Monitoring water and pollutant leaching at an industrial site using geophysics and a vadose zone monitoring system.

The development of protection and remediation plans for contaminated soil and groundwater require a detailed understanding of water flow dynamics and contaminant distribution in the subsurface. However, the retrieval of such information across the vadose zone is challenging, as most field technologies have limited accessibility beyond the first meters of subsoil and industrial environments pose additional technical difficulties for installation.
The research presented here aims to provide effective in situ characterization tools that are capable of providing information about water flow dynamics, contaminant distribution and chemistry across the vadose zone of industrial contaminated sites. The Vadose Zone Monitoring System (VMS) was selected in conjunction with geophysical methods (Electrical Resistivity Tomography), given their adaptability for installation in polluted industrial sites. The VMS allows retrieval of in situ continuous hydraulic and chemical information of infiltrated water at different depths of the vadose zone through customized sensors installed in a slanted borehole. The spatial resolution of the subsurface is improved with geophysical methods, which are used to characterize structural heterogeneities in the subsurface and the spatial distribution of solutes therein.
The setup containing both the VMS and a geophysical system was installed at a former industrial contaminated site in the west of Belgium. Soil and groundwater at the study site are contaminated with heavy metals, Polycyclic Aromatic Hydrocarbons (PAH) and inorganic contaminants, among others. Upon installation, the site was monitored under natural recharge conditions. Results from water content sensors installed in the VMS reveal quick rises in water content as a response to rainfall events to depths reaching 3.65m. From such water fluctuations, macropore, micropore, matrix and preferential flow mechanisms are identified. At greater depths, slower flow dynamics and matrix mechanisms are dominant. Results from sampled waters across the vadose zone reveal the existence of two predominant chemical facies at different depths, which are related with water infiltration flow mechanisms. Ni is identified as the main contaminant that is leaching across the vadose zone.
Subsequent to such initial monitoring period, a saline tracer test was infiltrated on site and monitored via surface, cross-borehole ERT methods and the VMS to simulate the transport of a contaminant across the vadose zone. Results from a short term monitoring period (5 days) reveal the formation of a plume in the first meter of the subsurface. Slow vertical flow through matrix is found to be dominant. Measurements carried out 105 days after tracer infiltration vertical transport of the tracer towards depths that reached 4m. Results obtained from the VMS indicate that the tracer has reached such depths through the activation of matrix and fracture flow mechanisms following frequent rainfall episodes.
The implementation of the setup has provided detailed information about water flow dynamics and chemistry across the vadose zone, as well as the spatial characterization of structures and tracer distribution in the subsurface. From such outcome, it can be concluded that this setup could be used to improve site conceptual models which are essential for developing risk assessments and remediation plans.