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Environmental Sustainability Division

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Geostatistical Modeling of Subsurface Geology at the Hanford Site, Southeast Washington

Objectives: The objectives of this project were to use geostatistical methods to map the distribution of subsurface sediment types known to control the movement of groundwater at the Hanford Site, estimate the spatial and geologic uncertainty in their distribution, and provide alternative conceptual models of the subsurface geology that can be used as input to the Sitewide Groundwater Modeling program. This capability is being used to determine the effect of geologic uncertainty on the predicted transport of contaminated groundwater at the Site.

Approach: Indicator variogram analysis was used to study the spatial distribution of two important subsurface geologic features at the site: the distribution of discontinuous mud units that act as local aquitards and the distribution of sand and gravel facies within the uppermost aquifer unit at the Site. Sequential indicator simulations of the geologic facies distributions were then used in a Monte Carlo approach to provide refined geostatistical estimates of the subsurface geology at the Site and the uncertainty associated with those spatial distributions.

Results: Maps were produced for the client that provided estimates of the type of subsurface sediment present for each grid block in the map area. Maps were produced showing the probability that each sedimentary facies would be present in each grid block. Individual stochastic simulations of the mud unit distribution and of the distribution of facies in the uppermost aquifer unit are being used as input to the groundwater modeling program to determine the uncertainty in contaminant transport predictions caused by the effect of uncertainty in the subsurface geologic framework.

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