Vadose Zone Transport Field Study: Status Report

Abstract

SummaryStudies were initiated at the Hanford Site to evaluate the process controlling the transport of fluids in the vadose zone and to develop a reliable database for testing vadose-zone transport models. These models are needed to evaluate contaminant migration through the vadose zone to underlying groundwater at Hanford. A study site that had previously been extensively characterized using geophysical monitoring techniques was selected in the 200 E Area. Techniques used previously included neutron probe for water content, spectral gamma logging for radionuclide tracers, and gamma scattering for wet bulk density. Building on the characterization efforts of the past 20 years, the site was instrumented to facilitate the comparison of nine vadose-zone characterization methods: advanced tensiometers, neutron probe, electrical resistance tomography (ERT), high-resolution resistivity (HRR), electromagnetic induction imaging (EMI), cross-borehole radar (XBR), and cross-borehole seismic (XBS). Soil coring was used to obtain soil samples for analyzing ionic and isotopic tracers.Laboratory-scale experiments with hypersaline fluids in Hanford sediments suggest that fluid properties may influence transport behavior, to the extent of finger formation, through an interaction between fluid and hydraulic properties. Yet, the importance of these mechanisms to field-scale transport is largely unknown, thereby limiting the accuracy of contaminant-transport predictions. To assess the importance of these interactions in field-scale solute transport, tank leaks were simulated by performing a series of injections with dilute fluids in late spring and early summer of FY 2000 and with hypersaline fluids (36 wt% sodium thiosulfate) during the spring of 2001. In both tests, a suite of isotopic and ionic tracers was included in the injected fluids. The test in FY 2000 consisted of injecting to ground a series of five 3875 L (1000 gal) pulses of water and tracers, weekly, for five weeks. The FY 2001 test, which was designed partly to evaluate the effects of fluid properties and transport processes, involved the injection of 19,000 L (5000 gal) of hypersaline fluid over the course of 5 weeks. This was followed by 11,400 L (3000 gal) of solute-free water applied in a 2-week period. In FY 2000, infiltration and redistribution were monitored using the nine characterization methods over the course of the injections and for 2 months after the last injection. In FY 2001, all methods except EMI were used to monitor the infiltration and redistribution of the 30,000 L (7925 gal) plume over the course of 3 months.Thus far, field-measured distributions of soil water content have been analyzed using threedimensional spatial-moment analysis. Results clearly show that the subsurface distributions of both the dilute and hypersaline fluids are controlled by an interaction between small-scale horizontal stratification and fluid properties. The centers of mass for the two plumes were similar in the lateral and transverse directions, but were significantly d...

A Markov chain model for characterizing medium heterogeneity and sediment layering structure

“…Two field injection experiments were conducted at the site in 1980 [Sisson and Lu, 1984] and 2000 [Ward et al, 2000;Gee and Ward, 2001]. Several numerical simulations have been conducted, with mixed success, to simulate the 1980 and 2000 moisture plumes [Fayer et al, 1995;Rockhold et al, 1999;Ward et al, 2000;Gee and Ward, 2001;Zhang et al, 2004;Ye et al, , 2007Yeh et al, 2005;Kowalsky et al, 2005;Ward et al, 2006aWard et al, , 2006b]. …”

“…Field investigation and core sampling revealed the apparent sediment layering structure in site lithostratigraphy; the core samples are, however, insufficient to represent the layering structure throughout the S-L site. However, the site has an extensive q i database, which was measured about one month before a series of field injection experiments were conducted [Gee and Ward, 2001]. Since the q i measurements carry signature of medium heterogeneity, as discussed later, they play a key role in characterizing heterogeneity, especially the imperfectly stratified layers.…”

Vadose Zone Transport Field Study: FY 2002 Status Report

“…The study was conducted on a 60-m-long transect at the Army loop Road clastic dike site. The transect was instrumented with sensors for monitoring local-scale water content, matric potential, electrical conductivity, and tracer concentrations while at the same time, allowing larger scale measurements with non-invasive geophysical tools Gee and Ward 2001;Ward and Gee 2002). This report documents the experimental design and monitoring protocols and provides an overall status of the study at the end of the fiscal year.…”

Hydraulic Conductivity Distributions for Anisotropic Systems and Application to Tc Transport at the U.S. Department of Energy Hanford Site

“…The sediments in the vicinity of the vadose zone transport field study (Gee and Ward, 2001;Ward et al, 2000) consist principally of sand with interstitial silt and silt beds . Thus a compound upscaling procedure, from the pore size to the sample size and from the sample to the formation size must be applied.…”

“…The value log(V)=0 corresponds to the fundamental length scale in the problem. In both the fractured carbonate aquifer (Schulze-Makuch, 1996) and the Hanford subsurface, this value appears to be approximately 1 meter (pages 4.10-4.13 of Gee and Ward, 2001, aside from possible issues with experimental resolution). The ratio of the maximum to minimum K in the Hanford subsurface is much smaller than that depicted; instead of about eight orders of magnitude, it will be closer to two orders of magnitude at larger tensions, whereas at smaller values of the tension (near saturation) it is likely to be 1 (a flat, or constant, graph) (internal Hanford studies as well as Zhang et al, 2003).…”