This data release provides a comprehensive dataset of sampling-site characteristics and baseline groundwater-quality data collected from a network of multilevel sampling wells installed in a densely developed coastal neighborhood undergoing a conversion from onsite septic systems to municipal sewering. Groundwater samples were collected during multiple events from a total of 227 well screens at 15 locations using a peristaltic pump between June 2016 and March 2024. Samples were collected and preserved following methods documented in the U.S. Geological Survey National Field Manual (variously dated) and Savoie and others (2012). This dataset provides well construction information, groundwater levels, water-quality field parameters, and laboratory results from nutrient and major ion analyses. Savoie, J.G., LeBlanc, D.R., Fairchild, G.M., Smith, R.L., Kent, D.B., Barber, L.B., Repert, D.A., Hart, C.P., Keefe, S.H., and Parsons, L.A., 2012, Groundwater-quality data for a treated-wastewater plume near the Massachusetts Military Reservation, Ashumet Valley, Cape Cod, Massachusetts, 2006–08: U.S. Geological Survey Data Series 648, 11 p, available online at https://pubs.usgs.gov/ds/648/ U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. A1-A10, available online at https://pubs.water.usgs.gov/twri9A. First release: March 2019 Revised to include June 2019 and September 2019 data: November 2019 (ver. 2.0) Revised to include January 2020, June 2020, September 2020, and December 2020 data: March 2021 (ver. 3.0) Revised to include June 2021 and September 2021 data: February 2022 (ver. 4.0) Revised to include for February 2022, May 2022, August/September 2022, February 2023, May 2023, August 2023, and March 2024 data: May 2024 (ver. 5.0)