Using Electrical Resistivity Imaging to Map Saline Discharge Into Fresh-Water Streams in Eastern Nebraska

Electrical resistivity imaging (ERI) can provide an expanded understanding of groundwater distribution through the acquisition of a large number of resistivity measurements collected at the surface.  Upon inversion of these measurements, the distribution of resistivity can be displayed in cross-section and groundwater flow conditions can be inferred.  Conventional methods of characterizing groundwater flow typically include measurements of water table elevations and pressure heads, which reveal little information about the nature of groundwater movement at depth. Over the last several decades, anthropogenic inputs and alterations have put tremendous stress on the watersheds and ecosystems in eastern Nebraska.  In Lancaster and Saunders counties, a unique inland salt-marsh provides favorable conditions for a number of salt-tolerant plant and insect species to thrive.  The salt-marsh habitat is supported by discharge of saline water from the Dakota Aquifer into Salt, Rock, and Little Salt Creeks.  In recent years, many of the regions supporting salt-tolerant plant and insect species have begun to freshen, reducing the amount and availability of habitat vital for these species.  In order to better understand the groundwater flow dynamics in this region, electrical resistivity data were collected at a number of wetlands within the Little Salt Creek Watershed. Preliminary results show that ERI can provide insights into the dynamics of ground and surface water interactions in this region by mapping groundwater flow.  Groundwater in this region has high electrical conductivity values (700-18,000 µS/cm) which create sharp contrasts in resistivity values between fresh and saline groundwaters and the sediments through which they flow. Thus, distinct plumes of saline water migrating from depth to discharge into local streams can be tracked in the resistivity images collected at these sites. These images reveal the complicated three-dimensional pathways that the saline groundwater takes to surface water channels, and are helping researchers better understand groundwater-surface water interactions.