Vadose Zone Transport Field Study: FY 2002 Status Report Page: 37 of 80
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Penetrometer Technology (CPT) Tube #1. These cores provided information at the lower end of the
moisture scale. A 2-m (6.6-ft.)-diameter ring was then installed around the access tube, and water was
added to increase the moisture content. Sampling was repeated to obtain neutron count data at the higher
water content. A final set of cores was collected near CPT Tube #8 where sampling focused on the effect
of the fine-textured lens. Cores were analyzed to determine bulk density and water content. Neutron-
probe readings were taken at the same depths at which the samples were taken.
Figure 3.4a shows a plot of 0 as a function of count ratio, the ratio of hydroprobe counts in the soil, C, to
shield counts, Cs, collected in the two field campaigns. All measurements in the 0- to 10-cm (0- to 4-in.)
range were removed before analysis.
The relationship is clearly curvilinear and is best described by the power function
?=0.3735CR 18756 (3.1)
The coefficient of determination (r2) for the power function is 0.85, compared to 0.65 for a linear model.
Given the relatively poor fit and knowledge that the site is composed of two distinct soil types (sand
matrix and fine-textured composites in the dike), the data were analyzed as two distinct populations to
investigate the need for separate calibration curves. There are two main factors contributing to the
hydroprobe response: variability in mineralogic al composition and variability in water-content
distributions, typically due to variability in physical and hydraulic properties. Both sources of
heterogeneity can affect probe response, particularly in measuring vertical profiles of water content, and it
has been suggested that different calibration curves may be needed for different textures. Variations of a
few percent for most elements encountered in soils rarely cause variations in 0 of more than 0.015 m3 m3.
Extensive excavation at the site shows a 15- to 30-cm (6- to 12-in.) fine-textured layer at a depth of 3 m
(10 ft) and underlain by a finer sand. The top 3 m (10 ft) of soil is mostly coarse to medium sand with
thin fine-textured lenses (Dike images). The fine-textured layer contains 25% silt-sized particles, 10%
sand, and 65% clay-sized particles. The bulk density, determined by the clod method, was 1.8 g m3.
Experimental investigations on a variety of soils have established that, in addition to the moisture content
of the soil, the probe reading depends mainly on dry bulk density with chemical composition being of less
importance. Figure 3.4b shows a significant improvement in the quality of the fit when the data are
separated into two populations. There was no significant difference between linear and power function
models for the data collected in the sand matrix. The data were described equally well with a linear
model
? =0.3416CR -0.0566 (3.2)
with a coefficient of determination of 0.93 and the power function model
?=0.2858CR TAT (3.3)
with a coefficient of determination of 0.97. The power function is perhaps more meaningful as it suggests
a count rate of zero for 0 = 0. Given the limited number of data points, the choice of calibration functions
cannot be finalized, and it is planned that calibration will continue in the next set of experiments.3.4
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Ward, Anderson L.; Gee, Glendon W.; Zhang, Z. F. & Keller, Jason M. Vadose Zone Transport Field Study: FY 2002 Status Report, report, January 2, 2003; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc926570/m1/37/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.