Modeling of Carbon Tetrachloride Flow and Transport in the Subsurface of the 200 West Disposal Sites: Large-Scale Model Configuration and Prediction of Future Carbon Tetrachloride Distribution Beneath the 216-Z-9 Disposal Site

Abstract

Three-dimensional simulations considered migration of dense, nonaqueous phase liquid (DNAPL) consisting of CT and co-disposed organics in the subsurface beneath the two disposal sites as a function of the properties and distribution of subsurface sediments and of the properties and disposal history of the waste. Simulations of CT migration were conducted using the Water-Oil-Air mode of Subsurface Transport Over Multiple Phases (STOMP) simulator.A large-scale model was configured to model CT and wastewater discharge from the major CT and waste-water disposal sites. A base-case simulation was conducted using hydraulic property values used in the base simulations reported in Oostrom et al. (2004;2006a;2006b) for local-scale simulations of the individual CT waste sites. Wastewater disposal resulted in increased water saturations in the Cold Creek Unit and a water table increase throughout the computational domain. The increased water saturations in the Cold Creek Unit reduced the downward movement of DNAPL CT and gaseous CT. Two sensitivity simulations were also performed using reduced permeability values and increased nonwetting-fluid entry pressure values for the Cold Creek Unit. The simulations conducted with smaller Cold Creek Unit permeability values than for the base case resulted in reduced DNAPL CT movement across the water table. For each case, additional simulations were conducted from 1993 to 2007 considering soil vapor extraction (SVE) from the well system in the vicinity of the three CT disposal sites. Partly due to the assumption of equilibrium partitioning between the various phase, considerably more CT was removed in the simulations compared to field observations. In addition to the large-scale simulations, a series of seven local-scale multifluid flow and transport simulations have been completed to quantify DNAPL, aqueous, and gas CT transport in the subsurface of the 216-Z-9 disposal site. This site was chosen for these simulations because, of the three major DNAPL disposal sites, it received the most DNAPL and it has the smallest footprint. For those reasons, previous simulations have shown that DNAPL disposed at this site would be able to penetrate deep into the subsurface 2006a), potentially to below the water table. In addition, CT concentrations of approximately 350,000 µg/kg were reported in a 1-ft thick lens, located at 19.8 m (65 ft) below the ground surface DOE-RL (2006). Six of the seven simulations (imposed cases 1-6) were conducted with imposed CT quantities in the Cold Creek Unit and/or the 1-ft thick silt lens, located in the Hanford 2 unit below the disposal site. The simulation period for these six cases was from 2007 -2107. The seventh case (modeled case) simulated flow and transport following aqueous phase and DNAPL disposal at the site using inventory data and SVE of 53,000 kg after 1993. The simulation period for this case was from 1955 -2107. ivThe computed aqueous CT mass flow rate across the water table for the modeled case is considerably larger than for imposed cases. The ...

Remedy Evaluation Framework for Inorganic, Non-Volatile Contaminants in the Vadose Zone

“…For instance, simulations conducted examining moisture changes at the Hanford 200 Area 216-U-10 site (U Pond) are an illustrative example of how added water pulses to the vadose zone dissipate over time. Oostrom et al (2008) conducted modeling investigations to predict carbon tetrachloride contamination distribution and transport behavior at the Hanford 216-Z-9 waste site. In addition to and associated with the carbon tetrachloride modeling, this study also simulated large-scale infiltration and drainage that occurred in the area from 1954 to 2005.…”

Treatability Test Report: Characterization of Vadose Zone Carbon Tetrachloride Source Strength Using Tomographic Methods at the 216-Z-9 Site

“…However, uncertainty remains-specifically related to the location and distribution of source zones within localized lowpermeability units such as the CCU (Oostrom et al 2007). The mass flux and vapor transport associated with these zones controls the long-term contamination of the vadose zone and the groundwater.…”

Evaluation of Soil Flushing for Application to the Deep Vadose Zone in the Hanford Central Plateau

“…Above and below this unit are the higher-permeability units called the Hanford and Ringold Formations, respectively. The Hanford formation contains a sequence of interbedded sand and mud, coarse to medium sand, and sandy gravel (Oostrom et al 2008). The Ringold Formation is a sedimentary sequence of fluvial-lacustrine clay, silt, sand, and granule to cobble gravel deposited by the ancestral Columbia River, and the water table is located within the Ringold Formation at the 200 Area (Zachara et al 2007a).…”