Investigating Potential Groundwater Outflows with Electrical Resistivity Tomography: Cottonwood sub-basin, Joshua Tree National Park, California, U.S.A.
Abstract
The Cottonwood visitor center and southern area of Joshua Tree National Park, California, rely on one groundwater well within the Cottonwood sub-basin. Depth to water at this well was reported at 67m below ground surface (bgs) in 2017, a drop of almost 15m since installation in 1958. Due to the region's aridity, determination of the cause, either extraction or potential fault-controlled outflows to the neighboring Pinto Basin, is of utmost importance. Thus, this study focused on investigating these potential outflows.
Electrical Resistivity Tomography was employed using an electrical resistivity meter collecting 2-dimensional subsurface resistivity measures. Two surveys of data located along ephemeral washes and into the sub-basin proper were used to interpret potential groundwater connections. Use of a salt-water solution applied to ERT electrodes significantly improved contact resistance to provide reliable data in a very arid climate. Depth to static water readings were also collected using a camera and well sounder. Results of the surveys found a continuous low-resistivity zone at approximately 10m depth below both washes. One survey showed a low resistivity layer in vicinity of the well, consistent with the present depth to water. Higher resistivity areas were consistent with known geologic interpretations of the area including faults. As of November 2018, depth to water at the well was measured at 58m bgs. These results suggest the reported depth to water of 67m did not likely represent the static water level when it was collected in 2017. The shallow near surface low resistivity layer in both surveys was interpreted as a saturated zone caused by a precipitation event 6 weeks prior to the survey. The second survey showed no evidence of a saturated layer before bedrock with no contiguous resistivity layers. Thus, suggesting a lack of a groundwater connection between the basins. Therefore, this study concludes there is no groundwater connection or underflow at the boundary of the two basins. This conclusion contributes to completing a definitive water budget for a resource constrained and arid environment. Due to popularity of the park and the likely increase in tourism in the area, further characterization of this basin must be emphasized.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMNS41B0819B
- Keywords:
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- 0933 Remote sensing;
- EXPLORATION GEOPHYSICS;
- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1835 Hydrogeophysics;
- HYDROLOGY;
- 1880 Water management;
- HYDROLOGY