Modeling multiphase non-isothermal fluid flow and reactive geochemical transport in variably saturated fractured rocks: 2. Applications to supergene copper enrichment and hydrothermal flows

Abstract

Reactive fluid flow and geochemical transport in unsaturated fractured rocks has been of increasing interest to investigators in the areas of geo-and environmental-sciences. To test geochemical hypotheses based on petrologic observation and to predict geochemical reactions that occur through a complex dynamic interplay of physical and chemical processes, we use the methods presented in a companion paper (part 1, this issue p. 16 -33) to investigate two problems: (1) supergene copper enrichment in unsaturated-saturated media and (2) predicted effects of thermohydrology on geochemistry during the Drift Scale Heater Test at the Yucca Mountain potential nuclear waste repository, Nevada. Through these two examples we address the importance of the following issues on geochemical processes: (1) participation of gas phase in transport and reaction, (2) interactions between fractures and rock matrix for water and chemical constituents, (3) heat effects on fluid flow and reaction properties and processes. In the supergene enrichment system, oxygen gas diffusion from the land surface through fractured rock promotes the alteration of the primary sulfide minerals and the subsequent deposition of secondary minerals. Modeling of the large-scale heater test shows effects of fracture-matrix interaction, heat-driven vaporizing fluid flow, and CO 2 degassing on mineral alteration patterns. The two examples also serve as a demonstration of our methods for reactive transport in variably saturated fractured rocks. introduction Reactive fluid flow and geochemical transport in unsaturated fractured rocks have been of increasing interested to investigators in the areas of geo-and environmentalsciences such as mineral deposits, contaminant transport, groundwater quality, waste disposal, acid mine drainage remediation, sedimentary diagenesis, and fluid-rock interactions in hydrothermal systems. This is a challenging issue because of the complexity of multiphase fluid flow, water-gas-rock and fracture-matrix interaction mechanisms, and difficulties dealing with physical and chemical heterogeneities and the strong non-linearities in the governing equations. For oxidized sulfide ore deposits, microscopic and petrologic studies based on 3-dimensional field mapping offer unique evidence as to the present state of surficial geochemical processes, mineral distribution, and controlling geological, hydrological, tectonic, and climatic factors (Brimhall, Alpers, and Cunningham, 1985;Ague and Brimhall, 1989). In fact, for almost a century supergene leaching and enrichment have been at the core of understanding redox processes at the paleo-ground water table. Geochronological methods cast such hydro-chemical systems in a temporal frame of reference so that an assortment of known deposits may be related in both a relative and absolute sequence of progressive geochemical states.The scientific relevance of numerical modeling of irreversible natural phenomena depends upon the character of the systems being modeled: natural or anthropogenic. In natural ...

TOUGHREACT User's Guide: A Simulation Program for Non-isothermal Multiphase Reactive Geochemical Transport in Variably Saturated Geologic Media, V1.2.1

“…Mineral surface areas in the rock matrix were calculated using the geometric area of a cubic array of truncated spheres that make up the framework of the rock and reductions to those areas owing to the presence of alteration phases such as clays and zeolites. Full details are given in Sonnenthal and Spycher (2001).…”

“…Mountain (Sonnenthal and Spycher, 2001;Spycher et al, 2003a). Minerals considered in the simulations are calcite, gypsum, goethite, tridymite, cristobalite-α, quartz, amorphous silica, hematite, fluorite, albite, K-feldspar, anorthite, Ca-smectite, Mg-smectite, Na-smectite, illite, kaolinite, opal-CT, stellerite, heulandite, mordenite, clinoptilolite, and volcanic glass.…”

“…Initial pore water chemical concentrations were based on analyses of ultracentrifuged water (L. DeLoach, unpublished data) and chemical speciation calculations (Sonnenthal and Spycher, 2001;Spycher et al, 2003a). Except for perched water that lies well below the potential repository horizon, water has not been observed in fractures in the UZ.…”

“…TOUGHREACT was further enhanced with the addition of (1) treatment of mineral-water-gas reactive-transport under boiling conditions, (2) an improved HKF activity model for aqueous species, (3) gas species diffusion coefficients calculated as a function of pressure, temperature, and molecular properties, (4) mineral reactive surface area formulations for fractured and porous media, and (5) porosity, permeability, and capillary pressure changes owing to mineral precipitation/dissolution (Sonnenthal et al, , 2000(Sonnenthal et al, , 2001Spycher et al, 2003a).…”

“…(4) Coupled thermal, hydrological, and chemical processes in boiling unsaturated tuff for the proposed nuclear waste emplacement site at Yucca Mountain, Nevada (Sonnenthal et al, , 2001Sonnenthal and Spycher, 2000;Spycher et al, 2003a, b;Xu et al, 2001). …”