Search for the cause-effect relationship between Miocene silicic volcanism and hydrothermal activity in the unsaturated zone of Yucca Mountain, Nevada: Numerical modeling approach

摘要

Elevated temperatures of waters that circulated through the thick unsaturated zone of Yucca Mountain and deposited secondary minerals have been explained by conductive heating of the rock mass by a magma body emplaced in association with the activity of the Timber Mountain silicic volcanic center (the “conductive heating” conceptual model). The model was evaluated by means of numerical modeling. Two- and three-dimensional simulations of the conductive heat transfer, as well as three-dimensional simulations considering convective flow of water, did not match the target temperature-time “trajectory” established from fluid inclusion studies and radiometric dating of secondary minerals at Yucca Mountain. This result was obtained despite the fact that the modeling was nonconservative in that the initial conditions, boundary conditions, and parameters of the models were selected so that they would return higher temperatures and longer cooling times. We conclude that the “conductive heating” conceptual model does not appear to constitute a plausible explanation for the circulation in the past of thermal waters (up to 70–90°C, according to fluid inclusion data) in the vadose zone of Yucca Mountain. The finding has important implications for the safety and expected performance of the proposed high-level nuclear waste disposal facility at Yucca Mountain.