PNNL

Abstract

Changes in bulk electrical conductivity are governed in large part by two diagnostic parameters concerning the state of vadose zone contamination: saturation and pore fluid conductivity. As a robust, remote, non-invasive, three-dimensional, and scalable imaging tool, time-lapse electrical resistivity tomography (ERT) can provide extensive diagnostic information concerning vadose zone contaminant distribution and migration by imaging changes in bulk electrical conductivity. We demonstrate long-term surface ERT monitoring of a ~70-m-thick contaminated vadose zone to investigate whether rising groundwater nitrate levels are caused by unanticipated vadose zone flux. A three-dimensional time-lapse image spanning 10 years (2007-2017) reveals the impacts of meteoric surface infiltration events, but ultimately supports the adequacy of current conceptual understanding by indicating that rising nitrate levels are not sourced in the vadose zone.