The investigation and remediation of radioactive and other contaminants at the NTS are the focus of the U.S. Department of Energy Environmental Restoration Program (U.S. Department of Energy, 1991). One of the objectives of the long-term program is to assess the extent of contamination on, and potentially off, the NTS. Oasis Valley is one U.S. Department of Energy area of concern because it is downgradient from Pahute Mesa, one of the sites of intensive underground nuclear testing. In 1997, the U.S. Geological Survey installed a network of wells in the valley because ground water from parts of western and central Pahute Mesa may discharge into Oasis Valley.
Monitoring-well sites were selected with the purpose of developing a long-term ground-water monitoring network in the Oasis Valley area. The objective of the network is to define spatial and temporal changes in the chemical and isotopic character of the ground water. Information from the network may be used (1) to refine the current understanding of the ground-water flow system through a better understanding of the ground-water chemistry and subsurface geology and (2) to monitor potential changes in ground-water chemistry related to past underground testing activities at the NTS.
Seven sites were chosen in which to install single or multiple wells. Multiple wells, where present, were less than 30 ft from each other and screened at different depths. The selected sites are upgradient from the major spring-discharge area in Oasis Valley. Twelve monitoring wells (in 11 boreholes) were installed at these sites. The wells range in depth from 65 to 642 ft.
Boreholes in Oasis Valley were geophysically logged to define the lithology, stratigraphy, and geohydrologic character of the rock units penetrated during drilling. A typical suite of logs for each borehole includes a caliper log, natural-gamma log, conductivity log, resistivity log, resistance log, and spontaneous-potential log. Additionally, some of the boreholes have the following logs: short- and long-normal resistivity, lateral, acoustic velocity, fluid temperature, fluid resistivity, fluid specific conductance, acoustic televiewer, and heat-pulse flowmeter.
The rocks of hydrogeological significance that underlie the region surrounding Oasis Valley are grouped into several major units. These include a sequence of Paleozoic and older sedimentary and metamorphic rocks, an assemblage of Miocene volcanic rocks, and locally thick deposits of post-volcanic valley fill composed of gravel, sand, and silt. The major regional aquifers have been designated (1) the carbonate-rock aquifer, (2) the volcanic-rock aquifer, and (3) the valley-fill aquifer. Only volcanic rocks and valley-fill deposits were penetrated in boreholes drilled for this study. Seven geologic units were identified and described from samples collected during the drilling: (1) Ammonia Tanks Tuff; (2) Tuff of Cutoff Road; (3) tuffs, not formally named but informally referred to in this report as the "tuff of Oasis Valley"; (4) lavas informally named the "rhyolitic lavas of Colson Pond"; (5) Tertiary colluvial and alluvial gravelly deposits; (6) Tertiary and Quaternary colluvium; and (7) Quaternary alluvium.
Hydrologic data collected from the wells include two sets of water levels in October 1997 and February 1998. Water levels ranged from about 18 to 350 ft below land surface. Additional hydrologic data include flowmeter data from a borehole penetrating volcanic rock on the north side of Oasis Valley. Six water-producing zones were identified in the borehole with the aid of an acoustic televiewer and a heat-pulse flowmeter. The zones of greatest transmissivity in the borehole (as determined from the flowmeter data) are at depths of about 205 ft in the "rhyolitic lavas of Colson Pond" and 340 ft near the top of the "tuff of Oasis Valley."
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