GROUND-PENETRATING RADAR IMAGES HYDROLOGICAL PROCESSES WITHIN THE UNSATURATED ZONE AT THE SUSQUEHANNA-SHALE HILLS CZO

Dye tracer and ground-penetrating radar (GPR) were used to image preferential flow paths in the unsaturated zone on hillslopes in two adjacent watersheds within the Susquehanna-Shale Hills CZO. At each site we released 50 L of water mixed with brilliant blue dye (4 g/L) into a trench cut perpendicular to the slope to create a line infiltration source (100 cm long by 10 cm wide by 18 cm deep). GPR (800 MHz antennae with constant offset) was used to monitor the movement of the dye tracer down slope on a 100 cm x 20 cm grid with a 5 cm line spacing. The site was then excavated and the stained pathways photographed. We saw a considerable difference in the pattern of shallow preferential flow between the two sites despite their similar positions on the slope. Both sites showed dye penetrating down to bedrock (~50 cm); however, lateral flow migration between the two sites was different. At the first site lateral flow migration was ~50 cm and was fairly evenly dispersed. At the second site lateral flow was ~35 cm, dye was barely visible until the excavation reached ~10 cm, and there was more evidence of distinct fingering. Factors that may have contributed to the different flow patterns include antecedent soil moisture conditions, soil structure, clay content, sediment size, root density, fracture density and fracture orientation. Comparison of the radargrams with the dye patterns showed that although GPR captured the general extent of the vertical and lateral flow of the dye, it did not have sufficient resolution to resolve the fine scale fingering.