Hydrus-1D as a tool to understand contamination of an aquifer: A case study in the Central Valley of Cochabamba

Abstract: In water resource management, it is of importance to be able to predict how different activities might contaminate aquifers. For this kind of application, numerical modelling of water and solute transport can be a useful tool. The purpose of this study was to investigate if one-dimensional numerical modelling (Hydrus-1D) of water in the unsaturated zone of the subsoil is useful to assess the transport of contaminants from the soil surface to aquifer. More specifically, the goal set up was to develop a conceptual model of the hydrogeological and hydrogeochemical situation at the site of a deep well (Tiquipaya) located in the Central Valley of Cochabamba, Bolivia, and to adjust this to a Hydrus-1D model. In the study, the specific case that was considered was the subsoil of an agricultural field where onion is cultivated. The contaminant of interest was nitrate, which entered the subsoil through the application of urea-fertilizers dissolved in water from flood irrigation. The model attempted to capture the transport of water and solute through the unsaturated zone down to the groundwater level. The goal of the study was reached in the sense that a conceptual model was develop and adjusted to Hydrus-1D. However, it required several major assumptions and simplifications regarding the site-specific conditions and the input parameters associated with them. In particular, parameters related to soil characteristics were derived from a simple description, despite being considered as important for the simulation results. The limitations associated with the simplifications and assumptions made while setting up the model meant that the specific results of the simulations are of limited use. However, in line with the purpose of the study, they were considered plausible enough for deeming the model to have a potential for being used in water resource management. Thus, the main conclusion of the study was that numerical modelling (Hydrus-1D) is suitable for assessing groundwater contamination, but that further studies must be performed to improve the accuracy of the model for it to be able to provide useful results.