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Title: Simulating variably-saturated reactive transport of selenium and nitrogen in agricultural groundwater systems

Author
item BAILEY, RYAN - Colorado State University
item GATES, TIMOTHY - Colorado State University
item Halvorson, Ardell

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2013
Publication Date: 3/6/2013
Publication URL: http://dx.doi.org/10.1016/j.jconhyd.2013.03.001
Citation: Bailey, R.T., Gates, T.K., Halvorson, A.D. 2013. Simulating variably-saturated reactive transport of selenium and nitrogen in agricultural groundwater systems. Journal of Contaminant Hydrology. http://dx.doi.org/10.1016/j.jconhyd.2013.03.001.

Interpretive Summary: Selenium is a contaminant that often is present in groundwater and surface water in semi-arid watersheds in the western United States. Elevated concentrations can harm waterfowl, fish populations, and human populations. In order to provide a tool that can assess remediation strategies for selenium pollution, this study develops a computational model that accounts for the movement of Selenium in irrigated agricultural soil and groundwater systems. The model is tested against field-observed soil water concentrations at a test site in Rocky Ford, CO. Results indicate that the model can be a useful tool in assessing Selenium movement in soil and groundwater systems.

Technical Abstract: Selenium (Se) contamination in environmental systems has become a major issue in many regions world-wide during the previous decades, with both elevated and deficient Se concentrations in groundwater, surface water, soils and associated cultivated crops reported. To provide a tool that can assess baseline conditions and explore remediation strategies, this paper presents a numerical model capable of simulating the reactive transport of Se species in large-scale variably-saturated groundwater systems influenced by agricultural practices. Developed by incorporating a Se reaction module into the multi-species, variably-saturated reactive transport model UZF-RT3D, model features include near-surface Se cycling due to agricultural practices, oxidation–reduction reactions, and the inclusion of a nitrogen (N) cycle and reaction module due to the dependence of Se transformation and speciation on the presence of nitrate (NO3). Although the primary motivation is applying the model to large-scale systems, this paper presents applications to agricultural soil profile systems to corroborate the near-surface module processes that are vital in estimating mass loadings to the saturated zone in large-scale fate and transport studies. The first application jointly tests the Se and N modules for corn test plots receiving varying loadings of fertilizer, whereas the second application tests the N module for fertilized and unfertilized test plots. Results indicate that the model is successful in reproducing observed measurements of Se and NO3 concentrations, particularly in lower soil layers and hence in regards to leaching. For the first application, the Ensemble Kalman Filter (EnKF) is used to condition model parameters, demonstrating the usefulness of the EnKF in real-world reactive transport systems.