A numerical exploration of hyporheic zone solute transport behavior estimated from electrical resistivity inversions

Abstract

An increasing number of studies have combined saline tracer injections with time-lapse electrical resistivity (ER) surveys to explore solute exchange within the hyporheic zone. The reliability and resolution of ER surveys of the hyporheic zone are rarely discussed. Here, we use a numerical modeling approach to assess how ER imaging resolves changes in saline tracer concentration within the hyporheic zone given different synthetic scenarios. We create a 3-D synthetic stream and surrounding hyporheic zone with an ER electrode transect and solve the coupled equations for porous fluid flow and solute transport. Then, we solve for the 3-D conduction of current between electrodes to create synthetic time-lapse ER surveys and invert the simulated resistances to obtain 2-D bulk electrical conductivity (σb) images perpendicular to the stream, which we compare with the known synthetic σb fields. The σb fields in the inversion generally capture the shape of the simulated σb fields, but with smoothing and artifacts as a function of the inversion process. Consequently, the calculated cross-sectional area of tracer plume is inaccurate. At inversion pixels beneath the stream, the accuracy of the inverted σb breakthrough curves when compared to the synthetic “truth” varies with stream size, pixel depth, and to a lesser extent injection time. The tails of these pixel breakthrough curves beneath the stream are consistently underestimated compared to the synthetic “truth,” i.e., σb in the inversions appear to return to background faster by one to six hours. The time series of average apparent bulk electrical conductivity requires no inversion and captures lingering saline tracer better than bulk EC breakthrough curves at individual pixels in the inversions.