Characterization of an arid groundwater flow system using satellite imagery

Transcription

1 Characterization of an arid groundwater flow system using satellite imagery A. Barud-Zubillaga & D. Schulze-Makuch The University of Texas at El Paso, El Paso, United States of America ABSTRACT: This research addresses the hydrogeology of the White Sands National Monument (WSNM), characterizing a small portion of the south-central part of New Mexico s desert. Satellite imagery were utilized to characterize an arid shallow groundwater flow system. Landsat-7 data sets were used to identify the different terrains leading to a better understanding of the White Sands, New Mexico. Results showed that the ancient Lake Otero floor has been carved deeper in the western margin of the White Sands area by water and wind. In the Alkali Flat region ancient deposits of the Lake Otero floor are exposed. The entire White Sands Dune Field today is increasing in size as the Alkali Flats and Lake Lucero shrink due to dryer conditions as time goes on. The active dune field is growing on top of the ancient Lake Otero floor to the east. Groundwater flow is to the west and discharges in the hydraulic sink of Lake Lucero, a highly saline playa ephemeral lake. 1 INTRODUCTION White Sands National Monument (WSNM) is located in south-central New Mexico within the boundaries of the Tularosa Basin. Parts of the White Sands area have been extensively used as a military test facility after World War II. This military range continues to be an important testing site for experimental weaponry and space technology. Lake Lucero is the lowest point in the Tularosa Basin with an elevation of 1,186 m above mean sea level. This lake, which has surface water less than 10% of the time, is a receptor of runoff from the San Andres and Sacramento Mountains. The analysis of satellite imagery using Landsat-7 data resulted in a highresolution differentiation of very similar terrains. 2 HYDROGEOLOGY OF WHITE SANDS 2.1 Geology White Sands dune field has been estimated to be between 12,000 and 24,000 old (Weber and Kottlowski, 1959). Several types of dunes are found here due to the great variability of wind regimes (Ahlbrandt et al., 1994). Rates of deposition vary from zero to eleven meters per year (McKee and Douglass, 1971). Precambrian rocks in the Sacramento and San Andres Mountains are the oldest exposed rocks bounding the Tularosa Basin. The Tularosa basin has a down-faulted graben structure. The oldest exposed rocks found in the basin are the Paleozoic Hueco Limestone and the Yeso Formation. Lithospheric extension occurred during the late Cenozoic (Keller et al., 1990). Since then, the Tularosa Basin has been filling mostly with bolson and evaporite deposits. The middle Tularosa basin consists of two graben blocks that lie between the San Andres and Sacramento Mountains. The WSNM lies above the western block. Lake Otero, the predecessor of Lake Lucero, contains high amounts of salts that were deposited from Permian marine evaporites (Allmendinger and Titus, 1973). Active aeolian deposits are found in the White Sands dune field. They are mostly concentrated in the eastern part of the study area and overlie the lacustrine deposits. Preserved sets of dune cross-strata and interdune deposits can be attributed to scour of different dunes down to damp sand near the water table (Simpson and Loope, 1985; Fryberger et al., 1988). 2.2 Hydrology The WSNM station has reported a total monthly precipitation of up to 9 cm during September of 1999 (NOAA, 2000). During summer evaporation in the basin is very high. Based on a four-year record, an evaporation rate of cm per year was reported by the U.S. Weather Bureau, which would classify this site as a Class A evaporation area (Engineers, 1998). Lake Lucero is a discharge area for the middle Tularosa basin. Surface runoff from the San Andres Mountains reaches this ephemeral lake and slowly infiltrates into the strata below. No surface water from the Sacramento Mountains is able to discharge into Lake Lucero due to the gypsum sand dunes that block its way. Groundwater in the eastern Tularosa Basin and north of Alamogordo flows to the west towards Lake Lucero and then southwards.

