Modelling uranium leaching from agricultural soils to groundwater as a criterion for comparison with complementary safet
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Modelling uranium leaching from agricultural soils to groundwater as a criterion for comparison with complementary safety indicators Jacques D.(1), J. Šimůnek(2), D. Mallants(1), and M.Th. van Genuchten(3) (1) Waste and Disposal Department, SCK•CEN, Boeretang 200, B-2400 Mol, Belgium (2) University of California Riverside, Riverside, CA 92521, USA (3) George E. Brown, Jr. Salinity Laboratory, 450 W Big Springs RD, Riverside, CA 92507, USA ABSTRACT 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 which eventually may be leached from agricultural soils to underlying water resources. Field soils that receive Pfertilizers 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, 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 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 lowlevel nuclear waste repositories.
INTRODUCTION Phosphate (P) fertilizers are typically applied annually to agricultural fields, partly in inorganic form. In Flanders, Belgium, the amount of applied P that originates from mineral fertilizers has decreased over the years, from 13.9x106 kg P in 1990 to 2.38x106 kg P in 2003 [1]. Mineral P-fertilizers contain some alpha-activity due to the presence of 238U (among other alpha emitters). Uranium concentrations in P-bearing fertilizers have been reported to be in the range of 300 to 3000 Bq kg-1 of fertilizer [2] or 1700 to 9200 Bq kg-1 of fertilizer (for both 238U and 234 U) [3]. By comparison, the average concentration of U in West European soils is 40 Bq kg-1 of soil [4]. Experimental studies recently evaluated the contribution of U from fertilizers to leached U in a small drainage basin [5], and on the seasonal behavior of U in a fertilized corn field [6]. The migration of U and other elements in soils depends on a large number of processes, including their interactions with other aqueous components and the solid phase (cation exchange, surface complexation) as well as time-va
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