Chasing the tracer - combining conventional salt tracer testing with Direct Push electrical conductivity logging for the characterization of a highly permeable fluviatile aquifer

Abstract:

Tracer testing is a well-established and commonly applied technique for the hydraulic characterization of the subsurface. However, in areas with limited or no prior knowledge about the hydraulic regime, layout of the tracer test field, i.e. positioning of monitoring wells, can be challenging. This is especially true for complex and/or highly dynamic fluviatile systems, where hydrological conditions may impede tracer recovery. Typical examples for a complex and very dynamic system are the highly conductive gravelly and sandy deposits of the Tagliamento River, Italy. Field work was performed near the city of San Daniele del Friuli to characterize hydraulic properties of the channel-adjacent deposits. No information was available on groundwater gradient and flow-field prior to well installations and stream gauge level changes can be observed within short time periods. For enhanced subsurface characterization, conventional salt tracer testing was combined with Direct Push electrical conductivity logging. Therefore, a salt tracer was injected in a two inch direct push-installed well. After tracer injection, direct push high resolution vertical electrical conductivity profiling was performed in concentric hemicycles around the injection well to identify the main direction of tracer propagation, tracer velocity, and tracer plume geometry. Based on the results, positions of monitoring wells were rapidly adapted. Results show that the combined use of conventional salt tracer testing with Direct Push electrical conductivity logging is an efficient strategy for the characterization of complex and highly conductive sedimentary deposits with groundwater velocities of several meters per hour.