The US Geological Survey, in response to a request from the U.S Department of Energy's Pittsburgh Naval Reactors Office, Idaho Branch Office, sampled water from 13 wells during 1997-98 as part of a long-term project to monitor water quality of the Snake River Plain aquifer in the vicinity of the Naval Reactors Facility, Idaho National Engineering and Environmental Laboratory, Idaho. Water samples were analyzed for naturally occurring constituents and man-made contaminants. A total of 91 samples were collected from the 13 monitoring wells. The routine samples contained detectable concentrations of total cations and dissolved anions, and nitrite plus nitrate as nitrogen. Most of the samples also had detectable concentrations of gross alpha- and gross beta-particle radioactivity and tritium. Fourteen quality-assurance samples were also collected and analyzed; seven were field-blank samples, and seven were replicate samples. Most of the field blank samples contained less than detectable concentrations of target constituents; however some blank samples did contain detectable concentrations of calcium, magnesium, barium, copper, manganese, nickel, zinc, nitrite plus nitrate, total organic halogens, tritium, and selected volatile organic compounds.