Monitoring unsaturated flow and transport using cross-borehole geophysical methods

Improved understanding of unsaturated flow and transport processes is limited by the lack of appropriate in situ measurement techniques. This study was conducted to determine whether two noninvasive cross-borehole geophysical methods combined could be used to estimate two important unsaturated zone transport parameters, namely the pore water velocity and longitudinal dispersivity. Cross-borehole electrical resistivity tomography and ground penetrating radar were used to estimate temporal and spatial variation of electrical resistivity and water content, respectively, during a 20-d forced infiltration experiment. The resulting one-dimensional profiles and two-dimensional images of moisture content and electrical resistivity were subsequently combined to estimate solute tracer concentration. The results were used to analyze the downward migration and vertical spreading of water and tracer mass. The two geophysical methods provided independent estimates of soil moisture content and electrical resistivity that were spatially and temporally consistent. The observed changes in moisture content and electrical resistivity were, as a first approximation, used in a one-dimensional moment analysis. The transport behavior was found to be very susceptible to layering of the subsurface. Even slight reductions in grain size apparently lead to flow barriers and associated lateral flow, resulting in tracer mass loss, reduced vertical pore water velocity, and increased longitudinal dispersivity. Synthetic data showed that the estimated unsaturated transport parameters (i.e., pore water velocity and longitudinal dispersivity) and the mass estimate were influenced by the selected electrical resistivity tomography inversion routine. In effect, an overprediction of all three parameters was observed.