MONITORING INFILTRATION FROM NATURAL STORMS USING TIME-LAPSE ELECTRICAL RESISTIVITY TOMOGRAPHY

Abstract

Time-lapse electrical resistivity tomography (TL-ERT) enables an accurate characterization of the heterogeneity of flow through the unsaturated zone especially when compared to point measurements taken within the same survey area. The most powerful tool for understanding the unsaturated zone is a combination of several techniques. Many models of unsaturated zone flow assume a uniform wetting front even though the existence of preferential flow paths is well-documented in the literature. TL-ERT surveys were collected perpendicular to a stream at the Stroud Water Research Center in Chester County, PA to provide continuous measurement of unsaturated flow during two natural infiltration events. Dielectric sensors were installed along this transect to collect soil moisture data during these events. Additionally, slug tests and infiltrometer tests were collected along the transect to characterize the subsurface at the study site. TL-ERT successfully located sections with preferential flow, and these results were reproducible three months later. Other methods of measuring soil moisture content or infiltration rates were less successful at identifying preferential flow. The rates determined from point measurements often did not match where the TL-ERT identified zones of preferential flow. This comparison reveals that slow-infiltration points can exist within preferential pathways and exemplifies the importance of large-scale measurements in the unsaturated zone. Any scientific study looking at infiltration should consider utilizing TL-ERT to map where preferential flow may be occurring.