Subsurface transport and potential risk of phosphorus to groundwater across different land uses in a karst springs basin, Florida, USA

Abstract Excess phosphorus (P) loading due to anthropogenic activities has been identified as a primary cause of surface water eutrophication, but it is controversial whether P is a contributor to groundwater contamination. In this study, subsurface transport of P to groundwater and its environmental risk across different land uses was investigated in a karstic springs basin, Florida, USA. Eleven sites including 5 agricultural sites, 5 urban, and 1 pine plantation were selected. Boreholes were drilled to collect soils from surface, vadose zone, and aquifer at 0.6 m depth intervals. Water-extractable P (WSP), Mehlich 3-P, Al, and Fe, soil P storage capacity (SPSC), and P sorption isotherm parameters were obtained for soils from selected depths. We found that agricultural sites particularly horse farms, and urban sites with golf courses and septic tanks showed high risk of P leaching with TP concentrations in the groundwater above the numeric criteria for streams in the north central Florida (0.3 mg L −1 ). Besides land use which reflected P loadings in the surface soils, significant positive correlations of P sorption maximum (S max ) with M3-Fe, Al, and organic matter, and the precipitation of P by calcium bicarbonate, imply the mechanism regulating soil P attenuation in vadose and saturated zones, respectively. Negative SPSC values were significantly negatively correlated with WSP in all the zones, suggesting the applicability of SPSC in deep soils. Moreover, high TP concentration was observed in the groundwater at one site where the SPSC value changed from positive in the surface soils to negative in the subsoils, indicating that an additive SPSC value to water table would be more robust for evaluating the potential of P leaching before moving down to the limestone area.