Browsing by Subject "Chihuahuan Desert"
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Item Analysis of Unsaturated Flow Based on Chemical Tracers and Comparison with Physical Data, Chihuahuan Desert, Texas(1990) Scanlon, Bridget R.; Wang, Fred P. Â ; Richter, Brend C.Personnel from the Texas Low-Level Radioactive Waste Disposal Authority office in Fort Hancock, Texas, assumed responsibility for all field service of rain gauges and erosion pins in January 1990. Since then, they have monitored the rain gauges monthly and have measured the erosion pins five times: once in March, twice in July, once in August, and once in October. All monitoring activities have been carried out as prescribed in Specific Work Instruction (SWI) 3.6. Data collection has been quite satisfactory, with no data losses due to operator error, and only minor errors in record keeping. The system of repairing gauges has functioned reasonably well by balancing the economics of repairing gauges only when necessary against possible data loss caused by occasional lack of a spare gauge when one in operation fails unexpectedly.Item Analysis of Unsaturated Flow Based on Physical Data Related to Low Level Radioactive Waste Disposal Chihuahuan Desert, Texas(1990) Scanlon, Bridget R.; Wang, Fred P. Â ; Richter, Brend C.Unsaturated flow in arroyos and fissures in the Hueco Bolson of West Texas was studied to determine if downward fluxes are higher beneath these features relative to other geomorphic settings. Soil samples collected from five boreholes in two arroyos and in and adjacent to a fissure were analyzed for moisture content, water potential, and chloride content to evaluate moisture flux. In addition, three neutron-probe access tubes were installed in and adjacent to a fissure to monitor moisture content over time. A ponding test with CaBr2 as a tracer was conducted at a fissure to compare moisture movement in the fracture fill with movement in the surrounding sediments. The arroyos are floored by a shallow (~2-m [~6.6-ft]) layer of surficial gravel underlain by clays. Moisture content was low in the gravel (~4%) and much higher in the clay (21% to 33%). Water potentials decreased with depth, which indicates a potential for downward flux. A sharp increase in chloride concentrations below the contact between the shallow gravels and deeper clays is attributed to a larger mass of water influenced by evaporation from the low-permeability clay material. Soil texture and moisture content in the fissure were similar to that in surrounding sediments. Water potentials were close to 0 at depth in the fissure as well as in surrounding sediments and decreased toward land surface, which suggests a potential for upward liquid water flow. Chloride concentrations were approximately two orders of magnitude lower in the fissure than in the clays of the arroyos. These low chloride concentrations indicate that chloride is being flushed downward in the region of the fissures. The ponding test showed that the downward flux of chloride and bromide was greater in the fracture fill than in surrounding sediments. Water potentials in the fissures and arroyos are higher than those found in the ephemeral stream and interstream settings, and the higher potentials are attributed to wetter conditions in the former geomorphic settings. The upward decrease in water potentials in the fissures is similar to that found in the stream/interstream settings and suggests upward liquid water movement. These water-potential gradients contrast with those found in the arroyos, which are downward and indicate a potential for downward water movement. Chloride concentrations in the arroyos are similar to those found in the stream/interstream sediments; however, chloride concentrations in the fissure sediments are an order of magnitude lower than those in other geomorphic settings. These comparisons suggest that the downward flux of water is greater in the fissures than in other geomorphic settings.Item Evaluation of Subsurface Flow in Fissured Sediments in the Chihuahuan Desert, Texas(1995) Scanlon, Bridget R.; Goldsmith, Richard S.Fissures are surface features, or gullies, some of which are underlain by sediment-filled fractures. A previous study of subsurface flow beneath a fissure showed higher water fluxes beneath the fissure, which was attributed to infiltration of ponded water in the fissure. This study was conducted to investigate the vertical and lateral extent of increased flow associated with fissured sediments, to compare subsurface flow beneath fissures of different maturity, and to examine different techniques for evaluating flow in fissured zones. Boreholes were drilled directly beneath four fissures and at distances of 10 m and 50 m from the fissures, and soil samples were analyzed for various soil physics parameters and environmental tracer distribution. Electromagnetic induction was used to map apparent conductivity in transects perpendicular to the fissures. Fissures had higher water potentials and lower chloride concentration than surrounding sediments. Zones of high flux were restricted to the area directly beneath some fissures, whereas others also had high fluxes in the profiles 10 m distant from the fissure. Water potential and chloride fronts were found beneath two of the fissures in the upper 20-m zone, which indicates that most of the flow occurred in this zone. Water flux estimates, based on the position of the chloride front and an assumed age of the fissures of 50 yr, ranged from 28 to 48 mm yr^-1. High tritium levels were found throughout the fissured profiles (to maximum depth of 26.4 m) and in some cases in the profiles 10 m distant from the fissure, indicating post-1952 water. The occurrence of high tritium levels beneath the chloride front in one fissure indicates that some of the water is flowing preferentially. Minimum estimates of water flux based on the tritium data ranged from 28 to 120 mm yr^-1. Stable isotopes of oxygen and hydrogen were less enriched beneath the fissure, which is consistent with higher fluxes beneath the fissure. Plant water potentials were of limited use in delineating fissure flow. Apparent conductivities were higher across two fissures, whereas the other two fissures did not show any variation in apparent conductivity. The higher conductivity in some fissures is attributed to higher water content. Multiple independent lines of evidence indicate that subsurface water fluxes are higher beneath fissures.Item Evaluation of Unsaturated Flow in Fissured Sediments in the Chihuahuan Desert, Texas(1994) Scanlon, Bridget R.; Goldsmith, Richard S.; Gustavson, Thomas C.Localized flow in fissured sediments in arid settings has important