Cross-borehole ground-penetrating radar for monitoring and imaging solute transport within the vadose zone

A two-stage salt infiltration experiment was conducted to simulate vadose zone contaminant transport and to investigate mechanisms that affect the development of contaminant plumes in the unsaturated zone. A low-concentration sodium chloride solution (1024 ppm) was first infiltrated for 7 months through one third of a specially designed infiltrometer at a rate of 2.7 cm/d, with tap water infiltrating through the remaining two thirds at the same rate. This first stage was followed by 3 months of running tap water through the entire infiltrometer to flush the system while maintaining the same unsaturated infiltration conditions. Next, the concentration of the sodium chloride solution was increased to 6900 ppm, which was introduced at the same infiltration rate over a different one third of the infiltrometer for 3 months. This paper focuses on using cross-borehole ground-penetrating radar attenuation tomography to image the developing plume in a time-lapse sense. Attenuation is shown to increase with increasing salt concentration, and the time-lapse images indicate that regions of anomalous attenuation develop in locations where pockets of higher moisture content exist. This suggests that either preferential transport pathways develop within the finer-grained sediments exhibiting greater water retention capability or salt ions are being incorporated into the mineral surficial charge within the fine-grained materials.