Development of a New, Direct-Push-Based, Geophysical and Geochemical Approach for Groundwater Tracer Tests

Groundwater tracer tests are often employed to measure parameters used to model groundwater movement and contaminant fate. A new, direct-push-based approach to conducting tracer tests was evaluated in a sandy gravel aquifer next to the Gunnison River at the U.S. Department of Energy Office of Legacy Management's, Grand Junction, Colorado, Site. This aquifer is of interest due to past contamination from uranium processing, and the need for groundwater flow and contaminant transport parameters. The new method incorporates simultaneous vertical logging of electrical conductivity (EC), and hydraulic properties, along with groundwater sampling at specific intervals using a direct-push drilling rig. A lithium bromide tracer was injected into the alluvial sand and gravels and subsequently monitored using this new approach. The tools and approach were assessed for viability of mapping the tracer movement. A Geoprobe Hydraulic Profiling Tool – Groundwater Sampler (HPT-GWS) was used to log vertical changes in both EC and hydraulic properties using measured hydraulic back pressure. These logs, with extracted groundwater samples, were used to locate the tracer plume. Where logs indicated a spike in EC, a groundwater sample was obtained. Sample-specific conductance measurements were made in the field and used to approximate the position of the conductive bromide tracer. The presence of bromide was later verified through laboratory analysis. Vertical EC variations within the saturated zone were observed in three downgradient logs using the HPT-GWS. Comparison of EC and hydraulic pressure (HP) profiles allowed for identification of high conductivity zones and possible presence of the tracer. Continuous sediment cores were used to identify lithology that might influence the groundwater flow and tracer movement and resulting EC values. The use of an HPT-GWS for assessment of hydrogeological parameters such as groundwater flow direction and velocity, contaminant dispersion, and aquifer lithology, can enhance characterizations of chemical and physical processes occurring in the aquifer.