Brad Lyles

Validation Analysis of the Groundwater Flow and Transport Model of the Central Nevada Test Area

The Central Nevada Test Area (CNTA) is a U.S. Department of Energy (DOE) site undergoing environmental restoration. The CNTA is located about 95 km northeast of Tonopah, Nevada, and 175 km southwest of Ely, Nevada (Figure 1.1). It was the site of the Faultless underground nuclear test conducted by the U.S. Atomic Energy Commission (DOEs predecessor agency) in January 1968. The purposes of this test were to gauge the seismic effects of a relatively large, high-yield detonation completed in Hot Creek Valley (outside the Nevada Test Site [NTS]) and to determine the suitability of the site for future large detonations. The yield of the Faultless underground nuclear test was between 200 kilotons and 1 megaton (DOE, 2000). A three-dimensional flow and transport model was created for the CNTA site (Pohlmann et al., 1999) and determined acceptable by DOE and the Nevada Division of Environmental Protection (NDEP) for predicting contaminant boundaries for the site.

Central Nevada Test Area Monitoring Report

Water level measurements were performed and water samples collected from the Central Nevada Test Area model validation wells in September 2006. Hydraulic head measurements were compared to previous observations; the MV wells showed slight recovery from the drilling and testing operation in 2005. No radioisotopes exceeded limits set in the Corrective Action Decision Document/Corrective Action Plan, and no significant trends were observed when compared to previous analyses.

Investigations in Well EPNG 10-36 at the Gasbuggy Nuclear Test Site, Rio Arriba County, New Mexico

Well EPNG 10-36 began as a production well in the San Juan Basin of northwester New Mexico. In 1967, the Gasbuggy underground nuclear test was conducted neaby as part of an effort to study nuclear stimulation of low-permeability reservoirs. The proximity of the well to the nuclear test required it to be p lugged prior to the test. Re-entry into the well after the test was not possible due to the original producing horizon in the Pictured Cliffs Formatino, so the well was completed as a groundwater monitoring well in the Ojo Alamo Formation. The well was sampled annually and in the mid-1980s, tritium was detected. A pressure test and subsequent sampling and analysis confirmed that the casing had been compromised. There are unresolved issues as to the original source of the tritium and discrepancies in the location of tritium in the wellbore.

Evaluation of Potential Hydrocarbon Transport at the UC-4 Emplacement Hole, Central Nevada Test Area

Emplacement hole UC-4 was drilled in 1969 at the Central Nevada Test Area and left filled with drilling mud. Surface characterization samples collected from abandoned mud pits in the area yielded elevated concentrations of total petroleum hydrocarbon, thereby raising a concern that the mud-filled emplacement hole may be leaching hydrocarbons into alluvial aquifers. This study was initiated to address this concern. An analytical solution for flow near a wellbore was used to calculate the amount of time it would take for a contaminant to move through the mud-filled well and into the surrounding aquifer. No hydraulic data are available from the emplacement hole; therefore, ranges of hydraulic conductivity and porosity were used in 100 Monte Carlo realizations to estimate a median travel time. Laboratory experiments were performed on samples collected from the central mud pit to determine the hydrocarbon release function for the bentonite drilling mud. The median contaminant breakthrough took about 12,000 years to travel 10 m, while the initial breakthrough took about 300 years and the final breakthrough took about 33,000 years. At a distance of about 10 m away from the emplacement hole, transport velocity is dominated by the hydraulics of the aquifer and not by the emplacement hole hydraulics. It would take an additional 45,500 years for the contaminant to travel 800 m to the U.S. Department of Energy land exclusion boundary. Travel times were primarily affected by the hydraulic conductivity and porosity of the drilling mud, then by the hydraulic conductivity, porosity and hydraulic gradient of the alluvial aquifer, followed by the hydrocarbon release function.

Potential hydrologic characterization wells in Amargosa Valley

More than 500 domestic, agricultural, and monitoring wells were identified in the Amargosa Valley. From this list, 80 wells were identified as potential hydrologic characterization wells, in support of the US Department of Energy (DOE) Underground Test Area/Remedial Investigation and Feasibility Study (UGTA/RIFS). Previous hydrogeologic studies have shown that groundwater flow in the basin is complex and that aquifers may have little lateral continuity. Wells located more than 10 km or so from the Nevada Test Site (NTS) boundary may yield data that are difficult to correlate to sources from the NTS. Also, monitoring well locations should be chosen within the guidelines of a hydrologic conceptual model and monitoring plan. Since these do not exist at this time, recompletion recommendations will be restricted to wells relatively close (approximately 20 km) to the NTS boundary. Recompletion recommendations were made for two abandoned agricultural irrigation wells near the town of Amargosa Valley (previously Lathrop Wells), for two abandoned wildcat oil wells about 10 km southwest of Amargosa Valley, and for Test Well 5 (TW-5), about 10 km east of Amargosa Valley.