Transport of phosphorus in an alluvial gravel aquifer

The transport of phosphorus (P) and bromide (Br) in an alluvial gravel aquifer was investigated in two tracing experiments: (1) injecting pulses of orthophosphate and (2) 10% diluted municipal effluent spiked with orthophosphate designed to simulate the leaching of P via preferential flow after the application of fertiliser or effluent on to shallow stony soils. Results indicated that compared with Br, filtered reactive P (FRP) transport was retarded, and its mass recovery relative to Br was only 6%-28%. Attenuation was probably a result of P sorption on iron (Fe) and manganese (Mn) oxides present in the aquifer media and filtration of colloidal-P. As a result, FRP could not be distinguished from background concentrations in the aquifer system when sampled 38 m from the injection site. At the concentrations/loadings used, surface water recharged from groundwater at this study site may receive P loading via the preferential flow of P into groundwater if the distance between source and recharge point is less than 38 m, and possibly at greater distances under higher loadings from intensive land use.