Comparison of subsurface and surface runoff phosphorus transport rates in alluvial floodplains

Phosphorus (P) loading to streams can occur by both surface runoff and subsurface transport, with subsurface P transport often assumed negligible. Groundwater P concentrations in alluvial aquifers can be significant, especially in preferential flow paths (PFPs). The objectives of this research were to quantify subsurface P transport rates at two sites in northeastern Oklahoma and to compare them with surface runoff P transport rates derived from a hydrologic model, the Pasture Phosphorus Management Calculator (PPM Plus). Ozark ecoregion study sites were adjacent to the Barren Fork Creek and Honey Creek in northeastern OK, USA. Each site, instrumented with 24 observation wells, was monitored for several months for both groundwater levels and P concentrations. Using the flow and P concentration data, Monte Carlo simulations with Darcy's Law and a P transport rate equation were used to calculate the distributions of subsurface P transport rates across a transect within the well field containing a single identified PFP. Total subsurface P transport rates, through both the non-PFP flow domain and a single PFP, were estimated to be 0.04 kg year−1 and 0.03 kg year−1 for the Barren Fork Creek and Honey Creek field sites, respectively. Monte Carlo simulations for surface runoff P transport rates with PPM Plus resulted in average total P surface runoff transport rates of 0.07 kg year−1 for the Barren Fork Creek site and 0.08 kg year−1 for the Honey Creek site. For the groundwater at these floodplains, the P source was P-laden stream water flowing into the alluvial aquifer and a minimal quantity of P leaching from the surface. Results indicated that the subsurface P transport rates for small (3 ha) alluvial floodplain sites in the Ozark ecoregion were at least 0.03–0.04 kg year−1, although subsurface P transport rates may be higher in cases with greater numbers of PFPs and where the subsurface is connected to a larger P source.

Alluvial floodplains possess preferential flow pathways detected by electrical resistivity imaging (top). In these gravel dominated systems (bottom, left), subsurface transport, including transport through these preferential pathways, was quantified using observation well fields (bottom, center) in two alluvial floodplains and shown to be capable of transporting phosphorus at similar magnitudes (bottom, right) to surface runoff.Figure optionsDownload as PowerPoint slideHighlights
► Subsurface phosphorus transport significant in alluvial aquifers.
► Phosphorus transport impacted by stage-dependent preferential flow pathways.
► Subsurface transport rates on the order of 0.05 kg year−1 for small (3 ha) floodplains.
► Subsurface phosphorus transport rate significant compared to surface runoff.
► Subsurface phosphorus transport may increase with stream order.