Comparing electromagnetic and seismic geophysical methods: Estimating the depth to water in geologically simple and complex arid environments

An integrated electromagnetic (EM) and seismic geophysical study was performed to evaluate non-invasive approaches to estimate depth to shallow groundwater (i.e., < 5 m) in arid environments with elevated soil salinity, where the installation of piezometers would be limited or prohibited. Both methods were tested in two study areas, one serving as a control site with relatively simple hydrogeology and the other serving as the experimental site with complex hydrogeology. The control site is located near the shore of Utah Lake (Palmyra, Utah, USA) where groundwater is shallow and unconfined in relatively homogeneous lacustrine sediments. The experimental site is in Carson Slough, Nevada, USA near the Ash Meadows National Wildlife Refuge in Amargosa Valley. Carson Slough is underlain by valley fill, with variable shallow depths to water beneath an ephemeral braided stream system. The geophysical methods used include frequency domain electromagnetic induction with multiple antenna–receiver spacings. High-resolution P-wave seismic profiles using a short (0.305 m) geophone spacing for common depth-point reflection stacking and first arrival modeling were also acquired. Both methods were deployed over several profiles where shallow piezometer control was present. EM results at both sites show that water surfaces correspond with a drop in conductivity. This is due to elevated concentrations of evaporative salts in the vadose zone immediately above the water table. EM and seismic profiles at the Palmyra site accurately detected the depth to groundwater in monitoring wells, as well as interpolated depths between them. This demonstrates that an integrated approach is ideal for relatively homogeneous aquifers. On the other hand, interpreting the EM and seismic profiles at Carson Slough was challenging due to the laterally and vertically variable soil types, segmented perched water surfaces, and strong salinity variations. The high-resolution images and models provided by the geophysical profiles confirm the simple soil and hydrological structure at the Palmyra site as well as the laterally complex structure at Carson Slough. The integrated approach worked well for determining depth to water in the geologically simple site, but was less effective in the geologically complex site where multiple water tables appear to be present.     

Using fossil spring deposits in the Death Valley region,USA to evaluate palaeoflowpaths

Spring deposits reveal the timing and environment of past groundwater discharge. Herein, however, the potential for fossil spring deposits to infer water sources and palaeoflowpaths through trace elements and stable and radiogenic isotopes is examined. Past discharge (70 to 285 ka) in the Tecopa Basin in the Death Valley region of southeastern California is represented by tufa deposits, including mounds, pools, cemented ledges and rare calcite feeder veins. δ¹⁸O values indicate that spring discharge was a mixture of far‐travelled (regional) water with a significant, and perhaps dominant contribution of local recharge on a nearby range front and alluvial pediment, rather than simply representing an elevated regional water table. δ¹³C values indicate regional water had a high TDS, whereas solute data imply low overall solute contents, consistent with dilution by a large component of local recharge. Radiogenic isotope data (U‐series, ⁸⁷Sr/⁸⁶Sr) for tufa indicate that siliciclastic rocks (a regional aquitard) interacted with discharging water. To access this aquitard, regional flow was probably partitioned into a permeable north–south damage zone of a north–south range‐bounding fault along the foot of the Resting Spring Range, which ultimately controlled the location of groundwater discharge. Existing models for modern discharge in the Tecopa Basin, by contrast, call upon westward interbasin flow in carbonate rocks from the Spring Mountains through the intervening (and nearly perpendicular) Nopah and Resting Spring Ranges. Understanding the controls on regional groundwater flow is critical in this and other arid regions where water is, by definition, a scarce resource. Thus, although it is a case study, this report highlights a fruitful approach to palaeohydrology that can be widely applied. Copyright © 2007 John Wiley & Sons, Ltd.     

Shallow groundwater chemical evolution,isotopic hyperfiltration,and salt pan formation in a hypersaline endorheic basin: Pilot Valley,Great Basin,USA

Hydrogeology Journal - Endorheic basin brines are of economic significance as sources of boron, iodine, magnesium, potassium, sodium sulfate, sodium carbonate, and tungsten, and they are a major...     

Chemical evolution of shallow playa groundwater in response to post-pluvial isostatic rebound, Honey Lake Basin, California–Nevada, USA

The 1,750-km² endorheic Honey Lake basin (California–Nevada, USA) was part of the 22,000-km² Pleistocene Lake Lahontan pluvial lake system which existed between 5,000 and 40,000 years BP. The basin consists of two subbasins separated by a low elevation divide. Groundwater in the western subbasin has a maximum total dissolved solids (TDS) content of only ～1,300 mg/L; however eastern subbasin groundwater has a maximum TDS of ～46,000 mg/L. This TDS distribution is unexpected because 94% of surface water TDS loading is to the western subbasin. In situ reactions and upwelling thermal groundwater contributing to groundwater chemistry were modeled using NETPATH. The TDS difference between the subbasins is attributed to post-Lake Lahontan isostatic rebound about 13,000 years ago. Prior to rebound the subbasins did not exist and the low point of the basin was in the eastern area where hydraulic isolation from the larger Lake Lahontan and frequent desiccation of the basin surface water resulted in evaporite mineral deposition in accumulating sediments. After rebound, the terminal sink for most surface water shifted to the western subbasin. Although most closed basins have not been impacted by isostatic rebound, results of this investigation demonstrate how tectonic evolution can impact the distribution of soluble minerals accumulating in shallow basins.