Nevada Offsites Long-Term Hydrologic Monitoring Program - 9424

The U.S. Department of Energy (DOE) Office of Legacy Management has long-term stewardship responsibility for DOE’s Nevada Offsites Project. The Nevada Offsites consist of eight sites, outside the boundaries of the Nevada Test Site, where underground nuclear tests were conducted between 1961 and 1973. The eight Nevada Offsites are Amchitka (Alaska), Shoal and Central Nevada Test Area (Nevada), Rio Blanco and Rulison (Colorado), Gasbuggy and Gnome-Coach (New Mexico), and Salmon (Mississippi). The underground tests resulted in the release of multiple radionuclides to the detonation zone (cavity, chimney, and nuclear-fractured region); however, tritium is the most likely contaminant to migrate significant distances from the detonation zone because of its occurrence both as tritiated liquid water, which moves with ground water, and as tritiated water vapor. The U.S. Environmental Protection Agency has conducted environmental sampling and long-term monitoring for tritium and other radionuclides since 1972 at the Nevada Offsites under the Long-Term Hydrologic Monitoring Program (LTHMP). The objectives of the monitoring were to detect denotationrelated radionuclides, track the fate and transport of other constituents, ensure public safety, inform the public and the news media, and document compliance with state and federal regulations. By and large, the LTHMP has achieved its objectives, because monitoring results have shown that areas outside the withdrawn lands are unaffected by the underground nuclear detonations. The past two decades have witnessed a gradual change in land use near some of the Nevada Offsites locations. In Colorado and New Mexico, these changes have included increasing population and increased extraction of natural gas near the detonation sites. In Nevada, there is a growing interest in tapping the vast ground water reserves that are contained in the rural intermountain valleys and piping the water to areas where it can be put to beneficial use. Because of these changes, the LTHMP is being modified to provide more focused monitoring near the underground test locations. Distant monitoring locations are being phased out and are being replaced with monitoring networks closer to the detonation sites. The revised monitoring approach will emphasize early detection of radionuclide movement through a network that monitors potential transport pathways near the detonation zone, before potential users of ground water, surface water, or natural gas near the monitored sites could be exposed to radiological contaminants. This approach will provide DOE with sufficient warning to implement protective measures, if necessary.