USING ELECTRICAL RESISTIVITY IMAGING (ERI) TO MAP SALINE GROUNDWATER-SURFACE WATER INTERACTIONS

Electrical Resistivity Imaging (ERI) was used to map horizontal and vertical variations in porewater salinity beneath a saline wetland, and to delineate upwelling deep saline groundwater discharge into the Little Salt Creek in southeastern Nebraska. The study was conducted at the Whitehead Wetland Complex, a saline wetland located at the northernmost edge of the capital city of Lincoln which was historically a vast salt flat with saline groundwater discharging at the surface. Two roll-along surveys were conducted along two separate lines using an Advanced Geosciences, Inc. (AGI) Supersting^® R8/IP earth resistivity meter with a switchbox and passive electrode cable. This automated resistivity system can perform stationary surveys with the electrode cable laid along the surface of the wetland, and/or along the bottom of small shallow surface water ponds, or streams. Electrode spacings of 3 meters (WH01) and 5 meters (WH02) were used. Detailed field notes of plant type, and the presence, or absence of ponded water, and/or salt flats was recorded at each electrode location along both survey lines along with the electrical conductivity of the water. The resulting processed electrical resistivity images, expressed as color contoured psuedosections, show clear demarcation between fresh water in the shallow alluvial silts having resistivity values of 5 to 41 ohm-m and deeper saline water which had resistivity values of 0 to 2.5 ohm-m within the clay and sand units of the Dakota Formation. The WH01 results indicate that while saline water is at some locations present at or near the surface beneath existing salt flats, most of the near surface porewater is fresher and not saline as observed historically. The WH01 results also show a plume of saline water rising from depth and discharging as baseflow into the stream. Groundwater levels and water chemistry measured in monitoring wells located along the line confirm the presence of upwelling saline water at this location. The results of the WH02 survey, which had deeper subsurface penetration, indicate that there is a layer of fresher, though still salty, water beneath the more concentrated parcel of highly saline water in the Dakota sediments.