheds along the Connecticut coast. The model is intended to be used as a framework for evaluating groundwater flow and nitrogen loading to LIS.

 

 

Percentage of groundwater discharge to inland and coastal waters for watersheds in the study area on the north shore of Long Island Sound
Percentages of groundwater discharge to inland and coastal waters for watersheds in the study area on the north shore of Long Island Sound.

The recently published report by Barclay and Mullaney (2021) on the results of the first phase of the study provides estimates of groundwater budgets and travel times in watersheds on the north shore of LIS and a preliminary assessment of nitrogen loading in the Niantic River watershed. Simulated groundwater budgets for the study area indicate that most groundwater (90.6 percent of inflows) discharges to inland waters, with smaller fractions discharging directly to coastal waters (7.0 percent) or at wells (2.4 percent). Simulations also showed that groundwater travel times are relatively short in the study area; the median groundwater travel time was 1.9 years, with an interquartile range of 0.1 to 5.9 years. A second phase of the study, focused on nitrogen transport and discharge in all watersheds on the north shore of LIS, is scheduled to be completed in 2023.

 

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