PRELIMINARY COMPARATIVE ANALYSIS OF GEOPHYSICAL METHODS IN INVESTIGATION OF SHALLOW SUB-SURFACE HYDROGEOLOGY IN HYPER-ARID SALINE SOILS, AMARGOSA VALLEY, NV: PILOT STUDY

An arid environment where leaching has significantly modified soil salinity presents a challenging problem for classical geophysical methods of mapping a shallow (e.g, < 5 m) water table. The aim of this pilot study is to determine the effectiveness and feasibility of three geophysical methods in characterization and mapping of the shallow water table in the hyper-arid saline soils of Carson Slough near Ash Meadows National Wildlife Reserve (AMNWR) in Amargosa Valley located about 113 km northwest of Las Vegas, Nevada. The three geophysical methods used include: the electromagnetic method using Geonics EM instruments with multiple antenna-receiver spacings, ground-penetrating radar (GPR; using 200 and 400 MHz antennas in continuous mode), and nominal 48-fold P-wave seismic reflection with short (0.3048 m) geophone spacing. Each method was employed over five 2-D sections where shallow piezometer control was present. This approach was deployed near AMNWR to determine which method would be best to employ over a large area in AMNWR and other major discharge areas in Amargosa Valley where multiple piezometer construction would be prohibited or very expensive.

The local conditions present challenges in water table characterization due to the shallow water table depth and high soil and groundwater electrical conductivity. Preliminary interpretations of the electrical conductivity data actually suggest an inversion in conductivity at the water table (higher conductivity over lower) that may be useful in determining depth to water. This inversion is due to the relatively high conductivity of evaporative salts in the vadose zone in comparison with the lower conductivity of the phreatic zone. Preliminary interpretations of the seismic data show apparent reflections from the water table. The GPR data are likely of the least benefit due to diffusion of the electromagnetic field in the high conductivity soils; however, some areas of the profiles indicative moderate reflectivity that might be useful for general site characterization. Our results thus far suggest an integration of methods in this type of environment where short seismic reflection profiles and long EM traverses, calibrated to a shallow borehole, would be an optimum method for reliably mapping a shallow water table.