Uncertainty and variability of infiltration at Yucca Mountain: Part 2. Model results and corroboration

Abstract

[2] The U.S. Nuclear Regulatory Commission actively investigated climate and infiltration at Yucca Mountain for two decades to (i) understand important controls and uncertainties influencing percolation through the unsaturated zone on multimillennial time scales and (ii) provide flux boundary conditions for up to 1 million years in performance assessment models of the proposed Yucca Mountain repository. This second part of a two-part series describes site-scale model results for present and potential future conditions and confirmatory analyses for present-day conditions. At both the grid-cell and site-average scale, the calculated uncertainty distribution for net infiltration is approximately lognormal, and the coefficient of variation decreases with increasing net infiltration. Smaller relative but larger absolute responses to climate change occur where net infiltration is large. Comparisons of distributed model estimates with temperature and geochemical observations from the unsaturated zone suggest that average estimates are generally consistent but exhibit significant variability. An observed seepage event in the South Ramp of the Exploratory Studies Facility, combined with related subsurface observations across the site, suggests that subsurface spreading from zones of high infiltration to zones of low infiltration may occur in stratabound fractures, laterally extensive discontinuities, or at transitions between welded and nonwelded tuff units. Two conceptual models for unsaturated-zone flow each explain the subsurface observations, collectively providing bounding estimates for net infiltration. Model-predicted uncertainty distribution for decadal-average site-scale net infiltration is generally consistent with estimated percolation fluxes using the bounding hypotheses, suggesting that the model-calculated uncertainty is reasonably consistent with the uncertainty in interpreting site observations.

Average infiltration at Yucca Mountain over the next million years

“…These three response functions are developed below. The response function used to translate decadal‐average MAP and MAT into decadal‐average MAI is based on Infiltration Tabulator for Yucca Mountain (ITYM) model results [ Stothoff , ]. Output from the last response function in the sequence is averaged over the control surface (the repository footprint box) and over the time sequence.…”

“…A surface‐based infiltration model, ITYM, links decadal‐average net infiltration to decadal‐average climate. Stothoff [] describe the ITYM model and model corroboration, and develop an expected‐response function from ITYM Monte‐Carlo simulations. The ITYM model considers the uncertainty and variability of input parameters to estimate uncertainty and variability of decadal‐average distributed infiltration on a 30 m grid for a given reference MAP and MAT at an elevation of 1400 m on Yucca Mountain.…”

“…This manuscript describes the approach NRC used to derive a uncertainty distribution for million‐year‐average net shallow infiltration within a 14.7 km 2 repository footprint box. The footprint box consists of vertical east‐west and north‐south planes bounding the proposed repository [ Stothoff , ]. The specified uncertainty distribution for time‐averaged and area‐averaged deep percolation within the footprint box can be assumed numerically equal to the corresponding uncertainty distribution for net shallow infiltration when (i) net lateral flow across the vertical bounding planes is negligible compared to total vertical flow within the footprint and (ii) the mean transit time from the surface to the repository footprint is negligible compared to the averaging duration.…”

“…The specified uncertainty distribution for time‐averaged and area‐averaged deep percolation within the footprint box can be assumed numerically equal to the corresponding uncertainty distribution for net shallow infiltration when (i) net lateral flow across the vertical bounding planes is negligible compared to total vertical flow within the footprint and (ii) the mean transit time from the surface to the repository footprint is negligible compared to the averaging duration. Stothoff [] describes observational evidence suggesting that lateral redistribution above the repository horizon within the footprint is predominantly due to spreading at the wash scale, from ridge to wash, rather than in or out of the footprint. Age‐dated waters within and below the repository horizon show bomb‐pulse signatures in some locations and can be interpreted as late Pleistocene in other locations [ Stothoff , ], suggesting that travel times are several orders of magnitude shorter than the million‐year period of interest.…”

“…Stothoff [] describes observational evidence suggesting that lateral redistribution above the repository horizon within the footprint is predominantly due to spreading at the wash scale, from ridge to wash, rather than in or out of the footprint. Age‐dated waters within and below the repository horizon show bomb‐pulse signatures in some locations and can be interpreted as late Pleistocene in other locations [ Stothoff , ], suggesting that travel times are several orders of magnitude shorter than the million‐year period of interest.…”