Characterization of hydrostratigraphy and groundwater flow on the southwestern San Andres Mountains pediment, NASA-JSC White Sands Test Facility
Geoffrey C. Giles and John W. Pearson
National Aeronautics and Space Administration environmental investigations at White Sands Test Facility require the detailed characterization of groundwater flow through fractured bedrock. East-to-west groundwater flow from San Andre Mountain-front recharge areas across the pediment slope is complex as a result of variable hydrostratigraphy and fracturing within the bedrock aquifer. Previous investigations have indicated that flow across the pediment is enhanced by a saturated alluvial paleochannel eroded in the bedrock and infilled with post-Laramide detritus derived from the Bear Canyon fold and thrust belt. The paleochannel contains Santa Fe Group alluvial-fan deposits, and is locally intercepted by the groundwater table to create a saturated alluvial hydrostratigraphic unit. Elevated hydraulic conductivities within this proposed unit have been used to conceptualize groundwater flow within a three-dimensional site-wide bedrock groundwater model. A 1997 field study evaluated a narrow midsection of the paleochannel formed as a result of confining hydrogeologic conditions. A pumping well (IS-1), 1000 ft Westbay® multiport monitoring well (BLM-33), and several existing conventional wells were utilized for an aquifer test to delineate hydrostratigraphic units and groundwater flow. A stratified sequence of five hydrostratigraphic units (HUs) were correlated across the study area within Oligocene volcanic bedrock between the groundwater table at 300 ft and 1020 ft. These units were designated: Santa Fe Group alluvium, <30 ft thick (HU-1); trachyte, 50-60 ft thick (HU-2); rhyolitic ash-flow tuff, 120-250 ft thick (HU-3); interbedded rhyodacites/quartz rhyodacites, 250-320 ft thick (HU-4); and dacite, >150 ft thick (HU-5). Study results indicate a minimal thickness of saturated alluvium and no enhancement of groundwater flow. Pervasive carbonate cementation of the saturated alluvial matrix drastically reduces the porosity of the alluvium to that of fractured bedrock (<5%). A step drawdown test and continuous rate-pumping test were performed at well IS-1 within the relatively productive HU-4. Groundwater depths were monitored using an automated data-logging system that allowed the simultaneous measurement of formation fluid pressures at several locations within West bay® multiport and adjacent aquifer test wells. Pump test data calculations yielded hydraulic parameters for HU-4 of: K = 2.1 x 10-6 to 1.4 x 10-5 ft/sec; Ta = 58.6 to 386. 5 ft2/day; and S = 1.5 x 10-5 to 1.4 x 10-4. Groundwater is hosted within a leaking, semi-confined, fractured-bedrock aquifer that was dewatered during the pre-test development and post-test recovery phases. The hydrostratigraphic data generated for this study warrant revision of the existing groundwater model. Groundwater heads must be recalibrated to simulate the semi-confined nature of bedrock-hosted groundwater, as opposed to unconfined saturated alluvium. Further definition of HU-1 and HU-2 hydraulic parameters within the upper 120 ft of the aquifer are required to conceptualize the previously undefined hydrostratigraphic and groundwater flow data.
- Giles, Geoffrey C.; Pearson, John W., 1998, Characterization of hydrostratigraphy and groundwater flow on the southwestern San Andres Mountains pediment, NASA-JSC White Sands Test Facility, in: Las Cruces Country II, Mack, G. H.; Austin, G. S.; Barker, J. M., New Mexico Geological Society, Guidebook, 49th Field Conference, pp. 317-325.