USGS Home
Contact USGS
Search USGS

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

Back to Table of Contents

Water-Level Contours

Water-level contours are presented for each continuous aquifer to portray the predevelopment, hydraulic gradients that influence the rate and direction of ground-water flow and potentially the transport of test-generated contaminants away from areas of underground testing in the RMSM area. Contours are interpreted from water levels measured in wells located throughout the study and surrounding areas. The contour maps represent the upper part of the aquifer, which is the area of the aquifer most likely to receive test-generated contaminants from the overlying test media. This conceptualization assumes that transport from tests detonated in the tunnel complexes moves resident water downward and outward away from the low-permeability test media. On the basis of this assumption, contaminants introduced near Rainier Mesa would first encounter more-permeable saturated rock in the upper part of upper carbonate or volcanic aquifers, and any contaminants introduced into Shoshone Mountain would first encounter more-permeable saturated rock in the upper part of the lower carbonate aquifer.

The general direction of ground-water flow within each continuous aquifer, as indicated by interpreted contours, is shown with generalized flow arrows. Regional ground-water flow throughout the RMSM area and flow between continuous aquifers is described in terms of tributary flow systems, which combine flow paths of continuous aquifers. These intermediate flow paths can cross aquifer boundaries and are indicative of the most likely transport path from the source area toward major discharge areas. Because transient ground-water effects in the study area (for example, from pumping or nuclear testing) are short term and highly localized, the predevelopment flow paths depicted in this report also are assumed to closely approximate modern-day (1950–present) conditions.

Volcanic Aquifer

Permeable volcanic rocks are well connected hydraulically throughout the western part of the study area and together form a continuous volcanic aquifer that spans much of the western half of the study area. This aquifer constitutes one of the principal aquifers in the study area and is referred to in this report as the Pahute Mesa–Timber Mountain (PMTM) volcanic aquifer (section B-B' of fig. 7; fig. 8). In the eastern half of the study area, saturated permeable volcanic rocks are less continuous and more poorly connected hydraulically. These disconnected volcanic rocks form a few scattered volcanic aquifers that typically occur beneath the larger topographic valleys. These isolated aquifers typically are separated hydraulically from each other by the volcanic confining unit (fig. 7).

The PMTM volcanic aquifer is composed primarily of five HSUs (Bechtel Nevada, 2002; National Security Technologies, LLC, 2007). The Redrock Valley aquifer, which is one of many volcanic aquifers identified in the RMSM HFM (fig. 4), is one of the more prevalent HSUs in the northern extent of the PMTM aquifer. This welded ash-flow tuff unit is the oldest volcanic aquifer in the study area. The saturated part of the Redrock Valley aquifer reaches a maximum thickness of more than 3,000 ft near borehole WW-8 (see deep volcanic aquifer unit in section B-B' of fig. 7). The Belted Range aquifer, another prevalent HSU within the PMTM volcanic aquifer, overlies the Redrock Valley aquifer in the north. The Belted Range aquifer is composed of welded ash-flow tuff and lava flows. Nearly 6,000 ft of this aquifer is saturated in the northwestern part of the study area. The Belted Range aquifer forms the shallow aquifer within the PMTM volcanic aquifer near borehole WW-8 in section B-B' (fig. 7). This shallow aquifer is highly productive, having yielded about 1.5 billion gallons of water to WW-8 from 1963 to 2006. The central part of the PMTM volcanic aquifer consists of a thick (2,500 to 5,000 ft) section of the Fortymile Canyon and Timber Mountain composite units (see well “ER-30-1-1 (deep)” in appendix 3). Because these composite HSUs include, in part, permeable volcanic rock (National Security Technologies, LLC, 2007), they are mapped in this report as part of the PMTM volcanic aquifer. As portrayed in the base and alternative HFMs of National Security Technologies, LLC (2007), these composite units fill the Rainier Mesa caldera (fig. 2). This caldera is one of multiple calderas that make up the Timber Mountain caldera complex, centered about Timber Mountain, just west of the study area (fig. 1). The southernmost part of the PMTM volcanic aquifer is underlain by an HSU referred to as the Yucca Mountain/Calico Hills lava-flow aquifer. This lava-flow unit thickens westward, and in the study area, the saturated part of this aquifer reaches a thickness of more than 1,000 ft (National Security Technologies, LLC, 2007).

