MONITORING SHALLOW SUBSURFACE FLOW ON A HILLSLOPE USING TIME-LAPSE GPR AND ERT

Abstract

To improve our understanding of aquifer recharge, storm water management, contaminant transport, weathering, erosion and soil development, we need better ways to characterize subsurface flow in response to precipitation and snowmelt. Rainfall on a hillside does not simply partition into surface runoff and water that percolates vertically to the water table. Much of the flow is lateral through the root zone, controlled by networks of macropores, and these preferential flow paths can change from event to event depending on precipitation and antecedent moisture conditions. Our research focused on using geophysical monitoring tools to detect these shallow preferential flow paths. In July, 2012, we conducted a series of infiltration tests at the Shale Hills Critical Zone Observatory in central Pennsylvania. Working on a forested hill slope with a thin soil cover (approximately 30 cm thick Weikert soil series ) we conducted a set of small-scale infiltration experiments. In each case, we released 26.5L or 53L of water at constant head into a 1-m long, 10 cm deep trench located 20 cm upslope of our 1 m by 3 m geophysical survey area. The geophysical methods tested were time-lapse electrical resistivity tomography (ERT), time-lapse ground penetrating radar (GPR), and two other GPR methods sensitive to moisture changes: GPR ground wave velocity and GPR surface reflectance. While calibration of the geophysical data to obtain volumetric soil water content proved problematic, qualitatively all of the methods detected unexpectedly rapid lateral flow, with water traveling as far as 80 cm downslope within 15 minutes after the water injection ended. Introducing a second water pulse shortly after the first showed flow controlled by hillside microtopography and by hydrologic connectivity created by the first water release. While none of the geophysical methods had sufficient resolution to map individual macropores, they all showed promise for characterizing heterogeneous patterns of subsurface lateral flow.