Focused subsurface flow in the Amargosa Desert characterized by direct-current resistivity profiling

Environmental-tracer studies have shown that ground-water recharge in the thick alluvial fill of the Amargosa Desert is localized beneath ephemeral stream channels and anthropogenic sources of water, with little recharge beneath native vegetation on interfluvial areas under current climatic conditions. These borehole-based studies provided relatively robust but limited, one-dimensional (vertical) information that can be only tentatively regionalized using geomorphologic, pedologic, and vegetational mapping. The ability of direct-current (DC) resistivity profiling to complement and extend studies of the spatial distribution of subsurface flow was examined by making surface-based measurements ("soundings") along one transect normal to the depositional fabric in each of three geomorphologically distinct settings: a well-incised ephemeral channel system, a poorly incised (distributory) ephemeral channel system, and an interfluvial upland. Linear arrays of 32 to 80 electrodes were deployed with a uniform 2 to 5-m spacing between adjacent electrodes. A multiplexing 8-channel resistivity instrument made automated inverse-Schlumberger-array soundings along the deployed line, using up to 10 electrodes at a time. The line was shifted piecemeal until composite transects consisted of 168 to 232 electrode positions. This approach allowed rapid profiling of long transects at high resolution. Numerical inversions assumed horizontal constancy normal to the vertical slices being imaged, producing solution sets of optimized resistivity values for several thousand points within each modeled slice. Imaged slices were ∼30 to 80 m deep and ∼1 km wide. RMS errors between apparent resistivities in the model inversions and field-measured apparent resistivities were ∼10%. On the basis of borehole studies, inverted resistivity (ρ ) values denoted three categories of alluvium: (1) low-water-content coarse gravel and highly desiccated surface materials, with ρ > ∼200 Ω -m, (2) vertical plumes of moist alluvium in areas of active recharge, with ρ < ∼20 Ω -m, and (3) other low to low-medium water-content alluvium in areas without active recharge, with intermediate values of ρ . The upland profile revealed a laterally extensive gravel layer (category 1 alluvium) at a depth of ∼25 m that intersects instrumented boreholes at the Amargosa Desert Research Site. This layer coincides with peak concentrations of radioactive gases and volatile organic compounds moving through the unsaturated zone from a nearby waste-disposal facility. DC-resistivity profiling appears useful for locating areas of active ground-water recharge as well as potential passageways for preferential gas transport in the Amargosa Desert and similar environments.