Pahute Mesa, Rainier Mesa, and Belted Range together form a prominent highland in the northern part of the study area (fig. 1). Here, local precipitation infiltrates downward, recharging the underlying aquifers (fig. 2). In general, water levels in the PMTM volcanic aquifer are highest beneath this local recharge area and decrease in altitude to the west and south. Water levels used to construct contours in the PMTM volcanic aquifer range in altitude from 4,197 ft in well “ER-30-1-1 (deep)” to 4,996 ft in well “ER-19-1-2 (middle)” (fig. 8). Contours constructed from these water levels reflect this observed trend and range in altitude from 3,900 ft in the southern part of the aquifer to 5,000 ft in the Rainier Mesa area. The water-level altitudes indicated by the mapped contours in the recharge area adjacent to Rainier Mesa and in the southern part of the PMTM volcanic aquifer are uncertain because water-level data in these areas are lacking (fig. 8).

As portrayed by the mapped contours (fig. 8), much of the ground water that flows through the PMTM volcanic aquifer originates at the local highland in the north-central part of the study area. Local recharge from the highland area is evidenced by downward hydraulic gradients measured throughout the recharge area (see water levels measured in wells completed in boreholes UE-19c, WW-8, and TW-1 in appendix 3). The presence of elevated ground water in wells is common in the recharge area, especially in the immediate vicinity of Rainier Mesa (fig. 8). Confining units are prevalent in the shallow subsurface of Rainier Mesa (section B-B' of fig. 7) and likely impede the downward movement of water, thus elevating the water level. The presence of elevated water is consistent with close proximity to a local recharge area and is assumed representative of perched or semi-perched conditions (Thordarson, 1965). The rocks containing elevated water were not mapped as part of any major aquifer. Instead these rocks are considered to be local sources of recharge. Water contained within these perched and semi-perched zones moves laterally until encountering conditions that allow downward flow into an underlying aquifer.

The general flow direction within the PMTM volcanic aquifer is away from the local recharge area toward the west and south (fig. 8). In general, the horizontal hydraulic gradient decreases as the distance from the local recharge area increases (fig. 8). Small amounts of water flow into or out of the aquifer along its margins, with the direction being dependent on the local hydraulic gradient. Along the northeastern margin of the PMTM volcanic aquifer, the shallow elevated water that originates as precipitation in the local highlands and that ultimately recharges the aquifer is represented by the small inward arrows in figure 8. South of Rainier Mesa along the eastern margin of the aquifer, the interpretation is that the hydraulic gradient between the PMTM volcanic aquifer and adjacent confining units reverses. This reversal results in eastward and downward flow out of the PMTM volcanic aquifer into adjacent confining units.

A small part of another volcanic aquifer, referred to in this report as the Yucca Flat volcanic aquifer, is mapped along the east-central margin of the study area (fig. 8). Water levels are about 2,000 ft lower in altitude than levels measured in the PMTM volcanic aquifer. Ground water in the Yucca Flat volcanic aquifer has been interpreted to flow eastward toward the center of Yucca Flat (Winograd and Thordarson, 1975; Laczniak and others, 1996).

Several isolated volcanic aquifers occur between the PMTM volcanic aquifer and the eastern boundary of the study area (fig. 8). These isolated aquifers are separated from each other and from other continuous aquifers by the volcanic and siliceous confining units (fig. 7). The isolated volcanic aquifer mapped in the northern part of the study area (section A-A' in fig. 7; fig. 8) underlies an area of local recharge (fig. 2) and, therefore, is assumed to have elevated water levels; however, water-level data are not available to confirm this assumption. Another isolated volcanic aquifer, which includes saturated alluvium, occurs beneath Mid Valley in the southeastern part of the study area (section A-A' inn fig. 7; fig. 8). Water levels at the relatively low altitude of about 2,700 ft are consistent with the interpretation of an isolated aquifer in an area of limited local recharge.

The isolated volcanic aquifers in the east-central part of the study area have water levels intermediate to those measured in the PMTM volcanic aquifer on the west and the Yucca Flat volcanic aquifer on the east. Water levels measured in the siliceous confining unit (wells “ER-12-2 main (upper zone)”, “UE-1L (recompleted)”, “UE-16d WW (2117-2293 ft)”, “UE-16f (1479 ft)”, and “UE-17a”; appendix 2) and the large difference in measured water levels between the two bounding volcanic aquifers indicate a steep horizontal hydraulic gradient across the eastern half of the study area. This steep gradient suggests limited eastward flow from the western highlands through the siliceous confining unit and isolated volcanic aquifers into Yucca Flat.

