Nitrogen concentration data were collected from 20 wells near the Niantic River Estuary, during 18 sampling periods from 2005 through 2011, as part of a study to determine changes in nitrogen concentrations and loads as a result of sewering on the Pine Grove peninsula in Niantic, Connecticut. The Pine Grove peninsula area is a neighborhood of 35 acres containing 172 residences with onsite wastewater treatment systems at the beginning of the study in 2005. From 2008 through 2009, the residences were connected to a newly installed sewer system. Water-quality data collection continued from 2010 through 2011, after the sewers were installed.

The peninsula is underlain by glacial stratified deposits. The freshwater in this aquifer ranges from 10 to 45 feet (ft) in thickness and overlies saline groundwater. The mean water-table altitude was from 0.09 to 0.97 ft above the North American Vertical Datum of 1988, with a horizontal hydraulic gradient of 0.0004 to 0.0005.

Initial sampling of the wells included analysis for nutrients, major ions, boron, bromide, and dissolved gases. Concentrations of nitrate plus nitrite nitrogen from the initial sampling ranged from 0.94 to 20 milligrams per liter (mg/L) in samples collected spatially and with depth in the aquifer. The mean concentration of total dissolved nitrogen before the sewers were installed was 7.5 mg/L, and dissolved gas analyses indicated little or no denitrification in the aquifer. Chloride to bromide ratios and boron analysis of the initial water samples confirmed that wastewater was a source of groundwater recharge to most of the wells. Annual recharge from onsite wastewater-disposal systems in 2006 was 4.98 inches, based on analysis of water-use data.

Concentrations of total dissolved nitrogen decreased following sewering in samples from most of the wells that were identified as having nitrogen related to wastewater discharge. Concentrations of total dissolved nitrogen in individual wells decreased by as much as 11.7 mg/L between the periods before and after the sewers were installed, and the mean concentration of total dissolved nitrogen in all wells decreased by 2.3 mg/L to a mean concentration of 5.2 mg/L.

Nitrogen loads from groundwater in the Pine Grove peninsula area were estimated for three time periods by using the measured mean concentrations of total dissolved nitrogen and estimated recharge rates. The estimated nitrogen load before sewering was 1,675 pounds per year (lb/yr) and following sewering was 963 lb/yr. Mean concentrations of total dissolved nitrogen were assumed to have been reduced to 1.1 to 2.3 mg/L after the aquifer had stabilized and sewage-related nitrogen had been completely discharged from the system, with an estimated future load of 202 to 423 lb/yr.

Nitrogen loads from groundwater discharge to the Niantic River Estuary from the lower part of the Niantic River watershed, including Pine Grove, were estimated to be 18,800 pounds (lb) in 2011. This compares with an additional 51,000 lb from the surface-water tributaries to the estuary and an unknown quantity of nitrogen load from stormwater runoff in the lower Niantic watershed.