Reactive transport model for the ambient unsaturated hydrogeochemical system at Yucca Mountain, Nevada

Abstract

To assist a technical review of a potential application for a geologic repository, a reactive transport model is presented for the ambient hydrogeochemical system at Yucca Mountain (YM). The model simulates two-phase, non-isothermal, advective and diffusive flow and transport through one-dimensional matrix and fracture continua (dual permeability) containing ten kinetically reactive hydrostratigraphic layers. We developed a thermodynamic interpretation of ambient groundwater chemistry at YM from limited analytical data and then combined it with insights from experimental and site-specific data to craft a conceptual model describing thermodynamic and kinetic relationships in the mineral/glass–water–gas system. The model was calibrated by adjusting uncertain thermodynamic and kinetic parameters to reflect observed trends at YM. The following calibration criteria were used to ensure that model predictions are consistent with hydrochemical and petrologic data from YM: (1) calculated multicomponent matrix pore water compositions do not vary significantly from reinterpreted analytical data; (2) simulated variations in matrix silica concentrations with depth are bounded by the observed analytical range; and (3) feldspars and glass dissolve while clays and calcite precipitate. The largest changes in calculated species concentrations occur in response to glass dissolution. Simulated matrix groundwater compositions at the depth of the potential repository are largely inherited from percolating waters rather than controlled by in situ chemical reactions. Predictions about groundwater flow pathways in the ambient YM system are sensitive to assumptions regarding percolation flux and fracture–matrix interactions. Confidence in complex reactive transport models of the thermally perturbed YM repository system depends on successful representation of ambient system conditions.