Upper Carbonate Aquifer

The upper carbonate aquifer consists of blocks of carbonate rock that are stratigraphically or structurally above and hydraulically separated from rocks that are part of the lower carbonate aquifer. Based on the degree of hydraulic connection within upper carbonate blocks and with adjacent aquifers, an upper carbonate block is mapped as a continuous or isolated aquifer. Rocks associated with the upper carbonate aquifer are present primarily in the central and eastern parts of the study area (fig. 9). Uncertainties associated with the degree of hydraulic connection in the upper carbonate aquifer in the central part of the study area are addressed with three alternative interpretations. In the southeastern part of the study area, a thrust sheet of upper carbonate rock directly overlies and is believed to have a good hydraulic connection with the lower carbonate aquifer. In this area, the upper carbonate rock is grouped with the underlying carbonate rock and mapped as part of the lower carbonate aquifer. An example of this grouping can be seen on the southern part of section A-A' by comparing the upper and lower sections (fig. 7).

The Yucca Flat upper carbonate aquifer consists of carbonate rock that is part of the upper sheet of the CP thrust (fig. 2; National Security Technologies, LLC, 2007). Carbonate-rock thickness varies across the extent of the aquifer and reaches a maximum of about 5,000 ft (Bechtel Nevada, 2006) on its eastern edge. The aquifer is underlain by low-permeability siliciclastic rock that, in this report, is considered part of the siliceous confining unit (section B-B' in fig. 7). The aquifer may be in direct hydraulic connection with the underlying lower carbonate aquifer along the eastern boundary of the study area where faulting is more intense and complex. Predevelopment water-level measurements are not available for the Yucca Flat upper carbonate aquifer. However, other available water-level measurements provide information that can be used to constrain the interpretation of contoured altitudes. All water-level altitudes shown in figure 9A for the Yucca Flat upper carbonate aquifer are prefixed with a “less than” symbol, signifying that the contoured water level must be less than the posted value. On the basis of limited water-level data, sparse geologic information, and the presence of an underlying confining unit, water levels in the Yucca Flat upper carbonate aquifer are assumed to be slightly elevated from water levels in the underlying lower carbonate aquifer. Although this interpretation is reasonable, it is considered uncertain and is portrayed by a single dashed 2,500-ft contour in figure 9A. The interpretation indicates southeasterly flow toward central Yucca Flat. On the basis of this interpretation, water in the Yucca Flat upper carbonate aquifer probably exits the aquifer laterally and possibly vertically, where in contact with another aquifer east of the study area. The continuity of the Yucca Flat upper carbonate aquifer throughout its mapped extent is uncertain and will remain so without additional geologic and hydrologic data.

The Rainier Mesa upper carbonate aquifer consists of carbonate rock that is part of the upper plate of the Belted Range thrust (fig. 2; National Security Technologies, LLC, 2007). Carbonate-rock thickness in the thrust plate varies across the extent of the aquifer and reaches a maximum of about 2,500 ft (National Security Technologies, LLC, 2007). The degree to which this carbonate rock transmits water is unclear. Recent interpretations of aquifer-test data in wells “ER-12-3 main” and “ER-12-4 main” (Fryer and others, 2006; Stoller-Navarro Joint Venture, 2006) indicate hydraulic-conductivity values that are low relative to estimates of hydraulic-conductivity values in other carbonate rocks on the NTS (Belcher and others, 2001). For this analysis, the carbonate rock is assumed to be an aquifer.

The Rainier Mesa upper carbonate aquifer is penetrated by many boreholes drilled on and around the mesa (for example ER-12-1, ER-12-3, and ER-12-4 on sections A-A' and B-B’ in fig. 7); however, no boreholes penetrate the aquifer north or south of the mesa. Consequently, the continuity of the aquifer throughout the remainder of its mapped extent is highly uncertain. Uncertainties in flow direction and aquifer continuity related to this data deficiency are addressed by the three alternative interpretations shown in figure 9. Although Rainier Mesa is not the only area in the study with flow uncertainty, a presentation of alternatives for this area is merited because of the history of underground testing at and near Rainier Mesa. The alternative interpretations indicate significant differences in the direction of ground-water flow beneath the underground tests at and near Rainier Mesa, and thus, are presented here because each one has a different effect on any prediction of contaminant transport.