2 Figure1. Landsat 7 image showing in-situ spectral sites within the White Sands National Monument 3 METHODS Several sites were surveyed (Figure 1) for spectral reflectance values to construct a supervised classification of the White Sands field. Landsat-7 Enhanced Thematic Mapper plus (ETM+) data sets were provided by the NASA sponsored Pan-American Center for Earth and Environmental Studies (PACES) that is partially housed in the Department of Geological Sciences at the University of Texas at El Paso (UTEP). The satellite data set used was acquired on June 10, As part of a supervised classification process included in this study, ten training sites were selected. They correspond to areas with significantly different reflectance. In situ spectral reflectance measurements were taken throughout the Monument with a portable spectroradiometer model GER These sites were surveyed during the rainy season when soil moisture is relatively high due to high permeability of the sediments. Surveyed spectral reflectance curves for the training sites were resampled with ENVI software to match the response of the ETM+ Landsat-7 sensor. Training sites for this study were defined with the endmember collection tool. The supervised classification performed on this study was a parallelepiped classification, which uses a simple decision rule that defines the boundaries from an n-dimensional parallelepiped in the image data space. The boundaries of the parallelepiped were of 10 digital numbers (DN) from the maximum spectral reflectance curve measured at each training site. The output image using the 20 DN yielded the best results compared to the other images tested (10, 15 and 40 DN). In this study, the authors used two sets of band combinations in order to investigate lineaments, vegetation, and moisture content at the study area (Barud-Zubillaga, 2000). One of the combinations was bands 7 (red), 5 (green), and 4 (blue), and the other was bands 7 (red), 5 (green), and 2 (blue). These two combinations provided a good differentiation of terrains in the White Sands area that have never been reported before using satellite imagery. 4. RESULTS 3.1 Principal Components Analysis (PCA) The Landsat-7 data were subjected to a PCA using bands 7, 5 and 2 and entered in as first principal components as follows: PC 1 in red, PC 3 in green, and PC 2 in blue. Important different land surfaces were depicted in this processed satellite image (Figure 1). Ancient gypsiferous aeolian deposits characterize the area shown in light blue in the center. These aeolian deposits may have experienced some early diagenetic alteration. According to the PCA process, the same deposits are present southeast of site SR-06, with a semicircular shape trimmed to the east. The water table in the area of these ancient deposits is much deeper than in the rest of the park

3 (Conrod, 1999) due to higher topographic elevation of the surface. Based on the regional water table map (Barud-Zubillaga, 2000) and the topographic elevation in the area, depth to water table should be in the range of 5 to 30 m below land surface. Light green color shows the tallest dunes in the area. They are also the driest gypsum sand grains with the highest reflectance values from the region. A darker green represents the interdune flats within the dune field. These regions contain in some cases some scarce vegetation. In most of these interdune flats, foresets of newly eroded sand dunes are well exposed. These foresets belong to the base of eroded dunes that had migrated northeast and continue to do so to form new dunes. The red color indicates a much higher moisture content of the soils, which is characterized by lacustrine and fewer aeolian deposits. Hiking in the northwestern quarter of the park, these deposits are coarser toward the west between sites SR-05 and SR-04. In addition, elongated gravel 2 to 4 mm long and 1 to 3 mm wide was observed between sites SR-03 and SR-04. For the area shown in red at Lake Lucero, these deposits have a much finer grain size above mostly lacustrine deposits. Moisture content is less uniform at South Lake Lucero than in the northern part of the monument. The pink patches along the west and south park boundaries, at North Lake Lucero, and at the southeastern corner of the map shown in Figure 1 represent shadows. The patches in dark purple that are located about 1 km southeast of the pink patches are the clouds responsible for these shadows. Areas colored in light purple and cyan, are considered as lightly to sporadic vegetated areas, overlying Quaternary deposits. 