GEOPHYSICALLY-MONITORED DYE TRACER TEST OF INFILTRATION IN THE UNSATURATED ZONE AT THE SHALE HILLS CZO

We conducted a dye tracer test of artificial infiltration and monitored the hydrologic process using ground penetrating radar (GPR) on a hillslope at the Shale Hills CZO. After the geophysical surveys were conducted, the site was excavated to photograph the dye pathways. The goal of the project was to improve the understanding of how GPR can be used to interpret infiltration by comparing GPR images and photographed dye pathways and to evaluate the importance of the soil-bedrock interface and saprolite fabric on infiltration pathways. Brilliant blue dye (4 g/L) was used as the tracer, and 50 L was injected into the unsaturated zone through a 1 m long trench on a hillslope. We performed dye injection at two sites, one under wet antecedent conditions and one initially dry. A 3D GPR survey with an 800 MHz antenna was conducted downslope of the trench on a 1 m by 2 m grid at tightly spaced 0.05 m intervals. While GPR does not directly image the dye, its signals respond to changes in soil moisture content and changes in radar propagation in saprolite versus bedrock. Mapping the bedrock is important because its low permeability may control flow paths. The GPR bedrock map shows a highly irregular surface and shallow bedrock (approximately 0.5 m depth). The dye tracer photographs show preferential flow paths at both tracer test sites. Dye pathways were anisotropic and strongly influenced by the saprolite fabric (oriented along strike). Under wet antecedent conditions, dye also followed root traces but under dry conditions, only structural fabric controls were observed. GPR radargrams showed a fingering pattern similar to the dye trace photographs, but with less obvious resolution of the structural fabric. The depth of penetration of the dye (approximately 0.5 m) and lateral migration (about 0.2 m) were also captured by the radargrams. Although the depth of penetration was controlled by bedrock, the fingering observed in the dye did not seem to be influenced by undulations in the bedrock but was set by the fabric encountered in the saprolite. The GPR provided a rapid method for mapping bedrock, which was an important control on depth of infiltration. GPR also suggested fingering occurs, but detailed pathways were difficult to resolve without dye tracing and excavation.