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Title: MODELLING URANIUM LEACHING FROM AGRICULTURAL SOILS TO GROUNDWATER AS A CRITERION FOR COMPARISON WITH COMPLEMENTARY SAFETY INDICATORS

Author
item JACQUES, D - SCK-CEN, MOL, BELGIUM
item SIMUNEK, JIRKA - U.C. RIVERSIDE, CA
item MALLANTS, D - SCK-CEN, MOL. BELGIUM
item Van Genuchten, Martinus

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: 3/10/2005
Publication Date: 9/1/2005
Citation: Jacques, D., Simunek, J., Mallants, D., Van Genuchten, M.T. 2005. Modelling uranium leaching from agricultural soils to groundwater as a criterion for comparison with complementary safety indicators. In: 29th Symposium on the Scientific Basis for Nuclear Waste Management, p. 1-9, Sept. 12-16, 2005, Material Research Society, Ghent, Belgium.

Interpretive Summary: Phosphate (P) fertilizers are often applied annually to agricultural fields to preserve soil fertility. Unfortunately, many mineral fertilizers, such as phosphates and superphosphates, contain a certain amount of naturally occurring radioactive material such as Uranium (U) and Thorium (Th). The fate and transport of U and Th in soils is very complex since they are subject to many complicated geochemical reactions such as those with soil organic matter, the soil mineral phase, phosphate, and other constituents. These geochemical processes are additionally influenced by soil moisture and water flux variations in space and time caused by time-variable weather conditions. This study addresses the long term leaching of U (as naturally present in mineral P-fertilizers) through soil to the ground water table. A 30-year long time series of climatological data from Northern Belgium was used to define precipitation and evaporation rates. Predictions were made with the HP1 code, which was recently developed by coupling the HYDRUS-1D software for water and solute transport with the very general PHREEQC-2 geochemical code. Calculated uranium fluxes over a 200-year period to groundwater were used as a reference in comparison to long-term radionuclide release rates expected from a planned surface repository of low-level radioactive waste in Belgium. U fluxes resulting from long-term mineral P-fertilizer applications were estimated to be much larger than those expected from the planned repository. The present study was a first analysis in which most, but still not all, soil processes were incorporated. Future studies will expand the simulations by incorporating the effects of nitrates and carbonates (soil respiration) in the soil profile, and U uptake by plant roots.

Technical Abstract: Radiological assessments of waste repositories often use the dose to humans as an indicator of safety. There is a tendency, however, to invoke alternative or complementary safety indicators for evaluating and confirming the long-term safety of a repository. One example is the use of concentrations of naturally occurring radionuclides (U, Th) in soils and groundwater. Naturally occurring radionuclides can also end up in soils and 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. 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. Calculated uranium fluxes from agricultural fields to groundwater may serve as reference levels for radionuclide fluxes from waste repositories. Calculations were based on a new reactive transport model, HP1, which accounts for interactions between U and organic matter, phosphate, and carbonate. Solid phase interactions were simulated using a surface complexation module. Furthermore, all geochemical 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 climatological data for Belgium. Based 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 reaching the groundwater table.