A comparison of groundwater fluxes computed with MODFLOW and a mixing model using deuterium: Application to the eastern Nevada Test Site and vicinity

SummaryThe primary objective of this study was to verify groundwater flows in the vicinity of the eastern Nevada Test Site (NTS) computed with a hydraulically defined flow model against groundwater δD values computed by a steady-state mixing model. The United States Geological Survey’s Death Valley regional flow model (DVRFM) is a transient, three-dimensional, groundwater model that uses the public domain, finite-difference code MODFLOW. The mixing model (Discrete-State Compartment Model-Shuffled Complex Evolution, or DSCM-SCE) is a recently developed code that is able to autocalibrate groundwater fluxes (both magnitude and direction) to best match observed tracer concentrations in groundwater. To compare modeling approaches, DVRFM boundary conditions and cell-to-cell interactions were implemented into a previously developed 15-cell DSCM-SCE δD model of the eastern NTS. Analysis of δD and δ18Ο data conducted throughout the model domain suggests recharge and mixing are the dominant mechanisms for groundwater isotopic enrichment in the downgradient direction. Therefore, evaporation, at least at the regional scale, was ignored. Model results showed that DVRFM boundary fluxes and cell outflow volumes reproduced observed groundwater δD values in nine of the 11 hydrographic basins that contained deuterium data. The remaining four modeled basins did not have any deuterium data, however, modeling results closely matched estimated δD for two of these basins. The isotope mixing model independently verified that most of the hydraulically defined groundwater flows simulated by MODFLOW are reasonable. Optimization of the DSCM-SCE was then done by adjusting groundwater fluxes between cells to improve predicted groundwater δD values. This significantly lowered the weighted root-mean-squared error and helped to identify several basins where DVRFM–simulated boundary conditions and groundwater fluxes should be reexamined. Overall, the two modeling approaches complemented each other, such that the isotope mixing model verified the hydraulically based groundwater flow model and refined groundwater flux estimates to better understand the physical system.