Wetland‐Scale Mapping of Preferential Fresh Groundwater Discharge to the Colorado River

Abstract

Quantitative evaluation of groundwater/surface water exchange dynamics is universally challenging in large river systems, because existing methodology often does not yield spatially‐distributed data and is difficult to apply in deeper water. Here we apply a combined near‐surface geophysical and direct groundwater chemical toolkit to refine fresh groundwater discharge estimates to the Colorado River through a 4‐km2 wetland that borders the town of Moab, Utah, USA. Preliminary characterization of raw electromagnetic imaging (EMI) data, collected by kayak and by walking, was used to guide additional direct‐contact electrical measurements and installation of new monitoring wells. Chemical data from the wells strongly supported the EMI spatial characterization of preferential fresh groundwater discharge embedded in natural brine groundwaters and weighted to the southern wetland section. Inversion of the EMI data revealed sub‐meter scale detail regarding bulk electrical conductivity zonation across approximately 15.5 km of transects, collected in only 3 days. This electrical detail indicates processes such as salinization of the unsaturated zone and direct discharge through the Colorado River sediments and a tributary creek bed. Overall, the study contributed to a substantial reduction in fresh groundwater discharge estimates previously made using sparse existing well data and a simplified assumption of diffuse fresh groundwater discharge below the entire wetland. EMI will likely become a widely used tool in systems with natural electrical contrast as groundwater/surface water hydrogeologists continue to recognize the prevalence of preferential groundwater discharge processes.

Ground‐penetrating radar, electromagnetic induction, terrain, and vegetation observations coupled with machine learning to map permafrost distribution at Twelvemile Lake, Alaska

“…As the method can operate across a range of frequencies, depth‐specific resistivity structure information can be recovered, where lower frequencies and higher frequencies generally sense larger/deeper or smaller/shallower volumes, respectively. FDEM has recently grown in popularity for mapping shallow groundwater flow paths and groundwater/surface water exchange dynamics, 55–57 although applications to cold regions are still relatively novel.…”

Technical note: Efficient imaging of hydrological units below lakes and fjords with a floating, transient electromagnetic (FloaTEM) system

“…Non-invasive geophysical methods provide spatial information on these subsurface properties and processes across many environments; over the last few decades the methods have played a vital role in near-surface investigations (Hatch et al, 2010;Day-Lewis et al, 2006). However, deployment of surface-based geophysical investigations (as opposed to airborne systems) on water bodies has historically been difficult (Sheets and Dumouchelle, 2009;Briggs et al, 2019;Parsekian et al, 2015) while not insurmountable; this has limited the application range to some degree.…”

“…Applications of transient electromagnetic (TEM) and frequency domain EM tools are reported in previous studies, e.g., discharge of groundwater to lakes and brines (Ong et al, 2010;Briggs et al, 2019) and extraction of lithium from large-scale natural brine systems (Munk et al, 2016). Airborne techniques have proved capable of mapping beneath lakes, rivers, and near-shore seas (Fitterman and Deszcz-Pan, 1998;Dickey, 2018;Rey et al, 2019), but they are costly and provide lower vertical and lateral resolution than their ground-based counterparts (Hatch et al, 2010).…”

Technical note: Efficient imaging of hydrological units below lakes and fjords with a floating, transient electromagnetic system (FloaTEM)

“…decades the methods have played a vital role in near-surface investigations (Barker, 1980;Hatch et al, 2010a;Day-Lewis et al, 2006). However, deployment of surface-based geophysical investigations (as opposed to airborne systems) on water bodies has historically been difficult (Sheets and Dumouchelle, 2009;Briggs et al, 2019;Parsekian et al, 2015); while not insurmountable, this has limited the application range to some degree.Electrical and electromagnetic methods (EM) are the two most-extensively used geophysical exploration and characterization techniques for hydrologic applications (…”