The US Geological Survey, in response to a request from the US Department of Energy`s Pittsburgh Naval Reactors Office, Idaho Branch Office, sampled 12 wells 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. Sixty samples were collected from eight groundwater monitoring wells and four production wells. Ten quality-assurance samples also were collected and analyzed. Most of the samples contained concentrations of total sodium and dissolved anions that exceeded reporting levels. The predominant category of nitrogen-bearing compounds was nitrite plus nitrate as nitrogen. Concentrations of total organic carbon ranged from less than 0.1 to 2.2 milligrams per liter. Total phenols in 52 of 69 samples ranged from 1 to 8 micrograms per liter. Extractable acid and base/neutral organic compounds were detected in water from 16 of 69 samples. Concentrations of dissolved gross alpha- and gross beta-particle radioactivity in all samples exceeded the reporting level. Radium-226 concentrations were greater than the reporting level in 63 of 68 samples.