Electrical Resistivity Imaging To Explore Solute Transport In A Stream System

Abstract

Time-lapse electrical resistivity imaging (ERI) was used to map the movement of a conservative salt tracer through a stream system at the Leading Ridge Experimental Watershed in central Pennsylvania. A transect of 12 electrodes was established perpendicular to the direction of stream flow to allow for imaging of the stream, streambed, and surrounding aquifer. ERI measurements were taken at regular intervals during the 21-hour tracer test and for 23 hours after the injection ended. The purposes of this test were 1) to identify the movement of a saline tracer through subsurface preferential pathways and the extent of the hyporheic zone, or subsurface actively connected to the stream, and 2) to determine the behavior of saline tracer in the subsurface as based on spatial moments. At early time, the tomograms show a drop in resistivity of up to 10 percent in the immediate area of the stream relative to background measurements taken before the start of the salt injection; later, a region of low resistivity develops in a hypothesized abandoned streambed on the inside of a meander bend. Transport of the tracer through the subsurface occurs at a slower rate than its movement in the stream. Spatial moments indicate that a substantial quantity of solute is entrained in the subsurface and persists in the hyporheic zone well after the end of tracer injection. ERI effectively images the movement of a saline tracer through hyporheic pathways, providing a higher resolution image of transport behavior in the subsurface.