Direct current and self-potential monitoring of an evolving plume in partially saturated fractured rock

Hole–surface and hole–hole mise-à-la-masse (MALM) measurements and surface self-potential (SP) measurements were taken over a 173-day period during an 8-well, radial injection, tracer experiment in partially saturated, fractured, Columbia River basalt. A potassium chloride tracer stream was injected into the fractured basalt and was energized directly for the MALM experiments. The constant concentration, tracer solution was injected for 76 days above a perched water table at an average rate of 10.07 l/day. Tracer injection was followed by a 34-day period of tap water injection and a 63-day drainage period. The associated changes in potential distributions and SPʹs over time indicate that an asymmetrical, vadose zone plume evolved in the fractured basalt. Hole–surface and SP measurements were used to track the lateral evolution of the tracer plume; however, there was some noise and variability in the SP data. The hole–hole measurements helped delineate the depth of preferential tracer migration in the fractured basalt.