Evaluating the effects of replacing septic systems with municipal sewers on groundwater quality in a densely developed coastal neighborhood, Falmouth, Massachusetts, 2016–19

Abstract

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 effects, including installing municipal sewer systems. To increase the understanding of the effects of municipal sewering on groundwater quality discharging to coastal surface waters, a network of multilevel monitoring wells was established in a densely developed coastal neighborhood on the Maravista peninsula, Falmouth, Massachusetts, which was undergoing conversion from onsite septic disposal to municipal sewering. The geohydrology of the study area on the peninsula is generally characterized as consisting of fine to coarse, well-sorted sands containing 2.9 to 9.3 meters of fresh groundwater and a flow system characterized by a groundwater divide slightly west of the center of the peninsula. The magnitude of hydraulic gradients at the water table is gently sloping, ranging from 0.000032 to 0.00059, and affected by daily and bimonthly tidal fluctuations from adjacent coastal ponds. On the western side of the divide, upgradient from Little Pond, average linear groundwater velocities and traveltimes along shallow flow paths, estimated from observed hydraulic gradients and estimated aquifer hydraulic conductivity and effective porosity, range from 0.076 to 0.094 meters per day and 7.8 to 9.7 years, respectively. The groundwater monitoring network consists of 14 profile sites on the peninsula that each include a multilevel sampler for water-quality data collection and a shallow monitoring well for groundwater-level measurements. The study area encompasses about 230 residences that transitioned from onsite septic disposal to municipal sewering between spring 2017 and summer 2019. An additional multilevel sampler that was in a residential coastal setting but not undergoing sewering also was sampled periodically as a reference site. Elevated nitrogen, as compared to typical uncontaminated, fresh groundwater in the Cape Cod aquifer, predominately as nitrate, was measured in 15 water-quality profiles at nitrate concentrations as great as 26.2 milligrams per liter as nitrogen ( n =749; mean and median values were 5.1 and 4.1 milligrams per liter as nitrogen, respectively). At all 14 profile sites and the reference profile site on a nearby peninsula, wastewater effects were denoted by increased nitrate, boron, and specific conductance, and by decreased pH and dissolved oxygen. The highest concentrations of nitrate typically occurred in the deepest one-half of the freshwater zone and in intervals of suboxic and oxic groundwater. Thickness-weighted mean and maximum nitrate concentrations, and total nitrate mass from four sampling rounds, provided a metric to evaluate expected changes at the 14 profile sites on the peninsula. Nitrate concentrations varied moderately by site between sampling rounds through both the presewering (June 2016 and April 2017) and transitional periods (April 2018 and June 2019). Nitrate concentrations greater than the U.S. Environmental Protection Agency maximum contaminant level for nitrate in drinking water (10 milligrams per liter as nitrogen), were detected at 9 of the 14 profile sites and at the reference site. The average of the mean thickness-weighted nitrate concentrations for the four full sampling rounds was greater than 5.0 milligrams per liter as nitrogen at 8 sites (7 profile sites and the reference site) and greater than 8 milligrams per liter as nitrogen at 3 profile sites. The total nitrate mass per square meter of land area at each profile site ranged from 1,830 to 36,800 milligrams per square meter. Nitrate mass flux, across a 500-meter-long section upgradient from Little Pond and covering about 15 percent of the total pond shoreline length, ranged from 124.3 to 192.6 kilograms per year for the four full sampling rounds under three groundwater-flow conditions. The expected improvements in groundwater quality in the freshwater zone should be characterized by decreases in concentrations of dissolved total and inorganic nitrogen and common ions such as boron, chloride, and fluoride. A statistical analysis using the Regional Kendall test for sampling points grouped in specific depth ranges confirmed that water-quality changes were statistically significant in at least one depth group during the 3-year sampling period (nitrate: −0.76 milligram per liter per year; specific conductance: −12.1 microsiemens per centimeter at 25 degrees Celsius per year; dissolved oxygen: 0.82 milligram per liter per year); however, the rate at which the water-quality improvements will result in decreases in nitrate mass loads to the coastal ponds primarily depends on groundwater traveltimes and the rate of flushing of wastewater constituents from the aquifer.