3.2 Supervised Classification (SC) A SC process applied to the remote sensing data set is shown in Figure 2. It provides a characterization of the existing terrains that are called classes. Seven classes were identified with this process. Although colors on Figure 2 for the San Andres Mountains and in the Tularosa Basin are the same, they do not belong to the same classes. Colors at the San Andres Mountains shall be ignored as part of the SC study. Even though these colors do not correspond to the same classes, they might be greatly correlated because of land features such as high moisture content, density of vegetation, etc. The supervised classification image shown on Figure 2 contains seven classes. Spectral reflectance surveyed sites are located within a specific class explained below. Field observations from the authors were used to determine the seven classes generated with this process. The first class on Figure 2 is Class 1 shown in red. Spectral reflectance site SR-01 lies within the boundaries of this class. The surface cover are fine- grained aeolian deposits with some lacustrine deposits below that. The only plants in the area are shrubs no larger than 80 cm high. These shrubs are distributed about every 6 to 8 meters apart in all directions Class 2 of the SC is shown in blue. As part of the effort to define Class 2, spectral reflectance readings were acquired for site SR-02. Dunes in this area coexist with a lot of cottonwood trees and salt cedars. Most of this vegetation is located on the southeastern side of the dunes. Class 2 is characterized by active dry gypsum sand dunes that are located along the middle to eastern side of the monument and extend to the north into the WSMR. Flat areas between the dunes are also present throughout the dune field. Grass and some small bushes grow on them, while in some cases they are completely devoid of vegetation. These flat areas are part of Class 7 explained below. Class 3 is another important unit for understanding the hydrogeology of White Sands National Monument. The aquamarine color shows this class mainly in the south of the White Sands area, with some small areas southeast of SR-06. This area had a very similar spectral signature to areas from Class 2. In terms of the type of surface, they are both gypsum sand dunes, but the ones in Class 3 belong to an ancient gypsum sand dune field. These exposed ancient eolian deposits had some level of early diagenetic alteration. The area classified as Class 3 is also the highest land in the monument. Class 4 was determined in part by the in situ spectral reflectance readings at site SR-03. The surface consists of poorly consolidated gypsiferous and some gravel deposits, mostly from a lacustrine environment. In most cases, the Class 4 surface is a terrace forming terrain. The majority of these terraces are facing east towards the Alkali Flat. Close to site SR-03, terraces are about 10 to 13 m higher than Alkali flat. Canyons of up to 4 meters deep were cut through some of these terraces by running water from the San Andres Mountains. Even though the terrain close to site SR-03 looks very different than the terrain at the southern end of Figure 2, the SC process grouped these two areas in one due to the high similarity of reflectance signatures acquired by the satellite sensor. Class 5 is shown in green. Class 5 is the area with higher moisture content in its soils. During one of the visits in August, about 3 cm of surface water was present in irregular patches of about 150 to more than 1,000 m 2. Site SR-04 is also characteristic of Class 5. The Alkali Flat in this region has no vegetation at all; it is very flat and contains almost no aeolian deposits. Coarse-grained gravel with a diameter of about 2-4 mm with even coarser grained, less rounded and more elongated sediments to the east towards SR-05 were observed.

