Assessment of Hydrologic and Water-Quality Changes in Shallow Groundwater Beneath a Coastal Neighborhood Being Converted from Septic Systems to Municipal Sewers

Land disposal of sewage wastewater through septic systems and cesspools is a major cause of elevated concentrations of nitrogen in the shallow coastal aquifers of southern New England. The discharge of nitrogen from these sources at the coast is affecting the environmental health of coastal saltwater bodies. In response, local, state, and federal agencies are considering expensive actions to mitigate these impacts, including the installation of municipal sewer systems. To inform these considerations, efforts are underway to increase the understanding of the fate and transport of nitrogen and other wastewater constituents in groundwater along the transport pathway from source areas to coastal surface waters.

The study objectives are to:

• Establish a monitoring network to assess groundwater levels and water quality beneath a densely developed coastal neighborhood undergoing a conversion from septic systems and cesspools to municipal sewers; and
• Develop an understanding of water-quality conditions before and after installation of the sewers.

The Little Pond Sewer Service Area project is intended to improve water quality in Little Pond, Falmouth, Massachusetts, by connecting about 1,400 properties surrounding the pond to an existing town sewer system. Connections to the new sewer system were made available starting in the spring of 2017.    

In the spring of 2016, the USGS installed 18 monitoring wells and 14 multilevel samplers (MLSs) at 14 locations to monitor water levels and groundwater quality beneath the Maravista neighborhood, which is on a peninsula on the eastern side of the saltwater pond.  Cores were collected during drilling to characterize the subsurface sediment. The wells were installed over an area containing about 225 homes along seven streets that cross the width of the peninsula. 

Water-quality samples are collected annually from MLS sampling ports at all 14 sites and one background site. More frequent sampling at two MLSs (MA-FSW 760-M01 and MA-FSW  767-M01) to assess temporal trends in water quality occurs multiple times per year. The water samples are analyzed for concentrations of nutrients (nitrate plus nitrite, ammonium, nitrite, and phosphorus), major ions, boron, and chloride. Parameters measured in the field include specific conductance, dissolved oxygen, pH, alkalinity, and temperature.

A USGS Scientific Investigations Report (SIR-2021-5130), published in December 2021, describes monitoring results during the preconnection (baseline) monitoring period from June 2016 to June 2017, when sewer connections began, and during the transitional monitoring period from June 2017 to September 2019, by which point all the properties in the study area had been connected.

Sampling results to date (March 2024) are available in the project data release by McCobb and others (2024).