Estimation for the breakthrough curves of solute movement in the vadose zone using time-lapse cross borehole radar data

Time-lapse cross-borehole ground-penetrating radar (GPR) data were collected in an artificial groundwater recharge pit test and salt tracer test to monitor an infiltration process and its solute dynamics through the vadose zone. The field tests were conducted in the Makinohara upland area in Shizuoka Prefecture, Japan. We measured electromagnetic (EM) wave velocities and their attenuation with a geometry that transmitter and receiver antennas set at the same depth, and estimated changes in the water content profile based on a petro-physical relationship between apparent dielectric constant and water content. The velocity of wetting front and the increment of soil water were then estimated. When the water profile in the observed zone become in a steady state, we change the infiltration water to the salt water as a tracer. We monitored not only the velocity but also the attenuation of the EM wave. From the petro-physical relationship between attenuation and electrical conductivity of soil, we estimated breakthrough curve in the deep vadose zone from the bottom of infiltration pit to a depth of 5.5 m. From the shape of the breakthrough curve, we suppose that the injected tracer percolated down through the vadose zone relatively quickly, while the existing soil water with low mobility in the finer part of soil was contaminated gradually by a dispersion effect of tracer. We can estimate the velocity and dispersivity of solute plume through a statistical analysis of the breakthrough curves. They were controlled by heterogeneity of soil especially in the less permeable layer with finer grains. The time-lapse cross-borehole radar is applicable for the estimation of solute dynamics in the vadose zone.