4 Class 1. Eolian and some lacustrine deposits. Reasonably flat with a few bushes no higher than 80 cm high at site SR-01 at the southern edge of the Alkali flat. Smaller bushes present up north. (RED) Class 2. Active white gypsum sand dune field with deposition rates from 0 to 11 m per/yr. (BLUE) Class 3. Diagenetic old dune field of ancient poorly consolidated gypsiferous eolian deposits. Land with higher resistance to erosion made this region the highest land on the White Sands National Monument. ( AQUAMARINE) Class 4. Old lacustrine deposits from ancient Lake Otero. Mostly covered with high grass and different types of mesquites, creole, and rarely some cactus. (LIGHT BLUE) Class 5. Western Alkali Flat, which consists of lacustrine deposits. Soils with higher moisture content and even surface water during the rainy season. No vegetation at all. (GREEN) SR-05 SR-03 SR-04 Class 6. Middle Alkali Flat containing lacustrine and eolian deposits with a bumpy surface due to a more weather resistant preserved dune foresets. Scattered 1 to 1.5 m high bushes scares in this area. (YELLOW) SR-06 SR-10 SR-10 SR-09 SR-08 SR-07 Class 7. Eastern Alkali Flat and Interdune Flat. This class is well spread out west of every dune terrain. It coexists with Class 2 as an interdune space between dunes. Vegetaion varies between grass and small to medium moderately dense bushes. preserved dune foresets. Lonely 1 to 1.5 m high bushes scares in this area. (ORANGE) Clouds and their shadow SR-02 SR-01 MW-000 Monitoring well, Push piezometer or regular piezometer. Name. SR-00 In-situ spectral reflectance measurement. Name. PND-00 Other important site. Name White Sands National Monument boundary Figure 2. Supervised classification of White Sands, south-central New Mexico. The area within Class 5 correlates with low topography throughout the White Sands area. The surface elevation of the Alkali flats of Class 5 is about 1,187 m, but in adjacent areas the elevation is higher than 1,190 m. With this said, Lake Lucero and the area within Class 5 are the only areas with topographic elevations below 1,190 m. Since the region with lower elevations is class 5, it is also the area with higher moisture content, which may be covered by runoff during the rainy season. There are a few patches of green on Figure 2 that correspond to light cloud shadows in the area. Clouds appear in pink about 1 km southeast of their shadows. Class 6 occupies two main areas. One is in the middle of the Alkali Flats and the other is Lake Lucero. Site SR-05 is located in the middle of the Alkali Flat at the northern edge of the park. The surface in this area is very similar to SR-01 although vegetation is much less abundant. The terrain at site SR-05 is a little hummocky with a surface full of very small and low mounds, ranging in elevation from 7 to 10 cm high separated by distances of 5 to 7 m. Vegetation is almost nonexistent but still present. Bushes existing in this area are 1 to 1.25 m high and are distributed between 60 to 100 m from each other. Although the surface at Lake Lucero is much flatter than at site SR-05, both areas belong to the same class 6 shown in yellow on Figure 2. Even though it is very flat, some drainage channels exist south of site PZ-04, where a few channels trend mostly from north to south. Bigger channels are more active in the western side of the park, where they develop small alluvial fans like the one at site PZ-02. Class 7 is shown in orange on Figure 2. A flat surface with very dry gypsum sand deposits is found within this class. The area contains patches of very sparsely vegetated areas with small bushes less than 50 cm tall. The environment in this area is very hostile because of a lack of water. Surface runoff from the San Andres or the Sacramento Mountains does not reach this area. Part of the Sacramento Mountains runoff coming from La Luz Creek flows through the Lost River which is blocked by the dunes and disappears before getting into this area. Interdune surfaces where grass can grow easier than in the eastern Alkali Flat are included in Class 7. These areas are also the interdune spaces that are located further into the dune field (e.g. Site SR-09).

