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

In radiological assessments for waste repositories the dose to humans is often used as a safety indicator. There is a tendency to use complementary safety indicators for evaluating the long-term safety of a repository. Naturally occurring radionuclides can end up in groundwater owing to human practices, such as fertilizer application. Mineral fertilizers ((super)phosphates) contain small amounts of 238U and 230Th. U from P-fertilizers accumulates in field soils and leaches to the groundwater. The objective is to numerically assess U migration in soils. The calculated uranium fluxes to groundwater may be used as the reference levels in comparison with radionuclide fluxes from repositories. The reactive transport model HP1 accounted for aqueous complexation, U-organic matter interactions and surface complexation on soil oxides. All geochemical processes are coupled with soil water content and water flux variations in space and time. Natural fluxes of U to groundwater were calculated using a 30-year long time series of climatological data for Belgium. Based on an average fertilizer application amount, the input of phosphate, nitrogen 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.