Contours representing one interpretation of ground-water flow in the Rainier Mesa upper carbonate aquifer are shown in figure 9A. Available water-level altitudes used to develop this interpretation are posted next to the boreholes on the map and range from 4,317 ft at borehole ER-12-4 to 4,172 ft at borehole TW-1. Water-level measurements on and near the mesa area (at boreholes ER-12-4, ER-12-3, and TW-1) show about 130 ft of decline from north to south, indicating southerly flow from borehole ER-12-4 to ER-12-3. As portrayed in this interpretation, water from the area north of borehole ER-12-4 also is flowing south into the mesa area. This interpretation assumes that recharge in the north would sustain higher water levels, but this assumption remains unconfirmed by water-level measurements. The likely destination of southerly flowing ground water in the Rainier Mesa upper carbonate aquifer is to the PMTM volcanic aquifer in the south-central part of the study area. Flow in the Rainier Mesa upper carbonate aquifer can not discharge into the PMTM volcanic aquifer until water levels in the carbonate aquifer exceed those in the volcanic aquifer. As interpreted, the water-level gradient between these two aquifers reverses south of the boundary between NTS areas 18 and 30 (figs. 8 and 9A). The concept of southerly flow in the upper carbonate aquifer is predicated on the existence of continuous upper carbonate rock from Rainier Mesa to the southern part of the Rainier Mesa caldera (fig. 9A), such as is postulated in several alternative HFMs of National Security Technologies, LLC (2007).

The interpretation in figure 9A indicates that the dominant flow direction in the Rainier Mesa upper carbonate aquifer is southward. Water ultimately flows westward and discharges to the PMTM volcanic aquifer near the southernmost extent of the Rainier Mesa upper carbonate aquifer. As interpreted, limited flow into and out of the upper carbonate aquifer also occurs along the remainder of the western and eastern margins of the aquifer (small arrows on fig. 9A). Limited flow into the aquifer from adjacent volcanic, granitic, and siliciclastic rocks is expected along the western boundary, and limited flow out of the aquifer primarily into adjacent siliciclastic rock is expected along the eastern boundary. Vertical flow is assumed to be restricted and downward over the entire extent of the aquifer.

A second alternative interpretation portrays water-level contours in the Rainier Mesa upper carbonate aquifer as decreasing northward away from borehole ER-12-4 (fig. 9B). The basis for this interpretation is the assumption that the mesa area serves as the primary recharge area for the Rainier Mesa upper carbonate aquifer. In this alternative, the local influx of recharge creates a ground-water mound centered on the mesa area, although water-level measurements are not available north of borehole ER-12-4 to confirm its existence. However, this alternative interpretation is supported by the combination of carbonate rock at and near land surface near borehole ER-12-1 (Slate and others, 2000; appendix 3), high precipitation (Soulé, 2006), and likely infiltration (Hevesi and others, 2003). The presence of shallow carbonate rock in combination with relatively high annual precipitation could allow sufficient recharge through the near-surface, fractured, carbonate rock to create a local mound. The primary difference between this and the first interpretation is that some of the ground water in the Rainier Mesa upper carbonate aquifer would flow northward away from the mesa area rather than southward into the mesa area.

A third alternative interpretation portrays all the carbonate rock beneath Rainier Mesa as multiple disconnected blocks that form aquifers isolated not only from the regional flow system but from each other (fig. 9C). This interpretation is predicated on the notion that the significant and complex faulting in the area juxtaposes fractured carbonate rock against less permeable rock. The outcome of this is manifested in a rather complicated hydrostratigraphic framework and is evidenced at borehole ER-12-1, where the borehole penetrates three separate carbonate-rock zones interlayered with younger siliciclastic rocks. The complex stratigraphy encountered at borehole ER-12-1 is interpreted to have resulted from imbricate thrust faulting (National Security Technologies, LLC, 2007). The complexity of the local geology indicates that the three-dimensional configuration of the upper carbonate aquifer may be much more discontinuous than represented in the HFMs². Large differences in nearby water levels and the highly undulating surface of the local carbonate rock also support an interpretation whereby the carbonate rock beneath the mesa area forms hydraulically isolated carbonate blocks rather than a single continuous aquifer throughout the area. For example, the carbonate water level in well “TW-1 (3700–4206 ft)”, just south of Rainier Mesa, is about 100 ft lower than water levels measured in wells open to carbonate rock beneath the mesa. Additionally, the surface of the carbonate rock at borehole TW-1 is about 2,500 ft lower than encountered at boreholes drilled on Rainier Mesa. This relatively large decline in water level, in combination with a large change in the altitude of the carbonate-rock surface, could be interpreted to support a hydraulic disconnect between the carbonate rocks encountered at the two boreholes.

