No abstract

Because the continuum approach is relatively simple and straightforward to implement, it has been commonly used in modeling flow and transport in unsaturated fractured rock. However, the usefulness of this approach can be questioned in terms of its adequacy for representing fingering flow and transport in unsaturated fractured rock. The continuum approach thus needs to be evaluated carefully by comparing simulation results with field observations directly related to unsaturated flow and transport processes. This paper reports on such an evaluation, based on a combination of model calibration and prediction, using data from an infiltration test carried out in a densely fractured rock within the unsaturated zone of Yucca Mountain, Nevada. Comparisons between experimental and modeling results show that the continuum approach may be able to capture important features of flow and transport processes observed from the test. The modeling results also show that matrix diffusion may have a significant effect on the overall transport behavior in unsaturated fractured rocks, which can be used to estimate effective fracture-matrix interface areas based on tracer transport data. While more theoretical, numerical, and experimental studies are needed to provide a conclusive evaluation, this study suggests that the continuum approach is useful for modeling flow and transport in unsaturated, densely fractured rock.

No abstract

No abstract

This paper presents a numerical study on the response of the unsaturated zone (UZ) system of Yucca Mountain to heat generated from decaying radioactive wastes emplaced at the proposed repository. The modeling study is based on the current thermal-hydrological (TH) mountain-scale model, which uses a locally refined 2D north-south cross section and dual-permeability numerical approach. The model provides a prediction of the mountain-scale TH response under the thermal-load scenario of 1.45 kW/m, while accounting for future climatic changes and the effects of drift ventilation. The TH simulation results show that ventilation of the repository drifts has a large impact on thermal-hydrologic regimes and moisture-flow conditions at the repository. In both cases, with and without ventilation, the TH model predicts dry or reduced liquid saturation near the drifts for over 1,000 years, during which liquid flux through the drifts is reduced to either zero or less than the ambient flux. Without ventilation, the model predicts higher temperatures at the repository, but no major moisture redistribution in the UZ except in the areas very near the heated drifts.

No abstract

No abstract

Contact : Eric S onnenthal, (5 10)48 6-5 8 66, elsonnen thal @ lbl. gov Research ObjectivesThe objectives of this study were to evaluate the thermal-hydrological-chemical (THC) effects on flow and geochemistry in the unsaturated zone (UZ) at Yucca Mountain at a mountain scale. The major THC processes important in the UZ are (1) mineral precipitatiorddissolution affecting flow and transport to and from the potential repository, and ( 2 ) changes in the compositions of gas and liquid that may seep into drifts. ApproachThe conceptual model developed for THC processes provides the basis for modeling mineral-water-gas reactions, as they influence the chemistry of water and gas and associated changes in mineralogy and hydrologic properties. Data incorporated in the model include hydrologic and thermal properties, geologic layering from the UZ 3-D flow and transport model, geochemical data (fracture and matrix mineralogy, water and gas geochemistry), and thermodynamic and kinetic data. Simulations included coupling among heat, water, and vapor flow; aqueous and gaseous species transport; kinetic and equilibrium mineral-water reactions; and feedback of mineral precipitatiorddissolution on porosity, permeability, and capillary pressure for a dual permeability (fracture-matrix) system. Simulations were performed using TOUGHREACT V2.3.The effects of coupled THC processes on the evolution of flow fields and water and gas chemistry in the UZ were evaluated for an above-boiling thermal operating mode. A cross section was chosen from the UZ 3-D flow and transport model that follows a north-south trend through the potential repository. Minerals include calcite, silica phases, feldspars, zeolites, clays, gypsum, fluorite, and iron oxides with the relevant aqueous species. The gas phase consists of air, water vapor, and CO2. The composition of the initial and infiltrating water was derived from the matrix pore water extracted from the Topopah Spring welded tuff near the ongoing Drift Scale Test. AccomplishmentsThe simulations revealed that by 5,00Oyears, most of the region above and directly below the potential repository undergoes a small reduction in fracture porosity of less than 1 percent (Figure 1). Above the northern edge, a region of slight porosity increase persists, owing to enhanced vapor convection and condensation. The areas that show the most effect as a result of heating are in the zeolitic units below the potential repository, where calcic zeolites dissolve to form alkali feldspars, leading to increased aqueous calcium concentrations and increased porosity. The predicted range in pH from 7 to 9 is linked to changes in gas-phase CO2 concentrations induced by heating of pore waters.