implications for waste disposal in these regions. Fissures are surface features or gullies that are underlain by partially open or sediment-filled fractures. The objectives of this study were to compare unsaturated flow beneath different fissures, investigate the vertical and lateral extent of increased flow associated with fissured sediments, and examine different techniques for evaluating flow in fissured zones. Boreholes were drilled directly beneath four fissures and at distances of 10 and 50 m from the fissures. Sediment samples were analyzed for hydraulic parameters such as water content and water potential and environmental tracers such as Cl, 36Cl, 3H, 2H, and 18O. A trench was dug beneath one fissure for detailed sampling. Electromagnetic induction was used to measure apparent electrical conductivity in transects perpendicular to the fissures. Unsaturated flow is relatively high beneath fissures, as evidenced by higher water potentials and lower chloride concentrations there than in surrounding sediments. The lateral extent of high water flux was restricted to the zone directly beneath one fissure but extended to profiles 10 m from two other fissures. The profiles 50 m distant from all fissures had low water fluxes, as indicated by low water potentials and high maximum chloride concentrations. The vertical extent of high water fluxes was restricted to the upper 10 to 20 m, as shown by water potential and chloride fronts within the upper 10 m zone beneath one fissure and by chloride fronts in the upper 20 m zone beneath and 10 m distant from another fissure. Additional evidence for localized water flux was provided by high tritium levels, less-enriched 2H and 18O, and higher plant water potentials in fissured sediments than in nonfissured sediments. Apparent electrical conductivity was higher in two of the four fissures. Multiple independent lines of evidence indicate that subsurface water fluxes are higher at shallow depths beneath fissures; however, the various techniques differ in their effectiveness in delineating higher water fluxes beneath fissures. Multiple profiles drilled in one fissure indicate that there is large variability in flow along this fissure that is attributed to topographic variations and degree of ponding.Item Landscape Evolution of Eagle Flat and Red Light Basins, Chihuahuan Desert, South-Central Trans-Pecos Texas(1993) Langford, Richard P.This report documents the development of the landscape near the site proposed for the Texas low-level radioactive waste repository, located in southern Hudspeth County, Texas. It documents the geomorphic, depositional, and erosional features in the area. The Texas Legislature designated an approximately 400 square mile (1,035 square kilometer) area as the Eagle Flat Study Area, within which the site was to be selected. The study area consists of six U.S. Geological Survey 7.5-minute topographic quadrangles between the towns of Sierra Blanca and Van Horn, Texas. The six-quadrangle Eagle Flat Study Area contains a large part of Eagle Flat Basin and a smaller area in Red Light Draw. The Eagle Flat Study Area contains parts of the Belson and Sacramento subsections of the Mexican Highlands section in the Basin and Range physiographic province. The Belson subsection is characterized by broad, internally drained alluvial basins, interrupted by rugged, discontinuous fault-block mountains. The mountains are composed of Cretaceous and Permian carbonate rocks with scattered areas of Tertiary intrusives and volcanics, older Paleozoic shata, and Precambrian metamorphic rocks. In the Belson subsection, the uplands make up about one-fifth of the area, whereas uplands make up 27 percent of the entire Eagle Flat Study Area. The Eagle Flat and Red Light Draw Basins, in the Belson subsection, are floored by Pliocene and Pleistocene alluvial sands and muds. The Sacramento subsection is represented by the Diablo Plateau in the northern part of the study area. This plateau is an upland area of Cretaceous hills separated by broad alluvial valleys with thin sedimentary cover. The Eagle Flat Study Area contains the four geomorphic regions: the mountainous region, the upland region, the basin floor, and the Sacramento Plateau.Item Molecular systematics and the origins of gypsophily in Nama L. (Boraginaceae)(2012-05) Taylor, Sarah Elizabeth; Simpson, Beryl Brintnall; Jansen, Robert K.; Levin, Donald A.; Panero, Jose L.; Mueller, Ulrich G.Nama L. is a genus of approximately 50 species of herbs and subshrubs that occurs in habitats ranging from arid deserts to mesic woodlands in the New World and the Hawaiian Islands. The group has historically been divided into five or six subgeneric groups based on habitat as well as on the morphology of the anthers, styles, leaves and seeds. At least 14 species of Nama from the Chihuahuan Desert Region are either facultatively or obligately endemic to gypsum deposits. This dissertation examines interspecies relationships within Nama from a molecular phylogenetic perspective in order to evaluate historic morphology-based subgeneric classification systems of the genus and to examine the origins of facultative and obligate gypsophily within the genus. DNA sequence data from the chloroplast regions matK and ndhF and from the nuclear ribosomal region ITS were collected from 46 species of Nama as well as from four new species and several outgroups. Data were analyzed using maximum likelihood and Bayesian methods. Phylogenetic analyses recover seven strongly supported major lineages within Nama. These lineages do not correspond to traditionally recognized subgenera, although they are largely congruent with an informal system based on ultrastructural observations of seeds. Four of the seven major lineages include gypsophilous species; these range from two lineages that include a single facultative gypsophile each, to one lineage that is almost entirely comprised of gypsophiles. Gypsum endemism in general, as well as facultative and obligate gypsophily in particular, has arisen multiple times in Nama. Parametric bootstrapping rejected the hypothetical monophyly of gypsophiles across the genus as a whole and within each of the two clades that contain multiple gypsophiles. Because approximately 20 species have been described since the last major revision of Nama nearly 80 years ago, detailed morphological observations of herbarium specimens were made in order to produce a comprehensive key to the species of Nama as well as the revision of a lineage comprising eight gypsophiles and one limestone endemic.Item Proceedings of the Desert Fishes Council Special Publication 2021(Desert Fishes Council, 2021) Bean, Megan; Garrett, Gary; Hoagstrom, Christopher