4. The Regional Setting of the Baseline Study
The study area is in southeastern Nevada, approximately 80 miles west of Las Vegas and just north of the California border. It is approximately seven miles south of the Nevada Test Site (NTS) and 20 miles northeast of Death Valley (See figures 4.0.1 and 4.0.2). The study area is about 18 miles long and extends from the western portion of the Amargosa Farms area to the northern portion of the Ash Meadows National Wildlife Refuge.
4.1 Geology and Hydrology
The study area lies within the Death Valley regional flow system (DVRFS). Groundwater in the DVRFS generally flows southward and naturally discharges (surfaces) at Ash Meadows, Oasis Valley, Alkali Flat, and Death Valley[1] (See figure 4.1.1). Groundwater is also pumped via wells for agricultural, industrial, and domestic purposes.
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Figure 4.0.2 Key Features of Study Area
(Adapted from Belcher, 2004, Figure A-1)
Figure
4.1.1 Groundwater Flow Directions and Natural Discharge Areas
in the Vicinity of the Study Area
(Adapted from Belcher, 2004, Figure A-1; and Laczniak, 1996, Figure 6 and Plate 1)
4.2 Major Hydrogeologic Units in the Vicinity of the Study Area
Table 4.2 presents a generalized description of the major hydrogeologic units that exist in the region near the study area. The units are listed in descending order with the uppermost units listed first. The table also identifies those units that transmit significant amounts of groundwater (aquifers).
Table 4.2
|
Hydrogeologic Unit |
Aquifer? |
|
Valley Fill |
Yes |
|
Volcanic Rock |
Yes |
|
Elena Confining Unit |
No |
|
Lower Carbonate Aquifer |
Yes |
|
Precambrian Confining Unit |
No |
The hydrogeologic units listed in Table 4.2 do not extend uniformly throughout the region. Some of the units do not exist in some portions of the region near the study area. See Figure 4.2 for a schematic[4] cross-section of the region surrounding the study area. It extends from north of Yucca Mountain, through the study area, to the Funeral Mountains.
4.2.1 Valley Fill Aquifer
The valley fill aquifer primarily consists of alluvium eroded from the surrounding uplands. Water levels in the Amargosa Farms area range from about 60 to 120 feet below land surface[5]. At Amargosa Farms, water levels in the valley fill aquifer declined up to 30 feet between 1950 and 2000[6]. The alluvium is more than 3000 feet thick in some areas[7]. Water from the volcanic aquifers and the Lower Carbonate Aquifer is believed to flow into the valley fill aquifer[8]. All of the wells sampled for this report derive their water from the valley fill aquifer.
The volcanic rock aquifers are found to the north of the study area. They are composed primarily of tuff[9] with some lava flows[10]. In the vicinity of the study area, the major volcanic units are (in descending order) the Topopah Spring tuff[11], the Calico Hills Formation, and the Prow Pass tuff[12].
At Yucca Mountain, the top of the aquifer (water table) is about 2500 feet below ground surface[13]. The depth to water is shallower in the valleys. Just to the east of Yucca Mountain, in Jackass Flats, the water table is approximately 750 feet below land surface[14]. South of Yucca Mountain, depth to water is less than 400 feet[15]. The thickness of the aquifer ranges from a few feet to more than 3000 feet[16]. Water flows through fractures in the welded tuffs. Groundwater in the volcanic aquifers near Yucca Mountain is believed to flow into the valley fill aquifer[17].
Figure 4.2 Schematic Cross Section Through Study Area
(Based on Laczniak et al., 1996, Plate 2; Parizek, 2005; and DOE, 2002, Figure 3-18)
The Lower Carbonate Aquifer consists of limestone and dolomite, and is part of a large system that extends far beyond the study area. The aquifer underlies the carbonate-rock province, which covers approximately 100,000 mi2 in Nevada, Utah, Idaho, and California[18]. In the vicinity of the study area the aquifer may be up to 16,000 feet thick[19]. The depth to the top of the Lower Carbonate Aquifer varies from near land surface at Ash Meadows to more than 3000 feet beneath the thicker portions of the valley fill[20]. This aquifer transmits more groundwater than any other aquifer in the DVRFS[21]. Water flows through fractures and solution openings that form as the limestone dissolves. The Lower Carbonate Aquifer is believed to transmit water from the NTS to the springs at Ash Meadows[22].
4.3 Ash Meadows
Ash Meadows is the only natural discharge area in the study area (Figure 4.1.1). Groundwater emerges from more than 30 springs and seeps that occur along a ten-mile long trace that runs roughly northwest-southeast[23]. The largest spring, Crystal pool, discharges up to 3000 gallons per minute. Estimates of the combined discharge of all the springs range from 10,500 to 13,000 gallons per minute (17,000 to 21,000 acre feet per year)[24]. Both of the springs sampled for this report (Fairbanks and Longstreet) are in Ash Meadows.
Groundwater in the Lower Carbonate Aquifer flows toward Ash Meadows from the north and northeast. Groundwater is believed to be forced to land surface by a series of faults that underlie the area[25].
Springflows have been fairly constant except for a period in the 1960s and 1970s when large amounts of groundwater were pumped from the area for agriculture. This caused local water levels and springflows to decline, and threatened the existence of the pupfish, a federally listed endangered species. A ruling by the U.S. Supreme Court resulted in a large reduction in pumping. Water levels and springflows have recovered since the pumping was curtailed[26].
