Multi-variable mixing cell model as a calibration and validation tool for hydrogeologic groundwater modeling

Agricultural irrigation can make substantial changes to groundwater systems. This study focuses on an arid area in northern Nevada, where 100 yr of extensive irrigation has occurred. In particular, the impacts of long-term agricultural activities on the groundwater fluid balance, geochemistry and potential changes to the water quality of a nearby river were investigated. The study, conducted in the Fernley Basin in western Nevada, utilized two different modeling approaches to construct a conceptual model that honors both the chemical and physical aspects of the basin groundwater flow regime. A multi-variable mixing cell model was constructed to represent the hydrochemical approach, while a MODFLOW model was constructed to represent the hydrogeological model. A geochemical and isotopic analysis of the basin and its margin groundwater was conducted to identify all of the water sources that potentially flow through the basin and all water bodies within the basin. Those water bodies and sources were incorporated into a mixing cell model that allowed delineation of the general flow lines connecting the different water bodies within the basin with their sources. The mixing cell model results were used to calibrate the hydrogeological model until a general agreement between the flow lines of both models was achieved. The calibration of the hydrogeologic model included adjustments of the hydraulic conductivities and the use of various interpolation methods. The study showed that parallel application of two different modeling approaches, the mixing cell model along with a hydrogeological model, provides an excellent tool for the validation of the conceptual model. Moreover, it allowed calibration of the hydrogeologic model not only to the water heads, as usually done in hydrogeological models, but also to the general flow lines that honors the basin geochemistry.