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

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Regional-Flow Depiction and Implications

The transport of test-generated contaminants is not necessarily constrained to a single aquifer. To address the potential for contaminants to move across aquifer boundaries, the mapped continuous aquifers were viewed together in the context of an interconnected regional flow system. Potential geologic and hydraulic connections between aquifers were evaluated to identify the flow paths most likely to control transport away from areas of past underground testing at Rainier Mesa and Shoshone Mountain. Inherent in this evaluation are uncertainties that can confound any interpretation of regional flow. These uncertainties have implications for contaminant transport and generally result from a lack of data. The more relevant implications of these uncertainties are discussed in this section, as are some suggestions for additional data collection directed at reducing these uncertainties.

Continuous aquifers of each aquifer type were combined onto a single map to evaluate the interconnection between these aquifers and the potential for regional ground-water flow (fig. 11). Regional flow is described through a series of tributary flow systems. Tributary flow systems consist of a part of a continuous aquifer, an entire continuous aquifer, or a combination of parts of continuous aquifers (fig. 3) that when taken together form a regional flow path. One or more tributary flow systems make up a regional flow system (fig. 3). Four tributary flow systems were identified and mapped in the study area: the Pahute Mesa, Fortymile Wash, Shoshone Mountain, and Yucca Flat tributary flow systems (fig. 11). A potential fifth tributary flow system consists of the carbonate rock that forms the lower carbonate aquifer in the northeastern part of the study area. This system was not named because its flow paths and interconnection with other aquifers within and outside the study area are highly speculative.

The Pahute Mesa tributary flow system (fig. 11) consists of the northern part of the PMTM volcanic aquifer (fig. 8). Water in this system generally originates from recharge in the highland areas on and around Rainier and Pahute Mesas. Ground-water flow is dominantly southwest toward regional springs and seeps in discharge areas located outside the study area to the west and southwest (Laczniak and others, 1996). Timber Mountain (fig. 1) may coincide with a divide between southwesterly flow in the Pahute Mesa tributary flow system and southerly flow in the Fortymile Wash tributary flow system. The presence of a divide and whether this divide is caused by a ground-water mound from recharge on the mountain, by the low permeability of intra-caldera rock, or by geologic structures associated with the formation and presence of the caldera remain uncertain.

The Fortymile Wash tributary flow system (fig. 11) may consist of the Rainier Mesa upper carbonate aquifer (figs. 9A and 9B) and the southern part of the PMTM volcanic aquifer (fig. 8). If water in the northern part of the Rainier Mesa upper carbonate aquifer is assumed to flow south (the first water-level alternative, fig. 9A), then some water in the Fortymile Wash tributary flow system originates north of the study area. Significant amounts of water also enter the flow system as recharge in the Rainier Mesa area. The first two alternatives (figs. 9A and 9B) extend the Rainier Mesa upper carbonate aquifer into the southern half of the study area. In these alternatives, the Rainier Mesa upper carbonate aquifer is separated hydraulically from the overlying PMTM volcanic aquifer in the central part of the study area. Only at the southern end of the mapped extent of the upper carbonate aquifer can water discharge into the volcanic aquifer; at this junction, flows from the two aquifers would merge. If the Rainier Mesa upper carbonate aquifer consists of disconnected blocks of carbonate rock (the third water-level alternative, fig. 9C), then the Fortymile Wash tributary flow system in the study area consists solely of the PMTM volcanic aquifer. In this alternative, water originates in the highlands near Rainier Mesa and flows primarily in a southerly direction through the PMTM volcanic aquifer, as shown on figure 11 by the two westernmost arrows south of Rainier Mesa. For all three upper carbonate water-level alternatives, flow in the Fortymile Wash tributary flow system continues in a south-southwest direction under Fortymile Wash toward the Amargosa Desert (fig. 1). Ultimately, water along this flow path discharges to regional springs and seeps in discharge areas located south and southwest of the study area (Laczniak and others, 1996).

The Shoshone Mountain tributary flow system (fig. 11) extends beneath the Eleana Range and Shoshone Mountain and consists of carbonate rock that forms the western lobe of the YFSM lower carbonate aquifer (fig. 10). Recharge to this aquifer is restricted by the low permeability of the overlying siliceous confining unit. Ground water in the shallow part of this tributary system flows south, ultimately discharging at regional springs and seeps in discharge areas south or southwest of the study area (Laczniak and others, 1996).

The Yucca Flat tributary flow system extends throughout the eastern part of the study area (fig. 11) and includes volcanic rock that forms the Yucca Flat volcanic aquifer (fig. 8), carbonate rock that forms the Yucca Flat upper carbonate aquifer (fig. 9), and carbonate rock that forms the eastern lobe of the YFSM lower carbonate aquifer (fig. 10). Water in the Yucca Flat volcanic aquifer drains slowly eastward and downward into the YFSM lower carbonate aquifer east of the study area, near the center of Yucca Flat (Winograd and Thordarson, 1975). Water in the Yucca Flat upper carbonate aquifer flows east-southeast and enters the lower carbonate aquifer along the eastern edge of the study area, or possibly even further east, where the two carbonate aquifers may be in good hydraulic connection. Ground water in the Yucca Flat tributary flow system moves southeast through the YFSM lower carbonate aquifer and ultimately discharges at regional springs and seeps in discharge areas south of the study area (Laczniak and others, 1996).

