The Indian Wells Valley, a terminal basin in an arid region east of the southern Sierra Nevada, is 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), the GEM center undertook a pilot study in May 2016 to assess the combined use of 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). 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.
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 control and refine the TEM inversion in areas with appropriate wells for logging. Surface results suggest that this tool would be well applied in the portion of the NAWS where the water table is shallow and drilling is precluded due to base activities. Results of this pilot study suggest that the combination of TEM and NMR should prove a cost-effective way to compliment traditional well-based investigations in assessing the brackish water resource in this groundwater basin.