The US Geological Survey, in response to a request from the US Department of Energy`s Pittsburgh Naval Reactors Office, Idaho Branch Office, sampled 14 wells during 1991--93 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 Laboratory, Idaho. Water samples were analyzed for manmade contaminants and naturally occurring constituents. One hundred sixty-one samples were collected from 10 ground-water monitoring wells and 4 production wells. Twenty-one quality-assurance samples also were collected and analyzed; 2 were blank samples and 19 were replicate samples. The two blank samples contained concentrations of six inorganic constituents that were slightly greater than the laboratory reporting levels (the smallest measured concentration of a constituent that can be reported using a given analytical method). Concentrations of other constituents in the blank samples were less than their respective reporting levels. The 19 replicate samples and their respective primary samples generated 614 pairs of analytical results for a variety of chemical and radiochemical constituents. Of the 614 data pairs, 588 were statistically equivalent at the 95% confidence level; about 96% of the analytical results were in agreement. Two pairs of turbidity measurements were not evaluated because of insufficient information and one primary sample collected in January 1992 contained tentatively identified organic compounds when the replicate sample did not.