²Three carbonate zones were penetrated by borehole ER-12-1. The deepest of the three carbonate zones is interpreted in the base-case hydrostratigraphic framework model (National Security Technologies, LLC, 2007) as part of the lower carbonate aquifer, and alternatively, as an isolated sheet of thrusted carbonate rock that is part of the upper carbonate aquifer—the latter is the preferred interpretation in this report. The middle carbonate zone in ER-12-1 is interpreted in the framework model as insignificant relative to the scale of the model and is lumped in as part of a thrust sheet of the upper clastic confining unit. It is not known if this middle carbonate zone connects laterally with the upper carbonate aquifer. However, it is assumed for water-level contouring purposes, that the carbonate rock in the middle zone is disconnected from the upper carbonate aquifer and the water level is representative of the upper clastic confining unit. Based on this interpretation, water levels in the middle zone were not used for contouring the upper carbonate aquifer. The shallowest carbonate zone in ER-12-1 is unsaturated, but is connected to the Rainier Mesa upper carbonate aquifer. Only the shallow and deep upper carbonate zones in ER-12-1 are shown in cross section on figure 7.

The third alternative differs from the first two alternatives in that any regional lateral ground-water flow in the upper carbonate rock beneath Rainier Mesa would be restricted. Measured water levels in the carbonate rock and in the adjacent confining unit (appendix 2) indicate a steep vertical hydraulic gradient and limited vertical flow. Based on this observation, differences in water levels measured in the carbonate rock may not be associated with lateral flow but simply may reflect the steep downward gradient within the confining unit. For example, water levels measured in carbonate rock at wells “ER-12-3 main” and “ER-12-4 main” are similar in altitude to water levels measured in confining units at wells “WW-8 (5290–5490 ft)” and “ER-19-1-1 (deep)” (table 1). Water levels measured in these different units may be similar to each other, not because they are laterally connected, but because they coincidentally represent similar hydraulic heads in the steep downward vertical gradient within a regionally extensive confining unit. The isolation of the carbonate blocks predicated by this third interpretation restricts lateral and vertical flow in the saturated system beneath Rainier Mesa. This inability of the saturated system to transmit water necessitates that either (1) recharge on Rainier Mesa is less than historically estimated (for example, Blankennagel and Weir, 1973; Hevesi and others, 2003), or (2) recharge moves laterally within perched and semi-perched systems into adjacent volcanic aquifers.

Several other carbonate aquifers are mapped in each of the interpretations in figure 9 as being isolated from the each other and from the regional flow system (figs. 7 and 9). One of these isolated aquifers is encountered at the bottom of borehole ER-12-1 and is interpreted as a relatively small block of thrusted carbonate rock that lies beneath the shallower Rainier Mesa upper carbonate aquifer (fig. 9) and above the much deeper, lower carbonate aquifer (figs. 7 and 10). Although the lateral extent of this isolated carbonate block is highly conjectural, its isolation from the mapped Rainier Mesa upper carbonate aquifer and the underlying lower carbonate rock is supported strongly by water-level measurements. The water level measured in this carbonate block is at an altitude of 3,055 ft, which is about 1,200 ft lower than levels in the shallow Rainier Mesa upper carbonate aquifer and 600 ft higher than levels measured to the south and east in the deeper lower carbonate aquifer. Differences of this magnitude indicate that the carbonate block with this intermediate water level is hydraulically disconnected from both the Rainier Mesa upper carbonate aquifer and the lower carbonate aquifer.

