Abstract. Water flow and solute transport through a fractured vadose zone underneath an industrial contaminated site in Belgium were studied with a new methodological concept. The Vadose Zone Experimental Setup (VZES) combines a vadose zone monitoring system (VMS) with cross-borehole geophysics. The VMS provides continuous chemical and hydraulic information at multiple depths in the vadose zone. When combining such information with multidirectional subsurface imaging from geophysical measurements, flow and transport can be characterized at a scale that covers the spatial variability of the subsurface. The setup was installed on site and monitoring was carried out under natural recharge conditions. Results reveal quick rises in water content as a response to rainfall events in the upper and intermediate part of the vadose zone (down to 3.65 m depth). Macropore, micropore, matrix and preferential flow mechanisms are identified at these depth ranges. At greater depths, flow dynamics is slower and dominated by matrix flow. The governance of water flow mechanisms at different directions is controlled by the heterogeneous distribution of geological materials. Results from sampled waters across the vadose zone reveal that the chemistry of water collected from matrix is different from that collected from fractures. In addition, analysis of heavy metals indicates that Ni is leaching across the vadose zone, and its release might be a consequence of pyrite oxidation from backfilled materials. Results obtained from VZES indicate that the combination of different techniques providing in situ quantitative and qualitative information improves conceptual models of flow and transport in a heterogeneous subsurface.
Water and contaminants moving through the vadose zone are often subject to a large number of simultaneous physical and chemical nonequilibrium processes. Traditional modeling tools for describing flow and transport in soils either do not consider nonequilibrium processes at all, or consider them only separately. By contrast, a wide range of nonequilibrium flow and transport modeling approaches are currently available in the latest versions of the HYDRUS software packages. The formulations range from classical models simulating uniform flow and transport, to relatively traditional mobile-immobile water physical and two-site chemical nonequilibrium models, to more complex dual-permeability models that consider both physical and chemical nonequilibrium. In this paper we briefly review recent applications of the HYDRUS models that used these nonequilibrium features to simulate nonequilibrium water flow (water storage in immobile domains and/or preferential water flow in structured soils with macropores and other preferential flow pathways), and transport of solutes (pesticides and other organic compounds) and particles (colloids, bacteria and viruses) in the vadose zone.
Combining cross-hole geophysical and vadose zone monitoring systems for vadose zone characterization at industrial contaminated sites