Ground-water flow beneath Rainier Mesa is uncertain. The flow scenario portrayed in figure 11 would move any test-generated contaminants that were transported into the saturated rock directly beneath the Rainier Mesa testing area southward away from the mesa area. These contaminants would move south by way of the Fortymile Wash tributary flow system, first through the Rainier Mesa upper carbonate aquifer, as portrayed in figure 9A, and then through the PMTM volcanic aquifer.

Two additional alternative interpretations for transport of test-generated contaminants from the Rainier Mesa area can be derived by substituting alternative water-level contour configurations for the upper carbonate aquifer (figs. 9B and 9C). Assuming the alternative configuration presented in figure 9B, the transport of contaminants entering the Rainier Mesa upper carbonate aquifer from the northernmost tunnels in the Rainier Mesa area is northward. Assuming the alternative configuration presented in figure 9C, the transport of contaminants entering the upper carbonate aquifer is restricted to the locally isolated disconnected blocks of carbonate rock that underlie the tunnel complexes. Any transport beyond one of these locally isolated blocks is impeded and controlled by the low permeability of the siliceous confining unit.

A final alternative for transport of test-generated contaminants from the Rainier Mesa area assumes that transport of contaminants in the perched or semi-perched system is primarily lateral rather than vertical. In this alternative, contaminants in perched and semi-perched zones would migrate west to the PMTM volcanic aquifer rather than downward into the Rainier Mesa upper carbonate aquifer. Once in the volcanic aquifer, contaminants would move westward by way of the Pahute Mesa tributary flow system or southwestward by way of the Fortymile Wash tributary flow system.

The rapid transport of contaminants from the Rainier Mesa area into the lower carbonate rocks of the Shoshone Mountain tributary flow system is unlikely because no direct hydraulic connection is believed to exist. The only pathway for contaminant transport from Rainer Mesa into the Shoshone Mountain flow system is by downward leakage through 3,000 to 4,000 ft of the siliceous confining unit. Downward movement through this low-permeability confining unit is slow, and accordingly, travel times would be long (probably exceeding 1,000 years).

Contaminants introduced into the subsurface by testing in the tunnel complex beneath Shoshone Mountain must move downward through about 2,500 ft of mostly unsaturated, low-permeability volcanic and siliceous confining units and some unsaturated carbonate rock before reaching the ground-water flow system. In the unlikely event of this long-distance migration, contaminants would enter saturated carbonate rock that forms the upper part of the western lobe of the YFSM lower carbonate aquifer (figs. 7 and 10). Once in this aquifer, contaminants would move southward out of the study area through the Shoshone Mountain tributary flow system (fig. 11). Although flow paths in the YFSM lower carbonate aquifer appear, at first glance, to be reasonably certain, water-level contours and flow directions in the study area essentially hinge on a single data point at borehole ER-16-1.

Geologic and water-level data from several strategically placed drill holes could reduce flow-path uncertainties. A deep hole on the east side of the Redrock Valley caldera (fig. 2), an uncertain modeled caldera structure (National Security Technologies, LLC, 2007), could serve multiple purposes. First, the drill hole could prove or disprove the existence of the caldera. If the caldera is present, then the likelihood of the Rainier Mesa upper carbonate aquifer extending to the south and connecting to the PMTM volcanic aquifer diminishes considerably. The presence of the caldera increases the likelihood of a locally isolated upper carbonate aquifer beneath the mesa area (fig. 9C). If the caldera is absent, then the proposed hole could be drilled into underlying carbonate rock to determine the existence and thickness of the Rainier Mesa upper carbonate aquifer and the YFSM lower carbonate aquifer. A hole completed in the aquifers present at this location would provide important water-level information regarding flow in the area. The geologic and hydrologic information obtained from the completed well or wells would help determine the existence, extent and hydraulic connection of the upper carbonate aquifer between Rainier Mesa and areas to the south, determine the depth of the lower carbonate aquifer at the well location, and support or refute the favored interpretation of southerly flow through the Shoshone Mountain tributary flow system in the area north of borehole ER-16-1. A water level at this location also would help resolve uncertainties in the HFM as to the continuity of the lower carbonate aquifer with carbonate rock penetrated at the bottom of borehole ER-12-1.

A deep drill hole in the northeastern part of the study area, northeast of borehole ER-12-4, would reduce the uncertainty in the direction of flow in the northern part of the Rainier Mesa upper carbonate aquifer. This hole would provide information in an area where no data currently exist. The same drill hole, if strategically located, could be drilled deep enough to penetrate both the Rainier Mesa upper carbonate aquifer and the Belted Range lower carbonate aquifer. Water levels and geologic data from a hole having a multi-level completion would provide invaluable information. A water level in the upper carbonate aquifer would prove or disprove the presence of a water-level mound under the mesa area and whether flow in the upper carbonate aquifer in the northern part of the study area is to the north or south. A water level in the lower carbonate aquifer near this location would provide an estimate of the water-level altitude and flow direction, which currently are unknown. Water levels also could help evaluate potential interactions between the upper and lower carbonate aquifers in this part of the study area.

Drill holes sited south and southwest of borehole ER-16-1 would reduce uncertainties about the continuity of the PMTM volcanic aquifer and about the direction of ground-water flow in the Shoshone Mountain tributary flow system. Currently, rock that is part of the volcanic composite unit SHUT is assumed part of the PMTM volcanic aquifer. Geologic data from a well located in this area would provide information as to the water-transmitting properties of the volcanic rock present throughout this area. A well in the Shoshone Mountain tributary flow system at the far southern end of the study area would help confirm whether the predominant direction of flow in this system is southerly.

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