Another block of carbonate rock mapped as isolated upper carbonate aquifer is in the Syncline Ridge area of the NTS (fig. 9). This aquifer is mapped as isolated because measured water levels are elevated relative to other nearby carbonate water levels to the east, and the carbonate aquifer is underlain entirely by confining unit (see borehole UE-16d WW on section A-A' in fig. 7). The aquifer is composed of limestone of Mississippian age that is near or exposed at the surface throughout the area (Slate and others, 2000). Surface and near-surface exposure likely allows some recharge of direct precipitation and of runoff from the surrounding highlands (fig. 2). One well in the area, “UE-16d WW”, produced 760 Mgal of water from this fairly extensive but isolated aquifer between 1981 and 2006 (U.S. Geological Survey, 2008), indicating that the aquifer can be productive locally. Although data are sparse, the few available measurements indicate that the water level in the aquifer is intermediate between higher levels in volcanic and upper carbonate aquifers to the west and lower levels in volcanic and upper carbonate aquifers to the east (fig. 9). A possible hydraulic connection may allow some water to flow from this isolated carbonate aquifer to the isolated volcanic aquifer immediately to the east (fig. 8).

A small carbonate block in the west-central part of NTS area 2 also is mapped as an isolated upper carbonate aquifer (fig. 9). The HFM (National Security Technologies, LLC, 2007) portrays this aquifer as being geologically connected to the large block of carbonate rock to the east, which is mapped in this report as the Yucca Flat upper carbonate aquifer. However, water levels measured in well “UE-2ce” and measurements in other nearby wells open to carbonate rock suggest that the carbonate aquifer at well “UE-2ce” is hydraulically isolated from the Yucca Flat upper carbonate aquifer. The saturated part of this isolated carbonate aquifer is nearly surrounded by low-permeability rock and structures that could impede ground-water flow and account for the elevated water levels. Minor modifications in the HFM to the structural top of the carbonate unit, which changes rapidly across the local area, would allow complete isolation of this carbonate aquifer. Well “UE-2ce” was pumped between 1977 and 1984, producing about 11 Mgal of water from the aquifer. However, the aquifer sustained a pumping rate of less than 10 gal/min and the recovery of the water level took about 10 years (Fenelon, 2005). The low production and recovery rates associated with the carbonate rock open to the well support designating this upper carbonate aquifer as either isolated or of low permeability.

Lower Carbonate Aquifer

The lower carbonate aquifer consists of generally continuous, hydraulically connected, Cambrian- to Devonian-age, dolomite and limestone. The aquifer is present throughout most of the study area except beneath the major caldera complexes in the northwest and the Gold Meadows and Climax stock areas in the north-central and northeastern parts of the study area, respectively (fig. 10).Throughout its extent, the aquifer, as modeled (National Security Technologies, LLC, 2007), maintains a fairly uniform thickness that ranges from about 10,000 to 13,000 ft (fig. 7) and, in most areas, lies from 1,000 to 10,000 ft below land surface. The aquifer is overlain almost entirely by siliceous confining unit and typically is fully saturated. In some areas, the top part of the lower carbonate aquifer is unsaturated, as exemplified at borehole ER-16-1 (section A-A' of fig. 7 and appendix 3). The top part of the aquifer is believed to be unsaturated in the southeastern corner of the study area beneath CP Hills and in small areas that follow the spine of a northeast-trending anticline in the carbonate rock that roughly coincides with the large flow arrows shown in figure 10 west of the siliciclastic wedge. The lower carbonate rock has been delineated into two continuous aquifers in the study area—an extensive aquifer referred to as the Yucca Flat–Shoshone Mountain (YFSM) lower carbonate aquifer and a much smaller aquifer in the northeastern part of the study area referred to as the Belted Range lower carbonate aquifer (fig. 10).

Water-level data in the lower carbonate aquifer are sparse. Well drilling into the lower carbonate aquifer in the study area is restricted by the excessive depths required to reach the surface of the aquifer. Only three boreholes, ER-16-1, WW-2, and UE-10j, have water-level measurements taken from an interval or intervals open to the lower carbonate aquifer. Water levels in these intervals range in altitude from 2,414 ft at WW-2 and UE-10j to 2,501 ft at ER-16-1 (fig. 10). Contours shown in figure 10 portray the interpreted water-level distribution in the lower carbonate aquifer. This interpretation is consistent with water levels from several wells open to the lower carbonate aquifer east and southwest of the study area. Given the paucity of water-level data, more than one interpretation of the water-level distribution is possible.

