Electrical resistivity imaging (ERI) of subsurface hydrological connectivity controlling phosphorous transport in artificially drained agricultural landscapes

The phosphorous (P) index is an applied assessment tool that identifies agricultural fields considered critical sources of P due to hydrologic connectivity between P sources and surface water. There is a need to improve the depiction of subsurface hydrological connectivity in P risk assessment tools in flat, artificially drained landscapes. Time-lapse ERI is an established near-surface geophysical technique that is useful for "visualizing" dynamic subsurface hydrological processes in space and time. We are applying time-lapse ERI in conjunction with salt tracers to a site on the Delmarva Peninsula (MD) that is drained by a dense network of open ditches. Our objectives are to examine the relative effects of soil properties, drainage intensity, and management factors on shallow lateral flow generation. As a proof of concept, a sodium chloride solution was spray-irrigated and continuously injected into two shallow wells (0.6 and 0.3 m deep) within a 23 m2 plot located directly adjacent to a small field drainage ditch. Time-lapse ERI in the region of spray-irrigation showed increased conductivity with some evidence of lateral surface migration. The injection well at 0.3 m depth exhibited increased conductivity during the tracer application and appeared to be hydraulically connected to an adjacent well 0.6 m away where no tracer was applied. Following this proof of concept study, we automated an ERI system of 192 electrodes within a 72m2 plot to monitor the application salt tracers from a 25 cm deep trench during natural rainfall events. This system, operational since September 2015, reveals the presence of tracer within and slightly below the trench suggesting only limited hydrological connectivity with drainage ditches during storms. Continual ERI monitoring over a wider range of storm intensities is likely to advance our understanding of shallow subsurface flows contributing to P transport and their potential connectivity with ditch drainage waters.