GEOLOGIC REPOSITORIES, EARTH SCIENCES AT THE LIMITS, AND BEYOND

Regulatory requirements for the siting of Yucca Mountain as a radioactive waste repository have placed unusual demands on the earth sciences, demands that are likely to increase as Yucca Mountain passes to the licensing phase. At any given time, there will always be limits to what questions the earth sciences can answer–and in what detail and surety they can be answered, especially as the threshold of tolerance becomes very small as in the probabilities of 10**-8/yr associated with certain volcanic and ground-motion issues. Often as not, there is a wide range of informed opinion on such matters, and this range tends to get wider as probabilities get lower. As we argue among ourselves in the fashion of the research scientists that we are, we can leave the impression in some quarters that we don’t know what we are talking about, thereby marginalizing our science and defaulting to engineering "solutions" that may be less useful than what we do know, even at the limits of our knowledge. Indeed, according to Ewing and Macfarlane (Science 296, 659, 2002), this situation has become sufficiently acute that "the original concept of geologic disposal has been turned on its ear." This development should not surprise Ewing and Macfarlane, given the many "unresolved" earth-science issues they recount, but it will surprise those who know of the huge corpus of earth-sciences observations and analyses indicating that the natural attributes alone of the proposed Yucca Mountain repository will retard significant release of radionuclides to the biosphere for long periods of time. To the extent this latter view is not held in some quarters, both within and outside of the earth sciences, we have failed as a community to convey what we know and the (un)certainty with which we know it. While much has been done to be quantitative about what we know and what we don’t at the limits of our knowledge, we need to do more. In some instances, much can be achieved with probabilistic formats, as has been done for seismologic and volcanic issues. Expressing the physical limits of geologic processes, earthquake ground motion, for example, will make for trickier business. Quantifying the significance of conceptual matters such as geologic inference, negative evidence, and natural analogues will present even greater challenges.