Using Electrical Resistivity Imaging to Characterize Subsurface Phosphorus Movement to Drainage Ditches.

Long-term application of poultry litter to agricultural soils on Delmarva has led to the build-up of soil test phosphorus (P) and increased risk of P losses to sensitive water bodies. Subsurface lateral flow pathways can deliver significant P loads from high P agricultural soils with artificial drainage. However, the mechanisms of subsurface P transport are poorly understood. For this study, salt tracers were paired with electrical resistivity imaging (ERI) to determine the site characteristics and environmental conditions that affect subsurface lateral flow of water to drainage ditches. Initially, soil cores and water samples collected from an agricultural field with a history of poultry litter application at the University of Maryland Eastern Shore research farm were analyzed for water soluble P and total dissolved P. Within the same field, a potassium bromide tracer was introduced 10.5 m from a drainage ditch via a 25-cm deep trench. Movement of the tracer toward the ditch was monitored from May 2015 through August 2015 using ERI. Electrical conductivity sensors were placed in 1 m deep wells along the suspected flow path in order to confirm ERI-observed salt plume movement. Very little movement of the bromide tracer occurred during the initial observation period, suggesting that nutrient movement was minimal when the water table was deep. Continued observations should show more rapid movement of the tracer during wetter periods with higher water tables. Results of this study are expected to provide a better understanding of P movement in artificially drained systems to guide agricultural P management.