Hydrogeologic and hydrochemical framework, south-central Great Basin, Nevada-California, with special reference to the Nevada Test Site

Abstract

Intensely fractured Precambrian and Paleozoic carbonate and clastic rocks and block-faulted Cenozoic volcanic and sedimentary strata in the Nevada Test Site are divided into 10 hydrogeologic units. Three of these the lower clastic aquitard, the lower carbonate aquifer, and the tuff aquitard control the regional movement of ground water. The coefficients of fracture transmissibility of these rocks are, respectively, less than 1,000, 1,000 to 900,000, and less than 200 gallons per day per foot; interstitial permeability is negligible. Solution caverns are locally present in the carbonate aquifer, but regional movement of water is controlled by variations in fracture transmissibility and by structural juxtaposition of the aquifer and the lower clastic aquitard. Water circulates freely to depths of at least 1,500 feet beneath the top of the aquifer and up to 4,200 feet below land surface. Synthesis of hydrogeologic, hydrochemical, and isotopic data suggests that an area of at least 4,500 square miles (including 10 intermontane valleys) is hydraulically integrated into one groundwater basin, the Ash Meadows basin, by interbasin movement of ground water through the widespread carbonate aquifer. Discharge from this basin a minimum of about 17,000 acre-feet annually occurs along a fault-controlled spring line at Ash Meadows in east-central Amargosa Desert. Intrabasin movement of water between Cenozoic aquifers and the lower carbonate aquifer is controlled by the tuff aquitard, the basal Cenozoic hydrogeologic unit. Such movement significantly influences the chemistry of water in the carbonate aquifer. Groundwater velocity through the tuff aquitard in Yucca Flat is less than 1 foot per year. Velocity through the lower carbonate aquifer ranges from an estimated 0.02 to 200 feet per day, depending upon geographic position within the flow system. Within the Nevada Test Site, ground water moves southward and southwestward toward Ash Meadows. C2 HYDROLOGY OF NUCLEAR TEST SITES The scope of the report is broad in view of the complexities of the geology, the vastness of the study area, and the absence of previous detailed hydrogeologic studies of similar terrane. Yet, the types and quantity of data obtained during this investigation are seldom available in hydrogeologic studies. In addition to standard hydrologic data, a wealth of geologic, geophysical, geochemical, and isotopic data were used to supplement interpretations of the hydrologic data. To a first approximation, therefore, the objectives of the study are believed to have been accomplished. The development of groundwater supplies was an important byproduct of the investigation; more than half the test holes are used as water wells. This report does not discuss the exploration for, and development of, new water supplies, although many of the data and interpretations will aid others in such tasks.

Major structural controls on the distribution of pre-Tertiary rocks, Nevada Test Site vicinity, southern Nevada

“…carbonate rock units of.the principal regional aquifer system in southern Nevada (Winograd and Thord&son, 1975;Laczniak and others, 1996). This aquifer system conveys substantial ground water from central Nevada toward the southwest in what is known as the Death Valley~ound water flow system (Winograd and Thordarson, 1975;Dettinger, 1989). This regional carbonate aquifer is stratigraphically contained between quartzitic elastic confining units below (Middle Cambrian and older) arid shales and siliciclastic confining units above (Late Devonian and younger; Laczniak.and others, 1996;Trexler and others, 1996).…”

“…The continuity of the carbonate aquifer and its physical characteristics that influence ground water flow are strongly influenced by post-depositional folding, faulting, . and fracturing (Winograd and Thordarson, 1975).…”

“…This work clarifies the structural and stratigraphic framework of potential pathways that may facilitate -the migration of water-borne contaminants from the test areas to points of water discharge or extraction. In particular, the Paleozoic carbonate strata are capable of transposing large quantities of water quickly toward these points of water use (Winograd and Thordarson, 1975;Dettinger, 1989;Laczniak and others, 1996). Knowledge of the structural conditions and stratigraphic relationships within the carbonate aquifer is essential to evaluate risks associated with the manmade contamination resulting from nuclear-weapons testing.…”

Structure of pre-Cenozoic rocks in the vicinity of Yucca Mountain, Nye County, Nevada: A potential nuclear-waste disposal site

“…Seismic reflection studies (L. W. Pankratz and H. D. Ackermaa, written commun., 1982) have not proved useful with respect to subvolcanic relations. Hydrologic information has been derived from Winograd and Thordarson (1975), Doty (1980), andWaddell (1982).…”

“…The volcanic rocks are estimated to be as much as 4,500 m thick north of Yucca Mountain but are believed to thin abruptly under the mountain and probably to continue thinning southward beneath Quaternary cover in the Amargosa Desert, as suggested by gravity (Snyder and Carr, 1982) and aeromagnetic (Kane and Bracken, 1983) data. Regional-ground water flow is generally to the south, toward discharge areas in the southeastern Amargosa Desert (Winograd and Thordarson, 1975). Hydrologic studies indicate that effluents leaking from a repository in the unsaturated volcanic rocks could drain downward into the ground-water flow system and find pathways in the older rocks; consequently, it is important to know the lithology and structure of the rocks beneath the volcanic rocks, especially south of the site.…”