Hydraulic Conductivity Estimates from Particle Size Distributions of Sediments from the Los Alamos Chromium Plume

Abstract:

Chromium used in Los Alamos National Laboratory cooling towers was released as effluent onto laboratory property between 1956 and 1972. As a result, the underlying regional aquifer is contaminated with chromium (VI), a toxin and carcinogen. The highest concentration of chromium is ~1 ppm in monitoring well R-42, exceeding the New Mexico drinking water standard of 50 ppb. The chromium plume is currently being investigated to identify an effective remediation method. Geologic heterogeneity within the aquifer causes the hydraulic conductivity within the plume to be spatially variable. This variability, particularly with depth, is crucial for predicting plume transport behavior. Though pump tests are useful for obtaining estimates of site specific hydraulic conductivity, they tend to interrogate hydraulic properties of only the most conductive strata. Variations in particle size distribution as a function of depth can complement pump test data by providing estimates of vertical variations in hydraulic conductivity. Samples were collected from five different sonically-drilled core holes within the chromium plume at depths ranging from 732’-1125’ below the surface. To obtain particle size distributions, the samples were sieved into six different fractions from the fine sands to gravel range (>4 mm, 2-4 mm, 1.4-2 mm, 0.355-1.4 mm, 180-355 µm, and smaller than 180 µm). The Kozeny-Carmen equation (k=(δg/µ)(dm²/180)(Φ³/(1-Φ)²)), was used to estimate permeability from the particle size distribution data. Pump tests estimated a hydraulic conductivity varying between 1 and 50 feet per day. The Kozeny-Carmen equation narrowed this estimate down to an average value of 2.635 feet per day for the samples analyzed, with a range of 0.971 ft/day to 6.069 ft/day. The results of this study show that the Kozeny-Carmen equation provides quite specific estimates of hydraulic conductivity in the Los Alamos aquifer. More importantly, it provides pertinent information on the expected variations with depth in hydraulic conductivity not obtainable from pump tests. This information is a prerequisite for modeling the spatial variation of the chromium plume and formulate remediation strategies.