Hydrological connectivity of soil pipes determined by ground-penetrating radar tracer detection

Soil pipes are common and important features of many catchments, particularly in semi-arid and humid areas, and can contribute a large proportion of runoff to river systems. They may also significantly influence catchment sediment and solute yield. However, there are often problems in finding and defining soil pipe networks which are located deep below the surface. Ground penetrating radar (GPR) has been used for non-destructive identification and mapping of soil pipes in blanket peat catchments. While GPR can identify subsurface cavities, it cannot alone determine hydrological connectivity between one cavity and another. This paper presents results from an experiment to test the ability of GPR to establish hydrological connectivity between pipes through use of a tracer solution. Sodium chloride was injected into pipe cavities previously detected by the radar. The GPR was placed downslope of the injection points and positioned on the ground directly above detected soil pipes. The resultant radargrams showed significant changes in reflectance from some cavities and no change from others. Pipe waters were sampled in order to check the radar results. Changes in electrical conductivity of the pipe water could be detected by the GPR, without data post-processing, when background levels were increased by greater than approximately twofold. It was thus possible to rapidly determine hydrological connectivity of soil pipes within dense pipe networks across hillslopes without ground disturbance. It was also possible to remotely measure travel times through pipe systems; the passing of the salt wave below the GPR produced an easily detectable signal on the radargram which required no post-processing. The technique should allow remote sensing of water sources and sinks for soil pipes below the surface. The improved understanding of flowpath connectivity will be important for understanding water delivery, solutional and particulate denudation, and hydrological and geomorphological model development.