4.4 Groundwater Flow Rates
There are few estimates of groundwater flow rates for the aquifers in the study area. Laczniak gives a range of less than 40 ft/yr to more than 36,000 ft/yr, with the higher rates occurring in the Lower Carbonate Aquifer[27]. Flow rates in the volcanic aquifer in the northwestern portion of the NTS have been estimated to range from 7 ft/yr to 270 ft/yr[28].
Belcher presents hydraulic conductivities for the aquifers in the vicinity of the study area[29]. For the valley-fill aquifer, the estimates range from less than 0.001 ft/day to more than 400 ft/day. The low values probably represent silts and clays while the higher values probably represent sands and gravels. Using these hydraulic conductivities, and reasonable assumptions for porosities and hydraulic gradients[30], flow rates in the valley fill aquifer are calculated to range from 0.008 ft/yr to 3500 ft/yr.
[1] Laczniak, R.J., J.C. Cole, D.A. Sawyer, and D.A. Trudeau, 1996, Summary of Hydrogeologic Controls on Ground-Water Flow at the Nevada Test Site, Nye County, Nevada, USGS Water-Resources Investigations Report 96-4109, page 4.
[2] Laczniak et al., 1996, page 6.
[3] Alluvium is material deposited by streams. It consists of clay, silt, sand, and gravel.
[4] This schematic representation only depicts the broad outlines of the major hydrogeologic units. It does not show the faulting or the other units that exist in the area. A more detailed depiction of the subsurface may be found in Laczniak et al., 1996, plate 2.
[5] As reported by well drillers and owners of wells sampled for this report.
[6] Fenelon, J. M., and M. T. Moreo, 2002, Trend Analysis of Ground-Water Levels and Spring Discharge in the Yucca Mountain Region, Nevada and California, 1960–2000, Water-Resources Investigations Report 02-4178, page 53.
[7] Laczniak, R.J., J.C. Cole, D.A. Sawyer, and D.A. Trudeau, 1996, Summary of Hydrogeologic Controls on Ground-Water Flow at the Nevada Test Site, Nye County, Nevada, USGS Water-Resources Investigations Report 96-4109, plate 2; Belcher, W.R., C.C. Faunt, and F.A. D’Agnese, 2002, Three-Dimensional Hydrogeologic Framework Model for Use With a Steady-State Numerical Ground-Water Flow Model of the Death Valley Regional Flow System, Nevada and California, Water-Resources Investigations Report 01-4254, figure 9.(A).
[8] U.S. Dept. of Energy, 2002, Final Environmental Impact Statement for a Geologic Repository for the Disposal of Spent Nuclear Fuel and High-Level Radioactive Waste at Yucca Mountain, Nye County, Nevada, February 2002, pages 3-40 and 3-47; Fenelon and Moreo, 2002, pages 7 and 9.
[9] Tuff is solidified volcanic ash. When the ash is so hot that it melts after it falls to the ground, it is called welded tuff.
[10] U.S. Dept. of Energy, 2002, figure 3-17 and page 3-57.
[11] The proposed Yucca Mountain repository is in the Topopah Spring tuff. This unit is unsaturated at Yucca Mountain but is below the water table to the south and east of Yucca Mountain (DOE, 2002, page 3-57).
[12] U.S. Dept. of Energy, 2002, pages 3-52 and 3-57, and figure 3-16.
[13] U.S. Dept. of Energy, 2002, page 3-50.
[14] Fenelon and Moreo, 2002, figure 21.
[15] Nye County, 2006, Early Warning Drilling Program, information available at: http://www.nyecounty.com/index.htm. Well Completion Summary Information for EWDP Phases I and II, well 1S.
[16] Belcher et al., 2002, figure 12.(A).
[17] U.S. Dept. of Energy, 2002, pages 3-40 and 3-57.
[18] Prudic, D.E., J.R. Harrill, and T.J. Burbey, 1995, Conceptual Evaluation of Regional Ground-water Flow in the Carbonate-Rock Province of the Great Basin, Nevada, Utah, and Adjacent States, USGS Professional Paper 1409-D, page D1 and figure 1.
[19] Belcher et al., 2002, figure 24.(A); U.S. Dept. of Energy, 2005, Nevada Test Site Environmental Report 2004, October 2005, page A-15.
[20] Laczniak et al., 1996, plate 2.
[21] Laczniak et al., 1996, page 6.
[22] Laczniak et al., 1996, page 6.
[23] Laczniak, R.J, G.A. DeMeo, S.R. Reiner, J.L. Smith, and W.E. Nylund, 1999, Estimates of Ground-Water Discharge as Determined from Measurements of Evapotranspiration, Ash Meadows Area, Nye County, Nevada, USGS Water-Resources Investigations Report 99-4079, page 7 and figure 3.
[24] Laczniak et al., 1999, pages 7 and 47.
[25] Laczniak et al., 1999, page 8 and figure 4.
[26] Laczniak et al., 1999, page 7.
[27] Laczniak et al., 1996, page 10. High groundwater flow rates are not uncommon in limestone aquifers where water flows through solution channels. These are known as karst aquifers. In the Edwards Aquifer, a karst limestone aquifer in south-central Texas, groundwater flow rates have been measured at more than several thousand feet per day.
[28] Laczniak et al., 1996, page 21.
[29] Belcher, 2004, page 122.
[30] The following assumptions were used to calculate the flow rates: effective porosity = 0.25, hydraulic gradient = 0.006 (this is the slope of the Amargosa River in an 18 mile reach between Beatty and the study area).