Long term uranium migration in agricultural field soils following mineral P-fertilization

To preserve soil fertility, organic and mineral fertilizers are applied to agricultural fields. Mineral fertilizers, such as phosphates and super phosphates, contain a certain amount of long-lived alpha activity due to 238U, 230Th, amongst others. The fate of U in soil systems is quite complex. Since U forms aqueous complexes with soil organic matter, nitrate, phosphate, and carbonate, amongst others, U migration may be influenced by their cycles in the soil. Furthermore, surface complexation onto the soil solid phase strongly influences the fate of U in the soil profile, whereby U-surface complexation competes with the adsorption of protons and other cations. Moreover, all these geochemical processes are coupled with the water content and water flux variation in space and time, due to time-variable precipitation and evapotranspiration rates at the soil surface. This study addresses the long term leaching of U (applied as a naturally occurring radioactive material in mineral P-fertilizers) through the soil to the ground water under natural boundary conditions. A 30-year long time series of climatological data for the Campine region (Belgium) is taken to define precipitation and potential evaporation rates. This 30-year long time series is used to generate a long term time series of upper boundary conditions (e.g., 200 years). Based on an average fertilizer application amount, the input of phosphate and uranium in the soil were defined. The paper discusses the calculated U geochemistry in a soil profile together with calculated U fluxes at the soil/groundwater interface. The sensitivity of U-leaching with respect to the type of imposed flow rate (steady-state versus transient) is also discussed. Calculated uranium fluxes to groundwater were used as reference levels in comparison with the release of long-lived actinides from a planned surface repository of low-level radioactive waste.