A plume of contaminated ground water extends from former disposal beds at the Massachusetts Military Reservation wastewater-treatment plant toward Ashumet Pond, and farther southward toward coastal ponds and Vineyard Sound, Cape Cod, Massachusetts. Treated sewage-derived wastewater was discharged to the rapid-infiltration beds for nearly 60 years before the disposal site was moved to a different location in December 1995. Water-quality samples were collected periodically from monitoring wells and multilevel samplers during and after the disposal period to characterize the nature and extent of the contaminated ground water and to observe the water-quality changes after the wastewater disposal ceased. Data are presented here for water samples collected from 1994 through 2004 from 16 wells (at 2 locations) and 14 multilevel samplers (at 9 locations) along a longitudinal transect that extends through one of the disposal beds. Data collected from the treated-wastewater plume are presented in tabular format. These data include field parameters; concentrations of cations, anions, nitrate, ammonium, and organic and inorganic carbon species; and ultraviolet/visible absorbance. The natural restoration of the sand and gravel aquifer after removal of the nearly 60-year-long treated-wastewater source, along with interpretations of the water quality in the treated-wastewater plume on Cape Cod, have been documented in several published reports that are listed in the references.