Contrasting physical controls on phosphorus transport to shallow groundwater at the hillslope scale

Abstract. In well-drained agricultural catchments transport of phosphorus (P) to groundwater (GW) can be controlled by static and dynamic factors and where surface water is GW fed this can lead to elevated P concentrations at the catchment outlet. In order to better control P transport along hillslopes a spatial and temporal conceptual view of P loss to GW must be developed. Initially in the present study, hillslope GW quality and rainfall data were examined for 2017 utilising a transect of piezometers at upslope (US), midslope (MS) and downslope (DS) locations. Two dominant scenarios emerged where GW P concentrations at DS and MS were simultaneously low or at other times DS became elevated and MS remained low. To examine the potential reasons for such scenarios, a one-dimensional hydrological transport model was developed for the unsaturated zone at DS and MS using rainfall and depth specific soil physical and hydraulic data. Results indicated that the DS zone facilitated transport (higher sand content, soil saturated hydraulic conductivity (K_s) and lower soil compaction) with higher modelled concentration peaks towards higher GW P concentrations whereas the MS zone had more potential to attenuate transport (lower soil K_s and higher soil compaction). Moreover, inter-annual variations of GW P concentrations at DS were related to rainfall and GW level. Hence, mitigation strategies should particularly (but not exclusively) focus on reducing P sources in the DS zone. This also indicates a need to identify hotspots of facilitated transport to shallow GW using finer scale soil properties surveys. Here, this is defined by low soil compaction, high sand content and soil K_s. However, challenges arise as soil properties can vary in time with soil management and with the difficulty of assessing the transport potential of deeper soil.