WRIR 01-4210: Hydraulic-Property Estimates for Use With a Transient Ground-Water Flow Model of the Death Valley Regional Ground-Water Flow System, Nevada and California
Previous Section: Introduction |
Next Section: Data Analysis and Synthesis
Table of Contents | Conversion
Factors and Acronyms | Report Home Page
The Death Valley region has an active geologic history, including intermittent marine and non-marine sedimentation, large-scale compressive deformation, plutonism, volcanism, and extensional tectonics (Stewart, 1980; Mifflin, 1988). Much of the study area has undergone deformation, and some parts have experienced nearly continuous tectonic activity since the late Proterozoic (Grose and Smith, 1989). The structural features and faulting in the region are a result of the complex interaction of the North American and Pacific lithospheric plates (Smith and Sbar, 1974; Atwater and Stock, 1998). Combinations of normal, reverse, and strike-slip faulting and folding episodes (Carr, 1988) have resulted in a complex distribution of rocks. Consequently, diverse rock types, ages, and deformational structures are often juxtaposed and subsurface conditions are variable and complex. Knowledge of the geology beneath the alluvial basins is indirect in most of the region. The rocks of the Death Valley region are comprised of Proterozoic and Cambrian siliciclastics and metamorphics; Paleozoic siliciclastic and carbonates; Mesozoic siliciclastics and intrusives; Pliocene fluvial, paludal, and playa sedimentary deposits; Tertiary volcanics and alluvium; and Tertiary alluvium and colluvium; and Quaternary eolian deposits (Waddell, 1982). Plate 1 presents a generalized stratigraphy of the Death Valley region.
Hydraulic connection between basins within the DVRFS occurs through unconsolidated sediments present atop low interbasinal topographic divides and by deep interbasinal flow beneath valley floors and adjacent mountains through fractured Paleozoic carbonate rocks (Winograd and Thordarson, 1975; Prudic and others, 1995).
Faults can disrupt stratigraphic continuity, thereby diverting water in regional circulation to subregional and local outlets. Within the Death Valley region, faults and related fractures exert the greatest influence on ground-water flowing through bedrock aquifers (Faunt, 1997).
Ground-water flow is controlled also by lithologic variability along flow paths. In basin-fill sediments, changing depositional environments over short distances may result in substantial facies changes that can affect transmissivity and hydraulic conductivity, particularly where silt and clay become intermixed or interbedded with sand and gravel (Plume, 1996). In volcanic rocks, a characteristic change from lava flows to welded tuffs and, ultimately, non-welded and bedded tuffs with increasing distance from eruptive centers can cause hydraulic properties of the stratigraphic unit to exhibit great spatial variability (Laczniak and others, 1996).
Lateral facies changes within Paleozoic rocks might affect permeability. For example, a westward facies change in Mississippian rocks from predominantly limestone and dolomite to predominantly argillite and quartzite produce a barrier to regional ground-water flow in the vicinity of the NTS (Winograd and Thordarson, 1975). Cambrian and Proterozoic clastic, igneous, and metamorphic rocks force water upward into overlying aquifers and create flow-system boundaries throughout the Death Valley region (Winograd and Thordarson, 1975).
Factors other than lithology and structure in the Death Valley region that influence permeability and ground-water flow include increasing cementation of basin-fill sediments with age and decreasing fracture volume in bedrock aquifers (Winograd and Thordarson, 1975), alteration and welding in tuffs (Laczniak and others, 1996), and the effects of hydrochemical changes in response to thermal gradients (Moore and others, 1984).
Physical characteristics were used by Winograd and Thordarson (1975) to group geologic formations of hydrologic significance in the vicinity of the NTS into HGU's. The seven HGU's defined by Winograd and Thordarson (1975), from oldest to youngest are: the lower clastic aquitard (currently termed the lower confining unit); the lower carbonate-rock aquifer; the upper clastic aquitard (currently termed the upper confining unit); the upper carbonate-rock aquifer; the tuff aquifers (currently termed volcanic-rock aquifers); volcanic aquitards (currently termed the volcanic confining units); and the valley-fill aquifer (currently termed alluvial aquifer). The lower confining unit forms the basement and generally is present beneath the other units except in caldera complexes. The lower carbonate-rock aquifer is the most extensive and transmissive in the region, but does not control ground-water flow within the caldera complexes. The upper confining unit is present in the north-central section of the NTS and restricts flow between overlying and underlying units; this unit also is associated with many of the steep hydraulic gradients in and around the NTS. The upper carbonate aquifer exists where it is physically separated from the lower carbonate aquifer by the upper clastic confining unit. The volcanic-rock aquifers and the volcanic confining units form a stacked series of alternating aquifers and confining units in and around the Southwest Nevada Volcanic Field (SWNVF). The volcanic-rock aquifers are moderately transmissive and are saturated in the western section of the NTS. The alluvial aquifer, though discontinuous, forms an important regional aquifer.
The major HGU's originally defined by Winograd and Thordarson (1975) form the basis of HGU's used in previous modeling studies (D'Agnese and others, 1997; IT Corporation, 1996a), in the ongoing DVRFS transient modeling study (Claudia Faunt, U.S. Geological Survey, written commun., 2001), and in this report. Although all the major geological features were retained, many of the smaller geologic units were grouped into larger entities by generalizing lithologic and hydrologic properties of the formations (fig. 3). Furthermore, the categorization of aquifers and confining units as distinct strata fails to account for structurally and lithologically controlled variations in hydraulic properties within geologic units and vertical ground-water flow between geologic units with different lithologies. On a regional scale, those factors exert strong influences on ground-water flow. While these terms are a useful designation, readers are cautioned about inferring hydraulic properties for a particular HGU, generally obtained from local-scale tests, to the hydraulic connectivity regional scale.
The DVRFS transient modeling study has further subdivided the unconsolidated sediments and consolidated rocks into 19 HGU's (table 1). For the purposes of this study, several of the DVRFS transient model HGU's were combined into a single HGU, such that a total of 11 HGU's are used (table 1). Each of the 11 HGU's has a quasi-uniform geological, structural, and hydrological characteristic and is laterally extensive.
Previous Section: Introduction |
Next Section: Data Analysis and Synthesis
Table of Contents | Conversion
Factors and Acronyms | Report Home Page