Introduction
We acquired and analyzed 3-D multicomponent groundpenetrating radar (GPR) data to better understand the subsurface flow conduits at a hydrologic experimentation site. Previous workers conducted a set of shallow (<2.5 m) subsurface hydrology experiments during simulated rainfall events within fractured and karsted limestone of the Edwards Aquifer region near San Antonio, Texas. They observed at an observation trench located on the down slope side of the site that lateral subsurface flow is guided by open joints, bedding planes and karst features. They also utilized tracer experiments, which showed a high degree of variability in tracer recovery, advection speed, and concentration depending on the location of the application of the tracer. We utilized the 3-D multicomponent GPR data in an attempt to identify the main conduits of flow within the experimentation site in order to explain the observed tracer experiments. The GPR revealed that the most obvious conduits run nearly parallel with the observation trench, with some conduits able to quickly move water away from the trench. This information helps explain the high spatiotemporal variability in the tracer data. Our study demonstrates that multicomponent GPR provides a technique that can improve future subsurface hydrologic experimentation.
A hydrologic experimentation site (7 x 14 m) situated within the Edwards Aquifer region of Texas, approximately 30 miles north of San Antonio, has been established by rangeland ecologists to study the effects of brush removal on the hydrologic cycle (Taucer et al. 2006; and Dasgupta, 2006). Juniper brush invasion is hypothesized to adversely effect the recharge of the Edwards Aquifer and local stream flow (Dasgupta, 2006). The site features rainfall simulation equipment, a runoff gage, rain gages, soil moisture probes and a 2.5 m deep trench excavated on the downhill edge of the slope ( 2%) of the site that allows for sampling lateral subsurface flow (Figure 1). The Glen Rose formation was deposited in a shallow carbonate sea during the Cretaceous, cycling between regionally laterally continuous layers of marl and limestone. provide good conduits for lateral flow and the layers of marl act as a baffle to vertical flow. Structurally the area is associated the Balcones fault zone with the main strike of faulting at northeast-southwest. The faulting and jointing of the limestone rocks has allowed water to flow through the otherwise impermeable rock and dissolve karst features within the rock that dominate subsurface flow (Ferrill et al., 2004). The observation trench at the experimentation site shows that the top 30 cm is weathered limestone and organic soil, the next 150 cm is limestone with joints and karst features that provides for the bulk of the observed lateral flow, and the limestone is underlined with a low permeability layer of marl. Taucer et. al., 2006, showed that during rainfall simulations nearly all water that reached the surface of the plot infiltrated into the subsurface, even during very intense rainfall simulations (15.2 cm/hr). Therefore the subsurface conduits are capable of moving water at a very high rate.
