DIRECT-PUSH ELECTRICAL CONDUCTIVITY ASSESSMENT OF A HISTORIC BRINE PLUME IN THE EQUUS BEDS AQUIFER, KANSAS

The Equus Beds alluvial aquifer is the primary water source for Wichita, Kansas and surrounding communities. Hydrostratigraphic features may significantly influence the distribution and transport of an historic contaminant brine plume (Cl^->2000 mg/L) that threatens the aquifer. Conventional geophysical and drillers logs have been used to characterize aquifer geology, but do not allow for detailed assessment of preferential lateral flow and irregular plume margins in heterogeneous sediments. Direct-push electrical conductivity (DP-EC) logging can detect thin, interbedded layers, and also identify the presence of conductive contaminants. We evaluated the ability of DP-EC logging to refine existing models of contaminant distribution in the Equus Beds.

Five, 30 m DP-EC profiles were obtained along a 9 km transect across the plume margin and compared to adjacent conventional drillers lithologic logs. A baseline EC log from an uncontaminated part of the aquifer corroborated the presence of the three dominant sand layers and thin bounding clays identified in drillers logs, and also revealed new information about the layers’ thicknesses and textures. High EC values (>200 mS/m) associated with the clay layers in this profile were observed at similar depths in the other profiles, generally supporting previous assumptions of restricted vertical brine movement within the aquifer. In several cases, the EC logs identified additional clay layers or high permeability zones within clay layers, and clarified discrepancies between multiple adjacent drillers logs. Brine samples were collected at three depths near each EC log. The lateral specific conductance and Cl^- gradients observed across the site were coincident with an increase in electrical conductivity in EC logs at the same depths. Associated electrical conductivity values (<80 mS/m) in the most contaminated sand layers were well below those for clay, allowing variations in contaminant levels to be recognized.

Efforts to protect and replenish the aquifer include a large-scale, injection-well curtain, and a multi-million dollar artificial recharge system (ARS). Our results demonstrate that EC logs may be useful in efforts to better characterize and monitor brine encroachment, and assess the design and success of the ARS.