Combining Geophysical Imaging and Conservative Tracers to Characterize Subsurface Phosphorus Losses in Ditch-Drained Soils.

Long-term application of poultry litter application to agricultural soils on Delmarva has led to a build-up of soil test phosphorus (P) to high levels and an increased risk of P losses to sensitive water bodies.  Subsurface lateral flow pathways can deliver significant P loads from high P agricultural soils in artificially ditch-drained agricultural fields. However, the mechanisms of subsurface P transport are poorly understood.  For this study, salt tracers will be paired with Electrical Resistivity Imaging (ERI) to determine the site characteristics and environmental conditions that affect subsurface lateral flow of water to drainage ditches at two agricultural field sites with contrasting soil and hydrologic properties (poorly drained, intensively ditched vs. well-drained, low intensity ditching). Soils at each site will be characterized for soil physical (texture, bulk density) and chemical properties (routine soil analysis, water soluble P, total P). Bromide and chloride salt tracer solutions will be applied at the sites using a double-ring infiltrometer. Electrical resistivity imaging will be used to track movement of the salt plume towards agricultural ditches. Suction lysimeters will be placed along the suspected flow path in order to correlate the tracer-laden water samples with the ERI-observed salt plume movement. Results will provide information about how and when P moves in subsurface lateral flow, which can be used to more accurately assess risk for subsurface P transport in the Delaware P Index and improve management of legacy P.