Modelling uranium leaching from agricultural soils to groundwater as a criterion for comparison with complementary safety indicators

Naturally occurring radionuelides can also end up in soils und groundwater due to human practices, such as application of certain fertilizers in agriculture. Many mineral fertilizers particularly (super)phosphates, contain small amounts of ²³⁸U and ²³⁰Th which eventually may be leached from agricultural soils to underlying water resources. Field soils that receive P-fertilizers accumulate U and Th and their daughter nuclides, which eventually may leach to groundwater. Our objective was to numerically assess U migration in soils. Calculations were based on a new reactive transport model, HPI, which accounts for interactions between U and organic matter, phosphate, and carbonate. Solid phase interactions were simulated using a surface complexation module. Furthermore, all geoehemical processes were coupled with a model accounting for dynamic changes in the soil water content and the water flux. The capabilities of the code in calculating natural U fluxes to groundwater were illustrated using a semi-synthetic 200-year long time series of elimatological data for Belgium. Rased on an average fertilizer application, the input of phosphate and uranium in the soil was defined. This paper discusses calculated U distributions in the soil profile as well as calculated U fluxes leached from a 100-cm deep soil profile. The calculated long-term leaching rates originating from fertilization are significantly higher after 200 years than estimated release rates from low-level nuclear waste repositories.