Electrical geophysical monitoring of subsurface solute transport in low-relief agricultural landscapes in response to a simulated major rainfall event

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The Delmarva Peninsula contributes significantly to nutrient loading of the Chesapeake Bay predominantly from excessive Phosphorus (P) historically applied to low-relief agricultural fields with artificial drainage. Subsurface P transport is recognized as a primary pathway for P loss from these artificially drained agricultural systems, especially during high intensity rainfall events. We used time-lapse electrical resistivity imaging (ERI) to monitor movement of an injected salt tracer in a low-relief, artificially drained agricultural field in Princess Anne, MD during a simulated 25-year rainfall event. Conductivity breakthrough curves (BTCs) from the time-lapse electrical datasets were compared against relative concentration BTCs of a solute transport model to assess the geophysically-estimated solute transport behavior. We identified rapid lateral transport via high permeability pathways, emphasizing the role of soil heterogeneity to increase the rate of groundwater flux from the field to ditch waters with the potential for subsurface P transport during high intensity rainfall events. Based on our results, we suggest that critical source areas for P loss (i.e., where high soil P concentrations coincide with hydrologic connectivity) in ditch-drained fields may be farther from field edges than previously recognized.