Using Electrical Resistivity Imaging to Map Saline Groundwater and Subaqueous Spring Discharge: An Example From The Saline Wetlands of Eastern Nebraska

In order to better understand saline groundwater distribution and discharge dynamics within the saline wetlands of eastern Nebraska, electrical resistivity data were collected at three wetland sites within the Little Salt Creek Watershed. Electrical resistivity imaging (ERI) can provide an expanded understanding of saline groundwater distribution through the acquisition of a large number of resistivity measurements collected at the surface; upon inversion, the distribution of resistivity can be displayed in cross-section and subsurface processes serving to control salinity can be inferred. In recent years, several studies have used conventional methods of characterizing groundwater flow within the saline wetlands. These point measurements reveal little about the complex mixing dynamics between saline groundwater and fresh surface waters required to sustain the saline habitat vital to number of different species in this area. Groundwater in this region has high fluid electrical conductivity values (2,000-40,000 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 can be tracked in the resistivity images collected at these sites. The vast majority of ERI surveys were conducted in order to map saline groundwater; as such, survey locations often transected Little Salt Creek in order to assess where discharge may or may not be occurring. Results show that each wetland site is unique, and that local wetland geology and geomorphology exert strong controls on saline groundwater migrating from depth. Additional surveys were conducted in order to evaluate the effectiveness of ERI as a temporal monitoring tool, thus data were collected along a single location twice, when hydrologic conditions varied. Results of this preliminary effort have shown that ERI images can track changes in subsurface resistivity where time and weather conditions are varied; continued temporal monitoring studies have great potential in revealing how discharge dynamics change in response to various hydrologic conditions. Advisor: F. Edwin Harve