Questioning the conceptual model
Every geoscientist should be aware of the perils of assuming an incorrect conceptual model, urges Tim Harper
In their summer 2022 Viewpoint column, Max Dobson and Dennis O’Leary emphasised that progress on the potential high-level waste (HLW) repository at Yucca Mountain, USA, was largely halted by political and social objections. Yet Yucca Mountain should be remembered as one of the most contentious and remarkable controversies in the history of geoscience.
Transport of radionuclides to the biosphere by groundwater has long been a primary concern in HLW disposal. Yucca Mountain was identified as a candidate host based on the general assumption of low rates of groundwater transport essentially downwards through an unsaturated zone of rock with sufficient thickness to host a repository (Flint et al, 2001). In this classical hydrogeologic model, flow is driven by gravity and represented in a ‘box’ with no transfer of energy through the (impermeable) base. Rock permeabilities in the unsaturated zone are assumed invariant.
Jerry Szymanski, a geologist recruited as the site licensing officer by the US Department of Energy (DOE) in the early 1980s, became concerned with the gravity-driven conceptual model – it allowed no role for tectonic deformation and volcanism that could potentially drive fluid flow toward the surface.
At Yucca Mountain, undisputed active faulting occurs on the sides of the mountain. There are numerous cinder cones with a prominent one, Lathrop Wells, dated at 30,000–130,000 years BP (Taylor & Huckins, 1995). Personally, I was convinced of fluid upwelling partly by surface outcrops, the evidence for which includes mineral concentrations at the outcrops of, and accumulated adjacent to, active faults on the sloping sides of Yucca Mountain, as well as subvertical sheets of calcite in cohesionless sand covering an active fault at Busted Butte.
In 1989, Szymanski proposed an alternative, thermodynamic conceptual model (later published in Szymanski, 2018) that incorporated (tectonic) work done on the ‘box’ and heat transfer through the base of the ‘box’ He proposed that the rocks are deforming, experiencing an evolving permeability structure as fractures cyclically dilate and close. The depth of the water table varies as the permeability and storage distribution change. Szymanski’s model incorporates intermittent Raleigh-Bénard convection when the active faults are dilated to bring hot (gas-charged) brines to the surface.
Thus began an extraordinary geoscientific controversy. The extensive and voluminous deposits of minerals at the ground surface and throughout the subsurface became a focus of investigations at Yucca Mountain. Did these minerals, which include co-genetic calcite, chalcedony, quartz, fluorite, zeolite, barite and strontianite, derive from infiltrating meteoric waters containing wind-blown calcium carbonate dust (Taylor & Huckins, 1995) or from upwelling hydrothermal brines? Were they pedogenic (Taylor & Huckins, 1995) or magmagenic? Did the water flow down or up?
Subsequent fluid inclusion studies found mineral deposition temperatures up to 70–90°C (at 30–50 m from the surface), decreasing with distance from one of the horst-bounding faults (Dublyansky & Smirnoff, 2003). This evidence seemed to settle the matter until the DOE then proposed that conductive heating of infiltrating meteoric water by a magma body (7 km north, cooling for 5–8 Myr) could explain the elevated temperatures of the unsaturated zone, without the need to invoke more recent hydrothermal activity (Dublyansky & Polyansky, 2007). This explanation of long-term heat transfer from the intrusion was shown to be invalid when tested numerically (Dublyansky & Polyansky, 2007), yet its continued acceptance allowed the DOE to exclude the possibility of hydrothermal activity in the performance assessment (Dublyansky, 2014).
After an expenditure in the region of $15 billion, in 2009 a new Secretary of Energy announced plans to terminate Yucca Mountain for policy reasons, not technical or safety reasons (GAO, 2011). The relatively simple unsaturated zone model of flow driven by gravity, essentially an equilibrium model, had been espoused by the DOE as the basis for safe storage at Yucca Mountain from the start (Roseboom, 1983). The complexity of the non-equilibrium, innovative Szymanski model involved a wide span of technical disciplines, marrying thermodynamics, geochemistry, rock mechanics, hydrology and structural geology, making it inherently more challenging to describe, evaluate and hence defend. It is greatly concerning that such contrasting models could remain so disputed at a site of such potential significance and vast financial investment.
Tim Harper is Director and Principal Engineer with Geosphere Limited, providing research and consultancy services. His 50 years’ experience include risk assessments for nuclear sites, for facilities in active faulting environments and of seismicity induced by hydraulic fracturing.
- Flint, A.L. J.F. (2001) Development of the Conceptual Model of Unsaturated Zone Hydrology at Yucca Mountain, Nevada. In: National Research Council. 2001. Conceptual Models of Flow and Transport in the Fractured Vadose Zone. Washington, DC: The National Academies Press; doi: 10.17226/10102.
- Taylor, E.M. & Huckins, H.E. (1995) Lithology, Fault Displacement, and origin of secondary calcium carbonate and Opaline Silica at Trenches 14 and 14D on the Bow Ridge Fault at Exile Hill, Nye County, Nevada. U.S. Geological Survey, Open-File Report 93-477.
- Szymanski, J.S. (2018) Understanding the Containment and Isolation Potential of Yucca Mountain; the Coupled Non-Equilibrium Dynamical Systems Perspective, Page Publishing.
- Dublyansky, Y., & Smirnoff, S.J. (2003) Review of the report “ Thermochemical evolution of calcite formation at the potential Yucca Mountain Repository Site, Nevada”, Siberian branch of Russaian Academy of Sciences, Novosibirsk.
- Dublyansky, Y. & Polyansky, O. (2007) Search for the cause-effect between Miocene silicic volcanism and hydrothermal activity in the unsaturated zone of Yucca Mountain, Nevada, Numerical modelling approach. J. Geophys. Res. 112, B09201; doi:10.1029/2006JB004597.
- Dublyansky, Y.V. (2014) Evaluation of the US DOE’s conceptual model of hydrothermal activity at Yucca Mountain, Nevada. Geosci. Model Dev. 7, 1583–1607.
- GAO (2011) Commercial Nuclear Waste, Effects of a Termination of the Yucca Mountain Repository program and Lessons Learned. US Government Accountability Office, Report GAO-11-229.
- Roseboom, E. H., Jr. (1983) Disposal of high-level nuclear waste above the water table in arid regions. Alexandria, Va.: Geological Survey Circular 903, 21 p.