Assessment of Carbon Tetrachloride Groundwater Transport in Support of the Hanford Carbon Tetrachloride Innovative Technology Demonstration Program

Abstract

and Technology. This report was prepared to document the results of the modeling effort and facilitate discussion of characterization and remediation options for the carbon tetrachloride (CT) plume among the ITRD participants.As a first step toward implementation of innovative technologies for remediation of the CT plume underlying the 200 West Area, modeling was performed to provide an indication of the potential impact of the CT source on the compliance boundary approximately 5000 m away. The primary objective of the modeling was to bracket the amount of CT source that will most likely result in compliance/non-compliance at the boundary and the relative influence of the various model input parameters.The modeling was based on the assumption that about 750,000 kg of CT were discharged to the soil in the 200 West Area. Previous work has shown that, of this 750,000 kg, about 65% cannot be accounted for (after totaling atmospheric losses [21%]; unsaturated zone inventory in soil gas, soil moisture, and adsorbed phases [12%]; and the dissolved phase in the aquifer [2%]). Therefore, model simulations were performed using 65%, 30%, 10%, and 1% of the 750,000 kg as possible source amounts of CT that could reach the groundwater (approximately 1 to 2% of the original CT inventory now exists in the distal plume based on averaged CT groundwater measurements). The modeling simulations conducted for this study examine the migration of CT from the source area to the compliance boundary. The simulations did not examine migration of the existing distal plume and did not attempt to match the historical disposal and migration of CT (i.e., did not attempt to reproduce the current CT plume distribution).Other model input parameters including the groundwater Darcy velocity; inlet concentration (the concentration of CT leaving the source area); porosity; soil/water equilibrium partition coefficient (K d ); abiotic degradation rate (K a ); dispersivity; and stream tube cross-sectional area were also varied to assess sensitivity of the results to each of the parameters.The CT transport simulation was conservatively modeled as a stream tube 1000 m wide by 5000 m long in one-dimensional flow (i.e., no vertical or transverse dispersivity or convection). Regional flow-modeling results using the Hanford Site-Wide Groundwater Model (SGM) flow grid provided groundwater velocity estimates for the simulation. The basic assumptions used in developing the model were as follows:• The major source of contamination is within a 500m x 500m box.• The contaminant plume was in equilibrium with the source immediately before pumpand-treat efforts.• Processes considered in the model are one-dimensional convective-dispersive transport of reactive solutes subject to adsorption and first-order abiotic degradation (hydrolysis).• Volatilization and first-order natural biodegradation are negligible.ivThe one-dimensional van Genuchten model simulated convective-dispersive transport of CT through a homogenous medium along the centerline of the contaminant plume f...

Abiotic Degradation Rates for Carbon Tetrachloride and Chloroform: Progress in FY 2010

“…A key aspect of these decisions will be to determine the relative sizes of the efforts devoted to active remediation and natural attenuation in order to stabilize the plume. Of the possible natural-attenuation mechanisms, biodegradation is not likely to contribute significantly (Truex et al 2001), and mechanisms such as dispersion, dilution, and sorption to Hanford Site sediments have been the focus of previous field and modeling efforts (e.g., Riley et al 2005). In contrast, abiotic-degradation processes such as hydrolysis and reduction, which are likely to contribute significantly to natural attenuation (Truex et al 2001), are not well understood or quantified.…”

“…Of the possible natural-attenuation mechanisms, biodegradation is not likely to contribute significantly (Truex et al 2001), and mechanisms such as dispersion, dilution, and sorption to Hanford Site sediments have been the focus of previous field and modeling efforts (e.g., Riley et al 2005). In contrast, abiotic-degradation processes such as hydrolysis and reduction, which are likely to contribute significantly to natural attenuation (Truex et al 2001), are not well understood or quantified. The work in this project is designed to partly bridge that gap by determining the mechanisms and rates associated with hydrolysis.…”

Recent Site-Wide Transport Modeling Related to the Carbon Tetrachloride Plume at the Hanford Site

“…The Hanford Carbon Tetrachloride Innovative Treatment Remediation Demonstration (ITRD) Program provided an initial evaluation of the nature and extent of carbon tetrachloride contamination in the unconfined aquifer in the 200 West Area of the Hanford Site (Truex et al 2001). After the ITRD program ended, subsequent studies more closely examined the transport of carbon tetrachloride (CCl4) in the unconfined aquifer system.…”

“…The ITRD modeling study examined carbon tetrachloride concentrations at an arbitrary boundary between the 200 East and 200 West Areas (Truex et al 2001). After that work was completed, questions arose concerning the concentrations reaching the Columbia River as well as the impacts of remediation options involving the removal or absence of the source.…”

“…To address those questions, additional modeling studies were conducted using the site-wide groundwater model (SGM) with the modeling domain extended to the Columbia River. The original SGM grid and the arbitrary compliance boundary are shown in Figure 2.1 along with other details of the ITRD analysis (Truex et al 2001 carbon tetrachloride (thin red lines), 2) the area of grid refinement for the ITRD study (orange line), 3) the compliance analysis boundary for ITRD study (thick red line), and 4) the flow tube used for the one-dimensional modeling to examine the effect of parameter uncertainty and develop sampling strategies to minimize the number of three-dimensional model runs (blue box).…”