Geochemical data were obtained to investigate the fate and transport of nitrogen in a subterranean estuary near East Falmouth, Massachusetts. The goal of this investigation was to assess nitrogen attenuation in the aquifer under the Eel River Estuary and the adjacent peninsula that was densely populated with residences having septic systems and legacy cesspool inputs of inorganic nitrogen. This estuary is one of many small embayments on Cape Cod where nutrient enrichment can lead to eutrophication and is a major concern for water-quality impairment. Groundwater water-quality data were collected from 3 transects from a total of 27 locations that were sampled between November 2015 and October 2016. Samples were analyzed for pH, dissolved oxygen, temperature, salinity, nitrate + nitrite, ammonium, total nitrogen, total phosphorus, and dissolved organic carbon. Selected samples were also analyzed for gaseous concentrations of nitrogen and argon. Water-level altitudes were measured during October 24 ? 26, 2016 in nine drive-point wells at three well clusters in one transect at low, high, and mean tides to determine hydraulic-head gradients. This data release is structured as a set of comma-separated values (CSV) files, which include U.S. Geological Survey (USGS) Site Name, date sampled, time sampled, and columns of specific analytical and(or) other data. The data files are as follows: Site_Information.csv, containing USGS Site Name, USGS 5-digit Site identification number (if applicable), site type, site description, site status, location in state plane coordinates, land surface elevation, measuring point heights, the depths of the bottom of the screens below land surface, altitudes of the top and bottom of the screens, mid-scree altitudes, location along the transect, tide level, and water depth; Field_Parameters.csv, containing specific conductance, dissolved oxygen, pH, water temperature, alkalinity, and membrane inlet mass spectrometry (MIMS ) number; Nitrogen_Species.csv, containing concentrations of nitrate+nitrite and ammonium; TKN_and_TP.csv, containing concentrations of total Kjeldahl nitrogen and total phosphorus; Organic_Carbon.csv, containing concentrations of nonpurgeable organic carbon and nonpurgeable dissolved organic carbon. N2_and_Ar.csv, containing data on gaseous concentrations of nitrogen and argon This data release also provides a Data Dictionary. Data_Dictionary.csv, containing information for each data file including variable name, description, and where applicable, units, analytical method, minimum reporting limit or limit of quantitation, method reference citation, minimum value, maximum value, median value, and mean value.