Identifying contrasting influences and surface water signals for specific groundwater phosphorus vulnerability

Two groundwater dominated catchments with contrasting land use (Grassland and Arable) and soil chemistry were investigated for influences on P transfer below the rooting zone, via the aquifer and into the rivers. The objective was to improve the understanding of hydrochemical process for best management practise and determine the importance of P transfer via groundwater pathways. Despite the catchments having similar inorganic P reserves, the iron-rich soils of the Grassland catchment favoured P mobilisation into soluble form and transfer to groundwater. Sites in that catchment had elevated dissolved reactive P concentrations in groundwater (>0.035mgl−1) and the river had flow-weighted mean TRP concentrations almost three times that of the aluminium-rich Arable catchment (0.067mgl−1 compared to 0.023mgl−1). While the average annual TRP flux was low in both catchments (although three times higher in the Grassland catchment; 0.385kgha−1 compared to 0.128kgha−1), 50% and 59% of TRP was lost via groundwater, respectively, during winter periods that were closed for fertiliser application. For policy reviews, slow-flow pathways and associated time-lags between fertiliser application, mobilisation of soil P reserves and delivery to the river should be carefully considered when reviewing mitigating strategies and efficacy of mitigating measures in groundwater fed catchments. For example, while the Grassland catchment indicated a soil-P chemistry susceptibility, the Arable catchment indicated a transient point source control; both resulted in sustained or transient periods of elevated low river-flow P concentrations, respectively. [Display omitted] •P transfer via groundwater to rivers was investigated in two agricultural catchments.•Fe-rich soils favour P mobilisation into soluble form and transfer to groundwater.•P concentrations in near-stream groundwater influence stream P concentrations.•Groundwater contribution to stream TRP flux was 50% and 59% in winter.•Susceptibility of P via groundwater should be considered for mitigation.