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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5044

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Summary

Accurate prediction of transport of radionuclides and other test-generated contaminants beneath the Rainier Mesa and Shoshone Mountain (RMSM) area requires an understanding of the rate and direction of ground-water flow within the major aquifers of the study area. The spatial distribution of the water-level altitudes across this area, a major control on the direction and rate of transport, has been portrayed historically by maps showing a single set of generalized water-level contours. These maps, by their very nature, ignore vertical flow components and depict the complex subsurface geology as a single, continuous, regionally extensive flow system. Contrarily, the ground-water flow system is made up of multiple aquifers that are separated hydraulically by confining units. The hydraulic separation creates multiple, semi-independent flow systems where flow is controlled by the head gradient within each aquifer.

The approach used to characterize ground-water flow was to construct water-level contour maps of the major aquifers forming distinct flow systems in the study area. Aquifers were identified and mapped by using a composite hydrostratigraphic framework model derived by merging previously published three-dimensional hydrostratigraphic framework models for the RMSM, Yucca Flat, and Pahute Mesa areas. Framework units with similar hydraulic properties and rock type were grouped together into seven subsurface hydrologic unit types (SHUTs): the alluvial aquifer, volcanic aquifer, volcanic confining unit, volcanic composite unit, upper carbonate aquifer, lower carbonate aquifer, and siliceous confining unit. Permeable SHUTs were grouped into three aquifer types (volcanic, upper carbonate, and lower carbonate) and mapped as either continuous or isolated aquifers.

Mean, predevelopment, water-level altitudes were calculated for 133 wells in the study area. Water levels associated with one of the three aquifer types were plotted and contoured to represent predevelopment conditions in each of the major aquifers. Contouring took into consideration water-level gradients, likely recharge areas, discharge areas, and lateral and vertical continuity of flow systems. Maps included in the report show the spatial distribution, dominant flow directions, and areas of lateral inflows to and outflows from each of the aquifers. Contoured surfaces were used to delineate regional and tributary (subregional) flow systems.

Hydraulically well-connected volcanic rocks form a continuous volcanic aquifer that spans the entire western half of the study area. This aquifer constitutes one of the major aquifers in the study area and is referred to as the Pahute Mesa–Timber Mountain (PMTM) volcanic aquifer. Much of the ground water that flows through this aquifer originates as recharge in the local highland in the north-central part of the study area and flows toward the west and south. The presence of elevated ground water in volcanic rocks is common on Rainier Mesa. Rocks containing elevated water are not mapped as part of the PMTM volcanic aquifer, but instead are considered as a source of local recharge. In the eastern half of the study area, saturated volcanic rocks are less continuous and less connected hydraulically. These disconnected volcanic rocks form a few scattered volcanic aquifers that typically occur beneath the larger topographic valleys.

The upper carbonate aquifer consists of continuous, hydraulically connected, carbonate rocks that are stratigraphically or structurally above and hydraulically separated from carbonate rock that makes up the lower carbonate aquifer. The upper carbonate aquifer is present primarily in the north-central part of the study area (Rainier Mesa upper carbonate aquifer) and along the eastern boundary of the study area (Yucca Flat upper carbonate aquifer). Several other disconnected carbonate blocks in the study area are mapped as aquifers that are isolated from each other and from the regional flow system. The mapped extent and continuity of the Rainier Mesa upper carbonate aquifer south of Rainier Mesa are highly uncertain. Uncertainties in flow direction and aquifer continuity are portrayed by alternative interpretations. In two interpretations, water from the Rainier Mesa upper carbonate aquifer flows south and likely discharges to the PMTM volcanic aquifer in the south-central part of the study area. In a third interpretation, the upper carbonate rock beneath Rainier Mesa is portrayed as disconnected blocks that form isolated aquifers.

The lower carbonate aquifer persists throughout most of the study area except beneath major caldera complexes and granitic stocks. In most places, water levels in overlying aquifers are 1,000 to 2,000 feet higher than in the lower carbonate aquifer. The steep gradient and large difference is maintained by a thick intervening siliceous confining unit that allows only small amounts of inflow into the lower carbonate aquifer. The primary section of lower carbonate aquifer is the extensive Yucca Flat–Shoshone Mountain (YFSM) lower carbonate aquifer. This aquifer is overlain by a north-south trending wedge of thick siliciclastic rock in the central part of the study area. The wedge is interpreted to restrict eastward flow in the shallow part of the carbonate aquifer, forming a western and an eastern lobe of carbonate aquifer.

A series of tributary flow systems are used to describe regional flow in the study area. Tributary flow systems consist of a part of a continuous aquifer, an entire continuous aquifer, or a combination of parts of continuous aquifers. One or more tributary flow systems make up a regional flow system. Four tributary flow systems were identified and mapped in the study area: Pahute Mesa, Fortymile Wash, Shoshone Mountain, and Yucca Flat tributary flow systems. The Pahute Mesa tributary flow system consists of the northern part of the PMTM volcanic aquifer. Water in this system originates from recharge in the highland areas on and around Rainier and Pahute Mesas and flows southwest. The Fortymile Wash tributary flow system consists of the southern part of the PMTM volcanic aquifer and, potentially, the Rainier Mesa upper carbonate aquifer. One water-level interpretation allows for water at the southern end of this upper carbonate aquifer to discharge into the volcanic aquifer, where it continues in a south-southwest direction under Fortymile Wash. The Shoshone Mountain tributary flow system consists of carbonate rock that forms the western lobe of the YFSM lower carbonate aquifer. Ground water in the shallow part of this system flows southward out of the study area. The Yucca Flat tributary flow system extends throughout the eastern part of the study area and includes volcanic aquifer and upper and lower carbonate aquifers. Ground water in this system flows southeast out of the study area, primarily through the YFSM lower carbonate aquifer.

Ground-water flow beneath Rainier Mesa is uncertain and several alternative scenarios are proposed to illustrate the potential for movement of test-generated contaminants that are transported into the saturated rock directly beneath the Rainier Mesa testing area. One alternative is southward movement away from the mesa area by way of the Fortymile Wash tributary flow system. Other alternatives include (1) northern transport of contaminants from the northernmost tunnels in the Rainier Mesa area, (2) minimal movement of contaminants because of containment within disconnected blocks of upper carbonate rock that underlie the tunnel complexes, and (3) westward movement through the volcanic aquifer. Contaminants introduced into the subsurface by testing in the tunnel complex beneath Shoshone Mountain must move downward through about 2,500 feet of mostly unsaturated, low-permeability rock before reaching the ground-water flow system. Any contaminants that enter the saturated carbonate rock will move southward out of the study area through the Shoshone Mountain tributary flow system.

Several strategically placed drill holes are proposed that could provide data to prove or disprove alternative HFM models, and consequently, reduce uncertainties in ground-water flow and contaminant transport. One area of uncertainty is in the vicinity of the proposed Redrock Valley caldera, and a second area is in the northeastern part of the study area in the northern part of the Rainier Mesa upper carbonate aquifer.

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