New Applications of Differential Electrical Resistivity Tomography and Time Domain Reflectometry to Modeling Infiltration and Soil Moisture in Agricultural Sinkholes

Sinkholes are commonly utilized for agriculture, and are thus subject to application of fertilizers, including animal manures and biosolids. Because sinkholes are hydrologically connected to underlying aquifers, fertilizer application within sinkholes may adversely impact groundwater quality. Few scientific data are available to support effective management of fertilizer application that will minimize karst groundwater contamination. Our research aims to characterize the hydrogeology and recharge mechanisms in agricultural sinkholes. Initial work includes physical characterization of several sinkholes at the Virginia Tech Kentland Experimental Farm, Whitethorne, VA. Nearly 100 2-D Electrical Resistivity Tomography (ERT) profiles, topographic mapping, and physical observations were used to generate 3-D sinkhole models of the shallow subsurface. Preliminary results generally show a clear resolution of changes in soil composition, and in some cases, the bedrock-soil interface. We conducted an experiment utilizing 2-D Differential ERT to monitor vadose water movement and to delineate preferential flowpaths during a recharge event. Repeated measurements in the same location before, during, and after a large storm, show significant resistivity differences interpreted to be related to changes in soil moisture due to infiltration. Similar experiments using 3-D Differential ERT are underway that will attempt to model infiltration and delineate preferential flowpaths over larger areas. 3-D Differential ERT results will be compared with simultaneous soil moisture measurements collected using down-hole Time Domain Reflectometry (TDR). If modeled ERT values can be correlated with the TDR soil moisture values, this technique has the potential to provide high resolution 3-D soil moisture data that can be used to greatly improve the visualization of vadose flow in shallow sinkholes.