|Van Genuchten, Martinus|
Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2007
Publication Date: 5/1/2008
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2237.pdf
Citation: Jacques, D., Simunek, J., Mallants, D., Van Genuchten, M.T. 2008. Modeling coupled hydrological and chemical processes: long-term uranium transport following phosphorous-fertilization. Vadose Zone Journal. Vol 7:698-711 Interpretive Summary: Accurate investigations of the mobility of contaminants in the vadose zone require consideration of all pertinent physical and biogeochemical processes. Coupling of fluid flow, solute diffusion, transport with flowing water, and multi-component geochemistry, into one integrated simulator provides a much-needed tool for studying the nonlinear feedbacks that occur between the different processes involved. In this paper we illustrated these feedbacks by using the HP1 reactive transport simulator to predict the migration of calcium (Ca), phosphorus (P), and uranium (U) in a multilayered soil profile following long-term inorganic phosphate fertilization. P fertilizers are often applied annually to agricultural fields to preserve soil fertility. Unfortunately, many mineral fertilizers, such as phosphates and superphosphates, contain small amounts of naturally occurring radioactive materials such as Uranium (U) and Thorium (Th). The fate and transport of U and Th in soils is subject to many complicated geochemical reactions, including with soil organic matter, the soil mineral phase, phosphate, and other constituents. The geochemical processes are additionally influenced by varying soil moisture conditions caused by time-variable weather conditions. Uranium fluxes to groundwater were calculated with HP1 over a 200-year period using climatological data from Northern Belgium to define transient precipitation and evaporation rates. Leaching of U from the soil profile was found to occur much faster in transient flow simulations (i.e., subject to time-dependent weather conditions) than in steady-state simulations (using long-term average weather conditions) as a result of the interplay between changing hydrological and geochemical soil conditions. Results demonstrates how transient water contents and fluxes affect soil pH, and hence the retention, bioavailability, and fluxes of various solute components. We note that HP1 is a public domain software package that can be downloaded freely from www.sckcen.be\hp1. The model provides theoretical and applied scientists and engineers with an important tool for predicting multi-component reactive contaminant transport in soils and groundwater.
Technical Abstract: Contaminants in the vadose zone are affected by the physical processes of water flow, heat movement and multicomponent transport, as well as generally by a range of interacting biogeochemical processes. Coupling these various processes within one integrated numerical simulator provides a process-based tool for investigating the mobility of inorganic and organic contaminants as affected by changing hydrologic regimes and geochemical conditions. Such a simulator allows detailed studies of various feedbacks between the different processes involved. In this paper we first review interactions between physical and biogeochemical processes in the vadose zone and their mutual feedbacks, and then present a case study that demonstrates these complex interactions. A hypothetical application is presented of the HP1 multicomponent reactive transport simulator to predict the subsurface transport of two major elements (Ca and P) and one trace element (U) applied annually for 200 years to a field soil in the form of an inorganic P-fertilizer. Interactions of Ca, P, and U with the solid phase are described using multi-site cation exchange and surface complexation reactions. Simulations carried out assuming both steady-state and transient flow conditions were analyzed in terms of temporal variations of the linear distribution coefficient, Kd, which depends strongly on pH and the composition of the aqueous phase. If the composition of the aqueous phase is constant, adsorption of Ca and U increases with increasing pH. Due to the annual addition of Ca, P, and U, and competition between P and U for sorption sites, the Kd of these elements decreased over time in the upper soil horizons despite an increase in pH due to annual Ca and P additions. Deeper in the soil profile, the Kd of U followed the pH increase because of a lack of competition from P. Because of the combined effects of changing hydrological and geochemical conditions, the Ca and U distribution coefficients and solute fluxes during the transient simulation exhibited large short-time variations of up to three orders of magnitude.