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Colorado State University (2018)

Simulating the fate and transport of salinity species in a semi-arid agricultural groundwater system : model development and application

Tavakoli Kivi, Saman

Titre : Simulating the fate and transport of salinity species in a semi-arid agricultural groundwater system : model development and application

Auteur : Tavakoli Kivi, Saman ;

Université de soutenance : Colorado State University

Grade : Doctor of Philosophy (Ph.D.) 2018

Résumé
Many irrigated agricultural areas worldwide suffer from salinization of soil, groundwater, and nearby river systems. Increased salinity concentrations, which can lead to decreased crop yield, are due principally to the presence of salt minerals and high rates of evapotranspiration. High groundwater salt loading to nearby river systems also affects downstream areas when saline river water is diverted for additional uses. Irrigation-induced salinity is the principal water quality problem in the semi-arid region of the western United States due to the extensive background quantities of salt in rocks and soils. Due to the importance of the problem and the complex hydro-chemical processes involved in salinity fate and transport, a physically-based spatially-distributed numerical model is needed to assess soil and groundwater salinity at the regional scale. Although several salinity transport models have been developed in recent decades, these models focus on salt species at the small scale (i.e. soil profile or field), and no attempts thus far have been made at simulating the fate, storage, and transport of individual interacting salt ions at the regional scale within a river basin. The required model must be able to handle variably-saturated groundwater systems ; sources and sinks of groundwater within an agricultural system such as canal seepage, infiltrated water from flood and sprinkler irrigation, groundwater pumping, and evapotranspiration from both the unsaturated and shallow saturated zones ; root zone processes such as salt ions cycling, crop uptake, and leaching to the water table ; addition of salt mass via fertilizer and irrigation water ; chemical kinetics affecting salt ions such as the influence of dissolved oxygen and nitrate on the chemical processes of anions such as sulfate (SO4) ; and equilibrium chemistry processes such as precipitation-dissolution, complexation, and cation exchange. This dissertation develops a physically-based, spatially-distributed groundwater reactive transport model that simulates the fate and transport of major salt ions in an agricultural groundwater system and can be applied to regional scale areas to address salinity problems. The model is developed by 1) constructing an equilibrium chemistry model that includes all the fate and transport processes that affect salt ions in an agricultural soil-groundwater system, including precipitation-dissolution of salt minerals, ions complexation, and cation exchange ; and 2) coupling the module with UZF-RT3D (Bailey et al., 2013) a MODFLOW-based numerical modeling code that simulates the transport of multiple interacting reactive solutes in a variably-saturated soil-groundwater system.

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