High-resolution Electrical Resistivity Tomography monitoring of a tracer test in a confined aquifer

Abstract

A permanent geoelectrical subsurface imaging system has been installed at a contaminated land site to monitor changes in groundwater quality after the completion of a remediation programme. Since the resistivities of earth materials are sensitive to the presence of contaminants and their break-down products, 4-dimensional resistivity imaging can act as a surrogate monitoring technology for tracking and visualising changes in contaminant concentrations at much higher spatial and temporal resolution than manual intrusive investigations. The test site, a municipal car-park built on a former gas-works, had been polluted by a range of polycyclic aromatic hydrocarbons and dissolved phase contaminants. It was designated statutory contaminated land under Part IIA of the UK Environmental Protection Act due to the risk of polluting an underlying minor aquifer. Resistivity monitoring zones were established on the boundaries of the site by installing vertical electrode arrays in purpose-drilled boreholes. After a year of monitoring data had been collected, a tracer test was performed to investigate groundwater flow velocity and to demonstrate rapid volumetric monitoring of natural attenuation processes. A saline tracer was injected into the confined aquifer, and its motion and evolution were visualised directly in high-resolution tomographic images in near real-time. Breakthrough curves were calculated from independent resistivity measurements, and the estimated seepage velocities from the monitoring images and the breakthrough curves were found to be in good agreement with each other and with estimates based on the piezometric gradient and assumed material parameters.

Geophysical-Geotechnical Sensor Networks for Landslide Monitoring

“…Monitoring at the site is based around the ALERT (Automated time-Lapse Electrical Resistivity Tomography) survey concept Ogilvy et al, 2009;Wilkinson et al, 2010a). The ALERT system provides the full capability for remote measurement, storage and transmission of geoelectrical data.…”

Imaging and quantifying salt-tracer transport in a riparian groundwater system by means of 3D ERT monitoring

“…For most ERT monitoring experiments, the 1-2 h required to record each suite of data is short compared to the movement of the tracer (on the order of days), thus justifying the common assumption that significant changes do not occur during data acquisition (Binley et al, 2002;Kemna et al, 2002;Cassiani et al, 2006;Wilkinson et al, 2010). However, for experiments in fast dynamic environments, such as ours, the timing of each measurement has to be taken into account (Day-Lewis et al, 2003).…”

“…Geophysical monitoring of salt-tracer tests using ERT has been successful in the laboratory (Slater et al, 2000) and in the field using 2D and 3D crosshole techniques (Singha and Gorelick, 2005;Wilkinson et al, 2010). Studies in which a salt tracer has been monitored from the surface have so far mainly been restricted to measurements along single or several 2D lines (Cassiani et al, 2006;Monego et al, 2010;Ward et al, 2010;Cardenas and Markowski, 2011).…”

Long-term time-lapse microgravity and geotechnical monitoring of relict salt mines, Marston, Cheshire, U. K.

“…Time-lapse geophysical surveys have been undertaken for other applications. For example, electrical resistivity surveys have been used to monitor active landslides (Wilkinson et al, 2010a), toxic leachate migration (Rucker et al, 2011), hydrological infiltration (Cassiani et al, 2009), aquifer exploitation (Chambers et al, 2007), and contaminated land (Wilkinson et al, 2010b). Time-lapse ground penetrating radar (GPR) has also been used for fluid migration studies (Birken and Versteeg, 2000), but GPR techniques typically do not have sufficient penetration to monitor mine-related subsidence.…”