WATER-TABLE FLUCTUATIONS BENEATH YUCCA MOUNTAIN, NEVADA, USA — HOW HIGH HAS IT RISEN?

A semi-arid climate and thick (400–700 m) unsaturated zone (UZ) are positive attributes of Yucca Mountain (YM) for long-term waste storage placed 200–300 m both below the surface and above the modern water table. Groundwater inundation of YM would have dire consequences for waste containment, but, has this phenomenon ever occurred? Hydrologic responses to cooler, wetter climate during Pleistocene glacial periods include increased recharge, greater groundwater fluxes, and higher water tables compared to hotter, drier interglacial periods. Evidence from beneath YM and adjacent areas indicate that water-table rise in response to climate has been <50 m.

The thick UZ also hosts secondary calcite and opal deposits that record evidence of climate change over the last 300 ka. Mineral coatings grew into air-filled voids at extremely slow, uniform rates (typically <1 µm/ka based on ²³⁰Th/U dating), and preserve climate-driven compositional fluctuations of meteoric water sources. ²⁰⁷Pb/²³⁵U dating of older U-rich silica layers in the same coatings show similarly slow growth that started once the 12.8-Ma tuffs cooled to <~100°C. Fluid inclusion and δ¹⁸O data indicate temperatures varied from ~80°C in the oldest layers to ~25°C in the youngest layers. Combined age-temperature data indicate a steady, but protracted, cooling history, with near-modern temperatures present since ~4 Ma.

However, temperature data from UZ calcite continue to be used as evidence that upwelling hydrothermal water inundated the UZ in the past (Dublyansky and Spötl, 2010, EPSL 289). This hypothesis ignores the dating framework and erroneously posits that the warmest temperatures apply to the entire depositional history. Published 2‑D paleogeothermal gradients do not consider age and stratigraphic data, and thus have no meaning. The inundation model relies on a seismic pumping mechanism; however, seismic stresses are incapable of lifting water 100s of m into the UZ using reasonable rheologic properties. Water-table responses to mega-earthquakes observed in the last 100 years have been measured in cm to a few 10s of m rather than 100s of m. The inundation model is not valid because it ignores critical geochronological data and lacks a defensible hydrological mechanism. Therefore, groundwater saturation has never been >50 m above modern levels.