ELECTROMAGNETIC IMAGING OF LANDSCAPE-SCALE GROUNDWATER/SURFACE WATER EXCHANGES

Groundwater/surface water exchange typically is investigated in detail at the bedform- to stream-reach scale. Larger scale (e.g., kilometer) lateral hydraulic connections commonly are inferred from head measurements in piezometers, which indicate only the potential for flow. When strong electrical contrast exists between shallow groundwater pathways and surrounding material, electromagnetic induction imaging (EMI) can be used to map zones of preferential flow and confining features across the landscape. Multi-frequency EMI tools also allow for depth-dependent sensing, so gridded surface data collection can be used to image the subsurface in three-dimensions. EMI data were calibrated and inverted to yield quantitative measurements of subsurface bulk conductivity using FEMIC, new software developed in collaboration between the USGS and Rutgers University. Examples from Moab, Utah and central Alaska are presented to illustrate the utility of hand-held EMI data collection in different hydrologic settings. Approximately 25 km of EMI data collection on land and water in Moab were used to identify preferential flow of fresh groundwater to the Colorado River through shallow natural brines. This information helped to locate observation wells and refine Darcy-based estimates of groundwater discharge to the river. In central Alaska, EMI was used in a region of discontinuous permafrost to map thawed zones that permit groundwater flow, and identify areas of new permafrost aggradation that impede flow between arctic lakes. These examples demonstrate the utility of EMI in hydrologic research to efficiently develop landscape-scale understanding of lateral connections between groundwater and surface water.