Flow and transport in fractured tuff at Yucca Mountain: numerical experiments on fast preferential flow mechanisms

• Published in: Journal of contaminant hydrology Vol. 43; no. 3; pp. 211 - 238
• Main Authors: McLaren, Robert G., Forsyth, Peter A., Sudicky, Edward A., VanderKwaak, Joel E., Schwartz, Franklin W., Kessler, John H.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.05.2000; Elsevier Science
• Subjects: Dual permeability; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Fast preferential flow; Fracture–matrix interaction; Hydrogeology; Hydrology. Hydrogeology; Numerical modeling; Pollution, environment geology; radioactive wastes; simulation models; transport processes; unsaturated flow; Unsaturated zone; USA, Nevada, Yucca Mt; waste disposal sites; Yucca Mountain; Nevada; Nye County Nevada; Yucca Mountain; United States; USA, Nevada, Yucca Mt; Numerical modeling; Dual permeability; Unsaturated zone; Fracture–matrix interaction; Yucca Mountain; Fast preferential flow; possibilities; sensitivity analysis; Cl-36; simulation; permeability; ground water; flow mechanism; North America; radioactive waste; underground storage; fractures; advection; transient flow; volcanic rocks; igneous rocks; transport; finite element analysis; infiltration; unsaturated zone; fractured environment; pyroclastics; chloride ion; numerical models; tuff; solution transport; dispersion
• ISSN: 0169-7722; 1873-6009
• DOI: 10.1016/S0169-7722(00)00085-1

Recent discovery of bomb-related 36Cl at depth in fractured tuff in the unsaturated zone at the Yucca Mountain candidate high-level waste (HLW) repository site has called into question the usual modeling assumptions based on the equivalent continuum model (ECM). A dual continuum model (DCM) for simulating transient flow and transport at Yucca Mountain is developed. In order to ensure properly converged flow solutions, which are used in the transport simulation, a new flow solution convergence criteria is derived. An extensive series of simulation studies is presented which indicates that rapid movement of solute through the fractures will not occur unless there are intense episodic infiltration events. Movement of solute in the environs of the repository is enhanced if the properties of the tuff layer at the repository horizon are modified from current best-estimate values. Due to a large advective–dispersive coupling between the matrix and fractures, the matrix acts as a major buffer which inhibits rapid transport along the fractures. Consequently, fast movement of solutes through the fractures to the repository depth can only be explained if the matrix–fracture coupling term is significantly reduced from a value that would be calculated on the basis of data currently available.