Impact of Preferential Flow Paths on Alluvial Groundwater Flow Patterns and Phosphorus Transport

Abstract

While surface runoff is considered to be the primary transport mechanism for phosphorus (P), subsurface transport through coarse subsoil to gravel bed streams may be significant and represent a source of P not alleviated by current conservation practices (e.g., riparian buffers). Previous research has documented P transport in a preferential flow path (PFP) identified as a buried gravel bar. It is hypothesized that PFPs, if connected to the soil surface, provide a rapid and efficient method of transporting P, and that these alluvial features are transient storage zones for nutrients, acting as a sink during high flow and a source during baseflow. The objectives of this project were to document the impact of PFPs on groundwater flow patterns on a field scale and to quantify potential P transport capacity through PFPs. Long-term monitoring was performed at floodplain sites adjacent to Barren Fork Creek and Honey Creek in northeastern Oklahoma. Based on results from subsurface electrical resistivity mapping, observation wells were installed both in PFPs and in non-PFP subsoils. Water levels and temperature in the wells were monitored real-time using pressure transducers for four months, which included multiple high flow events. Also, P samples were obtained from the observation wells and in the stream to document P concentration gradients over time. Contour plots showing direction of flow were generated based on water table elevation data. Results indicated spatial heterogeneity in hydraulic conductivity and zones of groundwater convergence and divergence. The activity of PFPs depended on the elevation of the water table and the interaction between the stream and the groundwater. The PFPs that rapidly transported P had groundwater total P concentrations that mimicked the stream and exceeded 0.20 mg/L during some high flow events. The pathways with rapid P transport did not necessarily correlate to subsurface zones of high hydraulic conductivity. Pathways of high hydraulic conductivity must be connected to the surface water source and be hydraulically activated for preferential transport to occur.