Subsurface Transport of Phosphorus In Riparian Floodplains: Tracer And Phosphorus Transport Experiments

Abstract

The primary transport mechanism for phosphorus (P) movement from upland areas to surface water systems is typically surface runoff, with subsurface transport assumed negligible. However, certain local conditions can lead to an environment where subsurface transport may be significant. The objective of this research was to determine the importance of subsurface transport of P along streams characterized by cherty or gravel subsoils. At a field site adjacent to the Baron Fork Creek, a trench was installed with the bottom of the trench at topsoil/alluvial gravel interface. Fifteen piezometers were installed at various locations surrounding the trench in order to monitor flow and transport. In three separate experiments, water was pumped into the trench from the Baron Fork Creek to maintain a constant head. At the same time, a conservative tracer (Rhodamine WT) and/or potassium phosphate were injected into the trench at concentrations ranging between 3 and 100 ppm for Rhodamine WT and at 100 ppm for P. Laboratory flow cell experiments were also conducted to determine the effect that flow velocity had on P sorption. Rhodamine WT and P were detected in some piezometers at equivalent concentrations as measured in the trench, suggesting the presence of preferential flow pathways. Phosphorus sorption was minimal (R = 1, where R is the retardation coefficient) along the preferential flow pathways but transport was retarded in non-preferential flow paths (R > 5 to 6). The effect that flow velocity has on P sorption was tested in the laboratory using flow through cells. Results suggested that velocity did have an effect on P sorption of the alluvial subsoil. The potential for nutrient transport shown by this alluvial system has implications regarding alternative management of similar riparian floodplain systems.