Monitored tracer experiment using the vadose zone experimental setup (VZES) for studying water and pollutant recharge processes in a brownfield

Contaminant transport characterization in the vadose zone of industrial contaminated sites requires in situ technologies that provide information representative of complex heterogeneous systems. However, finding the appropriate methodology is a challenge, as there is a risk of losing data resolution when capturing the spatial variability of the subsurface. An alternative method is provided by The Vadose Zone Experimental Setup (VZES) which combines surface and cross-borehole geophysical methods with a vadose zone monitoring system (VMS). When geophysical imaging is combined with in-situ hydraulic and chemical information at multiple depths of the vadose zone, detailed characterization of contaminant transport in heterogeneous systems is obtained.
The system was installed at an industrial contaminated site in Belgium. A saline tracer infiltration test was performed over a heterogeneous vadose zone composed of backfilled materials underlined by unsaturated fractured chalk. Surface and cross-hole Electrical Resistivity Tomography (ERT) measurements were carried out over a 5 day period, following tracer injection. Results from time-lapse imaging reveal high resistivity variations at 0-0.5m depth, indicating that most of the tracer remained in the upper backfilled deposits. This is coincident with the results from sampled waters across the vadose zone, as no tracer was detected below 0.5m depth. Lower resistivity differences were observed laterally, indicating tracer migration in different directions via preferential flow paths. Lateral migration was found to be dominant over vertical transport in the absence of rain events. Three months after the injection, a geophysical survey was performed and combined with in situ continuous hydraulic and chemical information at multiple depths of the vadose zone. Results from geophysical imaging and water sample analyses indicate vertical movement of the tracer, which reached 4 m depth. Information obtained from continuous measurements of water content reveal that the tracer was transferred via preferential flow. The activation of such flow mechanism occurred as a response to rainfall episodes, resulting in water percolation and tracer transport towards higher depths.

The results of the investigations demonstrate that the VZES is an effective method in identifying pathways and mechanisms of transport within a heterogeneous conductivity fields. The implementation of this methodological concept at industrial contaminated sites contributes to improve the development of site conceptual models for soil and groundwater protection and remediation.