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Title: LONG-TERM URANIUM MIGRATION IN AGRICULTURAL FIELD SOILS FOLLOWING MINERAL P FERTILIZATION

Author
item JACQUES, D - BNRC, MOL, BELGIUM
item SIMUNEK, JIRKA - UC RIVERSIDE, CA
item MALLANTS, D - BNRC, MOL, BELGIUM
item Van Genuchten, Martinus

Submitted to: Environmental Remediation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/2/2005
Publication Date: 9/1/2005
Citation: Jacques, D., Simunek, J., Mallants, D., Van Genuchten, M.T. 2005. Long-term uranium migration in agricultural field soils following mineral P fertilization. Environmental Remediation Conference Proceedings. In: 10th International Conference on Environmental Remediation and Radioactive Waste Management. Sept. 4-8, 2005. American Society of Mechanical Engineers. 8 p.

Interpretive Summary: To preserve soil fertility, organic and mineral fertilizers are often applied to agricultural fields. Unfortunately, most mineral fertilizers, such as phosphates and super phosphates, contain a certain amount of naturally occurring radioactive material such as Uranium (U) and Thorium (Th), among others. The fate and transport of U in soil systems is quite complex since they are subject to many complicated geochemical reactions such as those with soil organic matter, the solid mineral phase, nitrate, phosphate, and carbonate. These geochemical processes are additionally coupled with water content and water flux variations in space and time caused by time-variable precipitation and evapotranspiration rates at the soil surface. This study addresses the long term leaching of U (as naturally present in mineral phosphate (P) fertilizers) through soil to the ground water table under natural boundary conditions. A 30-year long time series of climatological data from Northern Belgium was used to define precipitation and potential evaporation rates. The paper discusses the calculated U geochemistry in the soil profile, together with calculated U fluxes to the groundwater table. 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 radionuclide (long-lived actinides) 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. Additional simulations are planned to study the effects of steady-state versus transient (time-dependent weather) conditions on leaching.

Technical Abstract: To preserve soil fertility, organic and mineral fertilizers are often applied to agricultural fields. Mineral fertilizers such as phosphates and super phosphates contain a certain amount of long-lived alpha activity due to 238-U, 230-Th, 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 water content and water flux variations in space and time due to time-variable precipitation and evapotranspiration rates at the soil surface. The simulation of these interacting processes requires an advanced reactive transport model incorporating modules to simulate (1) transient water flow in variably-saturated media, (2) transport of multiple components, and (3) mixed equilibrium / kinetic biogeochemical reactions. A recently developed code, HP1, couples HYDRUS-1D for water flow and solute transport with the geochemical code PHREEQC-2. HP1 can handle most of the coupled processes needed to describe the transport of U in the soil profile. 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.