Spatial Variability in Alluvium Properties at a Low‐Level Nuclear Waste Site

Abstract

AbstractGeological and statistical models for the spatial variability of soil properties are needed to predict field‐scale water flow and solute transport but only limited information is currently available on unsaturated soils below the root zone. Spatial variability of selected physical and hydrologic properties was quantified for fine‐ and coarse‐grained alluvial deposits at a low‐level nuclear waste disposal site on the Nevada Test Site. Gravimetric water content (w), bulk density (ρb), saturated hydraulic conductivity (Ks), and particle‐size distribution were determined for vertical and horizontal core specimens and bulk samples collected from 183‐m‐long horizontal transects in two existing waste disposal trenches located on a single alluvial fan. The transects were approximately aligned parallel and perpendicular to the principal direction of sediment transport. Properties were modeled as either normally or lognormally distributed random variables. Sample coefficients of variation were smallest for ρb and largest for log(Ks); a weak correlation was identified between log(Ks) and the grain‐size parameter d10. Particle‐size distributions for the fine‐ and coarse‐grained materials were different and significant differences in the natural logarithm of saturated hydraulic conductivity, log(Ks), existed between coarse and fine layers in an excavation aligned with the principal direction of alluvium deposition but not in a perpendicular direction. Differences in log(Ks) for vertical and horizontal cores were not significant. Sample variograms were described by combinations of pure nugget and spherical model structures with correlation lengths ranging from <2 to 61 m.