Electrical imaging of solute transport dynamics to understand nutrient transport from flat agricultural landscapes

Electrical and electromagnetic geophysical techniques are increasingly used to capture the dynamics of solute transport in the shallow subsurface. Three-dimensional electrical imaging techniques were used to investigate solute transport, including possible activation of preferential flow paths, at an artificially-drained site located on a flat agricultural landscape where excess nutrients discharge into drainage ditches. A saline tracer was injected into the saturated zone at a rate previously determined to limit groundwater mounding. Subsequently, a sprinkler system was used to simulate a 25-year, 12-hour storm event (~120 mm). A fan array of 96 electrodes, designed based on solute transport simulations, was used to acquire 3D electrical imaging monitoring datasets during tracer injection and over the subsequent simulated storm event. The time lapse electrical geophysical datasets suggest the tracer transport was through storm-activated preferential pathways in the subsurface. Calibration of hydrological models using the electrical geophysical datasets as proxies of changes in solute concentration provides unique insights into the role of preferential flow in providing groundwater-sourced nutrient inputs to drainage ditches from agricultural fields.