Investigating potential groundwater outflows: Cottonwood sub-basin, Joshua Tree National Park, California, USA
Date
2019
Authors
Boyle, John, author
Sanford, William, advisor
Ronayne, Michael, committee member
Omur-Ozbek, Pinar, committee member
Harvey, Forrest, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
This study utilizes Electrical Resistivity Tomography (ERT) to investigate potential groundwater outflows of the Cottonwood sub-basin (CsB) in Joshua Tree National Park (JOTR), California. Southern JOTR depends on one groundwater well (LUB-23) screened in an unconfined aquifer unlike the northern section which obtains its water from the municipal water system of the town of Joshua Tree. Depth to water was reported at 67m (219 ft) below ground surface (bgs) in 2017, a drop of almost 15m (49 ft) since installation in 1958. This variability in water level drives the need for a definitive water budget particularly since there is only one groundwater well. To contribute to developing a water budget, this study focused on investigating a potential groundwater connection between the CsB and the larger neighboring Pinto Basin. Interpretation of the subsurface resistivity models showed lack of a groundwater connection (or lack of contiguous low resistivity distribution) thus, this study concludes there is no groundwater connection or underflow at the boundary of the CsB and the Pinto Basin. Depth to water readings, which confirmed the water table was at 58m (190 ft) bgs. suggest the reported depth to water of 67m (219 ft) did not likely represent the static water level. Water quality and stable isotope analyses of groundwater samples were collected and compared to analyses performed from 2009 showing little variability over the 9-year period between sampling. Thus, no indications that pumping has modified chemistry or isotope composition.