The Indian Wells Valley, California
The Indian Wells Valley is a terminal basin in an arid region east of the southern Sierra Nevada. Stakeholders in the valley are seeking to better understand their local groundwater basin. In an effort to attain sustainable management, stakeholders are considering using brackish water from the lower portion of the aquifer system to help meet overall water needs in this groundwater-dependent basin. In specific, local stakeholders need tools to assess the location, depth, and overall geometry of the main aquifers and aquitards in the system, and would need tools to differentiate brackish groundwater from fresh groundwater.
In partnership with the Indian Wells Valley Water District, the town of Ridgecrest, and the China Lake Naval Air Weapons Station (NAWS), we undertook a pilot study in 2015-2016 to assess the combined use of time-domain electromagnetics (TEM), surface nuclear magnetic resonance (NMR) and logging NMR to characterize a complex fresh/brackish aquifer system in their groundwater basin. TEM is a surface geophysical technique that provides information on the electrical resistivity of the subsurface, which is affected by sediment texture (sands are more resistive than clays) and groundwater salinity (fresh water is more resistive than brackish water). NMR was used to augment the TEM survey, because this technique is sensitive to the volume of water in the subsurface without any effect from the salinity of that water.
Data were collected at five sites, representing a variety of geologic, hydrologic, and electromagnetic noise conditions at both the scale of individual sites (100s of meters) and the basin scale (10s of kilometers), including significant changes in depth to bedrock and lateral variations in the presence, depth, and thickness of fine-grained strata. Results of this pilot study indicate that TEM is viable across the varied geologic environments of this basin, providing clear indications of the water table, differentiating coarse sediments from silts/clays, identifying the transition to bedrock, and providing indications of increasing groundwater salinity in some areas. Downhole NMR proved to be a valuable tool to refine the TEM inversion in areas with appropriate wells for logging. Surface NMR results suggest that this tool would be well applied in the portion of the NAWS where the water table is shallow and where drilling is precluded due to base activities.
Building on the positive results of this study, we recommend resistivity mapping with TEM be implemented on a larger scale by stakeholders as a tool to better understand the Indian Wells Valley groundwater system. Larger data sets, including linear traverses and/or grids of surface TEM soundings, or ideally airborne TEM surveys, will allow for a detailed characterization of the groundwater system and development of a hydrogeologic conceptual model of the basin.
Geophysical results at site NAV A in the Indian Wells Valley. (a) Lithology. (b) Smooth inversion of TEM data. (c) NMR logging data from well 26S/39E-15D01. (d & e) Surface NMR inversion results in terms of the water content and relaxation time.
The WalkTEM system used for TEM data acquisition in the Indian Wells Valley