5 4 CONCLUSIONS The remote sensing results confirm that areas with lower elevations are the wettest in the region. Based on data from the surveyed sites, depth to water table is closer to the surface in these regions than in any other area in the Tularosa Basin. These lower and wetter regions are formed by incision into older lacustrine deposits. Areas further to the east are dryer regions that contain greater amounts of aeolian deposits rather than lacustrine deposits. The main areas assigned Class 4 are interpreted as ancient Lake Otero floor that has been carved deeper and deeper in the western margin of the White Sands area. Some authors suggest that the Alkali Flat region is carved into the Lake Otero floor that has being eroded by water and wind since the Pleistocene (Meinzer and Hare, 1915; Allmendinger, 1971; Langford, 2000). The active White Sands Dune Field today is increasing in size as the Alkali Flats and Lake Lucero shrink due to dryer conditions as time goes on. The processed satellite images, field observations, data collected and analyzed, and references used, suggest that the existing active dune field is growing on top of the ancient Lake Otero floor, while the ancient Lake Otero lacustrine deposits to the west are the source of these gypsum sand dunes (Allmendinger, 1971; Herrick, 1904). REFERENCES Ahlbrandt, T. S., Gautier, D. L., and Bader, T. A., 1994, Low- Angle Eolian Deposits in Coastal Settings: Significant Rocky Mountain Exploration Targets: The Mountain Geologist, v. Vol. 31, no. 4, p Allmendinger, R. J., 1971, Hydrologic control over the origin of gypsum at Lake Lucero, White Sands National Monument, New Mexico [Master of Science thesis], 82 p. Allmendinger, R. J., and Titus, F. B., 1973, Regional Hydrology and Evaporative Discharge as a Present-day Source of Gypsum at White Sands National Monument, New Mexico. New Mexico Bureau of Mines & Mineral Resources, Open file report 55. Barud-Zubillaga, A., 2000, A conceptual model of the hydrogeology of White Sands National Monument, Southcentral New Mexico [Master thesis]: The University of Texas at El Paso, 138 p. Bobba, A. G., Singh, V. P., and Bengtsson, L., 2000, Application of environmental models to different hydrological systems: Ecological Modeling, v. Vol. 125, p Conrod, W., 1999, Tour in the White Sands National Monument - personal communication. Elston, W. E., Clemons, Kelly, V.C. 1981, New Mexico Geologic Highway map: New Mexico Geological Society. Engineers, T., 1998, Proposed expansion of the German facilities at Holloman Air Force Base: Engineers. Fryberger, S. G., Schenk, C. J., and Frystinik, L. F., 1988, Stokes surfaces and the effects of near-surface groundwater-table on eolian deposition: Sedimentology, v. Vol. 35, no. 1, p Herrick, C. L., 1904, Lake Otero, an ancient salt lake basin in southeastern New Mexico: American Geologist, no. September, p Keller, G. R., Morgan, P., and Seager, W. R., 1990, Crustal structure, gravity anomalies and heat flow in the southern Rio Grande rift and their relationships to extensional tectonics: Tectonophysics, v. V. 174, p Langford, R., 2000, Assistant professor of the Geological Sciences Department at the Personal communication. Lanka, K., 1995, An integrated study of the subsurface structure of the Tularosa Basin, South-central New Mexico [Master of Science thesis]: University of Texas at El Paso, 64 p. McKee, E. D., 1966, Structures of dunes at White Sands National Monument, New Mexico (and a comparison with structures of dunes from other selected areas): Sedimentology, v. 7, p McKee, E. D., and Douglass, J. R., 1971, Growth and movement of dunes at White Sands National Monument, New Mexico: U.S. Geological Survey, U.S. Geological Survey Prof. Paper 750-D. Meinzer, O. E., and Hare, R. F., 1915, Geology and Water Resources of Tularosa Basin, New Mexico: U.S. Geological Survey, Water-Supply Paper 343. NOAA, 2000, Official website of the National Weather Service, National Oceanographic Atmospheric Administration. Orr, B. R., and Myers, R. G., 1986, Water resources in basinfill deposits in the Tularosa Basin, New Mexico: U.S. Geological Survey, Water Resources Investigations Report Simpson, E. L., and Loope, D. B., 1985, Amalgamated interdune deposits, White Sands, New Mexico: Journal of Sedimentary Petrology, v. Vol. 55, no. 3, p Weber, R. H., and Kottlowski, F. E., 1959, Gypsum Resources of New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin, v. 68, p. 68. Whitlock, C. H., LeCroy, S. R., and Wheeler, R. J., 1994, Narrowband angular reflectance properties of the alkali flats at White Sands, New Mexico: Remote Sensing of Environment, v. V. 50, no. 2, p