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

Abstract

Executive SummaryThe 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. After the ITRD program ended, subsequent studies more closely examined the transport of carbon tetrachloride (CCl4) in the unconfined aquifer system. Those studies were undertaken to support strategic planning and provide guidance for the more robust modeling needed to obtain a final record of decision (ROD) for the carbon tetrachloride plume. This report documents the technical approach and results of those studies.The ITRD modeling study examined carbon tetrachloride concentrations at an arbitrary boundary between the 200 East and 200 West Areas. After that work was completed, questions arose concerning the concentrations reaching the Columbia River as well as about the impact of remediation options involving source removal or absence. 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. This modeling analysis resulted in the following conclusions:With the assumption of a continuing source with no sorption or abiotic degradation (Case 1a), we observed: Initial conditions yielded an initial mass of about 542 kg in the aquifer, which grew to 58,050 kg after 1000 years (the year 3000).With the assumption of a continuing source with median value estimates of sorption and abiotic degradation (Case 1b), we observed:Limited development and migration of a carbon tetrachloride plume from source areas within the general vicinity of the 200 West Area. Predicted concentrations reached about 4.5 µg/L at the arbitrary boundary chosen for this analysis. Concentrations at discharge areas along the Columbia River did not reach any substantial levels during the 1000-year period of analysis.The combination of sorption and abiotic degradation rate significantly limits aquifer source loading and the aquifer area and volume affected by the carbon tetrachloride plume migration. The more important parameter of the two is the abiotic degradation rate because retardation alone will not reduce concentrations other than through dilution because of hydrodynamic dispersivity.Without a continuing source of carbon tetrachloride and no sorption or abiotic degradation, we observed results that were very similar whether the source area with the highest concentrations in the iv plume (i.e., above 3000 µg/L) was assumed to be removed from the aquifer (Case 2) or the existing plume was considered an initial condition of aquifer contamination (Case 3). In both cases we observed:A more limited development and migration of a carbon tetrachloride plume outside the 200 West Area toward the Columbia River than with the continuing source assumption evaluated in Case 1a.A predicted concentration profile reaching about 6.5 µg/L at the arbitrary boundary over a period of ...

Project Work Plan Carbon Tetrachloride and Chloroform Attenuation Parameter Studies: Heterogeneous Hydrolytic Reactions

“…Previous fate and transport studies have demonstrated that the abiotic degradation of carbon tetrachloride and partitioning to aquifer sediments are the two parameters where uncertainty in the parameter values most significantly affect the fate and transport of carbon tetrachloride Bergeron and Cole 2005). In these studies, both Monte Carlo simulations for one-dimensional transport modeling and directed studies using a three-dimensional model concluded that the range of potential abiotic degradation rates (i.e., homogeneous hydrolysis) includes values that would result in attenuation of the carbon tetrachloride plume such that natural attenuation would be an important component of the remedy along with active remediation.…”

Screening of Potential Remediation Methods for the 200-ZP-1 Operable Unit at the Hanford Site

“…Previous modeling analysis suggests that these mechanisms may significantly reduce concentrations within the plume and decrease the extent of carbon tetrachloride plume migration (Truex et al 2001;Bergeron and Cole 2005). Natural attenuation processes acting on trichlorethene include sorption, dispersion, and dilution.…”