Statistical Exploration of Matrix Hydrologic Properties for the Bandelier Tuff, Los Alamos, New Mexico

Abstract

The Bandelier Tuff forms the top of the thick vadose zone at Los Alamos National Laboratory (LANL). Subsurface pathway performance or risk assessments at Los Alamos require characterizing the unsaturated hydrologic properties of the Bandelier Tuff. The objective of this study was to analyze available unsaturated zone matrix hydrologic properties data for the Bandelier Tuff from the Los Alamos area for the purpose of developing estimates of properties and parameters for modeling vadose zone flow and transport. The hydrologic properties bulk density, saturated hydraulic conductivity, and the van Genuchten equation parameters, residual and saturated water contents, α, and n, were measured or estimated from measured data. A nonparametric statistical approach was used because of the small sample size leading to nonnormal distribution of the samples. The following differences were investigated: (i) between boreholes for a given unit within a site and (ii) between sites for a given unit. Results for borehole analyses within a location found that properties were different at one location and could be combined at another. The testing between locations found that samples could be readily combined. Linear correlation analyses of the one geologic unit that was the most consistent across the Los Alamos site found that there was essentially no relationship between matrix hydrologic properties or parameters to allow surrogate properties to be used for prediction. This preliminary analysis indicated that investigations at LANL will require more detailed sampling on a site‐specific basis because properties cannot be transferred or estimated with any confidence.