An assessment of direct push electrical conductivity for identifying choloride migration pathways in the equus beds aquifer.

Abstract

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 a contaminant brine plume (Cr > 2000 n1g/L) that threatens the aquifer. Conventional geophysical and drillers' logs have been used to characterize aquifer geology, but do not allow for detailed assessn1ent 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. This study evaluated the ability of DP-EC logging to refine existing models of contaminant distribution in the aquifer. Five DP-EC profiles were obtained along a transect across the plume margin and compared to adjacent conventional drillers' logs. A baseline EC log from an uncontaminated part of the aquifer corroborated the presence of three dominant sand layers and thin bounding clays identified in drillers' logs, and revealed new information about the layers. High EC values (>200 mSIIn) associated with the clay layers in this profile were observed at sin1ilar depths in the other profiles, suppotiing previous assmnptions of restricted vertical brine movement within the aquifer. In several cases, the EC logs identified additional clay layers or high pern1eability zones within clay layers, and clarified discrepancies between adjacent drillers' logs. Brine samples were collected at three depths near each EC log. The lateral specific conductance and cr gradients observed across the site were coincident with increases in EC logs at depth. Associated EC values ( <80 tnS/m) in the most contaminated sand layers were below those for clay, identifying possible contaminant levels variations. The study demonstrates that EC logs may be used to better characterize and monitor brine migration in the aquifer.