Tracer Experiment in a Brownfield Using Geophysics and a Vadose Zone Monitoring System

Abstract

A saline tracer infiltration test across the fractured vadose zone of an industrial contaminated site in Belgium was monitored by combining surface and cross-borehole electrical resistivity tomography (ERT) methods with a vadose zone monitoring system (VMS). The VMS provides in situ continuous hydraulic and chemical information on the percolating tracer at multiple depths in the vadose zone. The combination of such high-resolution data with timelapse geophysical images that capture the spatiotemporal variability of the subsurface improves interpretations of flow and transport, providing a better characterization of infiltration mechanisms and preferential flow paths. The tracer infiltration test was performed over a heterogeneous vadose zone composed of backfilled materials, sands and silts, and unsaturated fractured chalk. Monitoring results during a 5-d period revealed the formation of a tracer plume in the upper backfilled deposits, while some of the tracer migrated laterally following preferential pathways. Slow vertical flow through matrix pores was found to be dominant under dry conditions. Infiltration of small quantities of rain during the test was found to have an influence on the spatial distribution of the plume. Results from long-term monitoring revealed vertical transport of the tracer toward depths that reached 4 m during a time period of 105 d. During that period, fracture and matrix flow mechanisms across the vadose zone were activated as a response to frequent rainfall episodes. The study demonstrates that the interpretation of geophysical images is improved by in situ information from the VMS.Abbreviations: ERT, electrical resistivity tomography; FTDR, flexible time domain reflectometry; VMS, vadose zone monitoring system; VSP, vadose sampling port.The detailed characterization of the contaminant distribution and percolation mechanisms of industrial contaminated soils is particularly challenging. The presence of disturbed soil, artificial ground, and abandoned infrastructures modifies flow mechanisms in the vadose zone, generating preferential flows that are difficult to identify and quantify. Additionally, temporal and spatial coverage is required to capture the physical and biochemical changes of multiple contaminants in such a dynamic system (Atekwana et al., 2000).Electrical resistivity tomography (ERT) methods are suitable for flow and transport characterization because subsurface electrical properties are influenced by parameters such as solute concentration or water saturation (Slater et al., 2000;Johnson et al., 2013). Electrical resistivity tomography measurements allow monitoring of infiltrated water during precipitation events, which have an impact on contaminant transport across the vadose zone (Descloitres et al., 2008;Clément et al., 2009;Carrière et al., 2015). The combination of ERT methods with tracer tests allows the identification of preferential flow paths and solute transport processes (Ptak et al., 2004;Robert et al., 2012). However, information provided by ERT measurement...

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