The interpretation shown in figure 10 is focused primarily on the upper part of the lower carbonate aquifer. The upper part of the aquifer is referred to as “shallow” in this report and is that part of the aquifer that likely has the most influence on the transport of the test-generated contaminants. The shallow part of the lower carbonate aquifer is defined arbitrarily as the area where the aquifer is less than 7,500 ft below land surface and is mapped as the darker blue lower carbonate unit in figures 7 and 10.

The YFSM lower carbonate aquifer is overlain in the central part of the study area by a north-south trending wedge of thick siliciclastic rock (fig. 10). The rock making up the siliciclastic wedge includes low-permeability rock that is part of the siliceous confining unit (fig. 7). As interpreted, the wedge restricts the eastward flow of ground water in the shallow western part of the YFSM lower carbonate aquifer, resulting in a southerly flow direction (fig. 10). In the deep part of the YFSM lower carbonate aquifer (not contoured), where the influences of the siliciclastic wedge are negligible, an eastward flow direction is suspected. As the siliciclastic wedge thins approaching the southern boundary of the study area, the contours indicate that some of the water moving southward on the west side of the siliciclastic wedge moves in an east-southeast direction. Ground-water flow east of the siliciclastic wedge in the YFSM lower carbonate aquifer is predominantly east-southeast. Alternatively, if the siliciclastic wedge is not a major impediment to ground-water flow, then the entire shallow part of the YFSM lower carbonate aquifer within the study area likely would have a strong easterly flow component.

The direction of flow and the continuity of the carbonate rock in the Belted Range lower carbonate aquifer are uncertain (fig. 10). As interpreted, the Belted Range lower carbonate aquifer is hydraulically disconnected from the shallow part of the YFSM lower carbonate aquifer. The disconnect occurs just northeast of Rainier Mesa and may result from (1) a thick sequence of low-permeability rock in the area, which overlies carbonate rock where it is excessively deep, or (2) the presence of a geologic structure. Flow in the Belted Range lower carbonate aquifer is interpreted to flow northward; however, water-level data do not exist to support this interpretation. No wells are open to the aquifer throughout its mapped extent. Water levels in pre-Tertiary rock immediately east of the study area (east of the Belted Range lower carbonate aquifer that is mapped in figure 10) are highly elevated with respect to levels measured throughout the YFSM lower carbonate aquifer (Winograd and Thordarson, 1975, fig. 32). The elevated water levels in these rocks support the interpretation of a hydraulic disconnect between the Belted Range and YFSM lower carbonate aquifers in the study area.

The lower carbonate aquifer is the regional drain for water in the study area. In most places, water levels in overlying aquifers are 1,000 to 2,000 ft higher than in the lower carbonate aquifer. The steep gradient and large difference is maintained throughout the study area by a thick (generally greater than 1,000 ft) intervening siliceous confining unit (fig. 7). Although the vertical hydraulic gradient forces some water downward and outward through the confining unit and into the lower carbonate aquifer, the inflow into the lower carbonate aquifer is assumed small because it is restricted by the low permeability of the intervening confining unit.

A higher potential for recharge into the lower carbonate aquifer is possible in four relatively small areas where the top of the lower carbonate aquifer is at or near land surface (less than 1,000 ft) and near an area of potential recharge (fig. 2). These four areas are (1) on the eastern boundary of the study area north of NTS area 15; (2) just north of borehole UE-10j; (3) east of the southernmost extent of the siliciclastic wedge; and (4) in the highlands just west of Mid Valley (fig. 10). Potential evidence of local recharge into the lower carbonate aquifer is seen in hydrographs from wells “UE-10j (2232–2297 ft)” and “WW-2 (3422 ft)”. Both hydrographs show a marked rise in water level beginning in September 2005 (appendix 1). This water-level rise may be a pressure response related to recharge resulting from the much wetter-than-normal conditions on Rainier Mesa and other highland areas in the winter of 2004. From October 2004 to March 2005, about 20 in. of precipitation were measured on Rainier Mesa, whereas the long-term average precipitation for this period is less than 8 in. (Air Resources Laboratory, Special Operations and Research Division, 2007).

Back to Table of Contents