No abstract

Methods are developed to use data collected during cyclic operation of soil vapor extraction (SVE) systems to help characterize the magnitudes and timescales of mass flux associated with vadose zone contaminant sources. Operational data collected at the Department of Energy’s Hanford site are used to illustrate the use of such data. An analysis was conducted of carbon tetrachloride vapor concentrations collected during and between SVE operations. The objective of the analysis was to evaluate changes in concentrations measured during periods of operation and non-operation of SVE, with a focus on quantifying temporal dynamics of the vadose zone contaminant mass flux, and associated source strength. Three mass-flux terms, representing mass flux during the initial period of a SVE cycle, during the asymptotic period of a cycle, and during the rebound period, were calculated and compared. It was shown that it is possible to use the data to estimate time frames for effective operation of an SVE system if a sufficient set of historical cyclic operational data exists. This information could then be used to help evaluate changes in SVE operations, including system closure. The mass-flux data would also be useful for risk assessments of the impact of vadose-zone sources on groundwater contamination or vapor intrusion.

As a result of past practices, up to 580 m3 carbon tetrachloride (CT) was discharged to waste sites at the 200 West Area of the USDOE's Hanford Site near Richland, WA. Three‐dimensional modeling was conducted to enhance the current conceptual model of CT distribution beneath the major disposal site (216‐Z‐9). The simulations, using the STOMP code, focused on migration of dense nonaqueous phase liquid (DNAPL) consisting of CT and codisposed organics under scenarios with differing sediment properties, sediment distribution, waste properties, and waste disposal history. Simulation results support a conceptual model for CT distribution where CT in the DNAPL phase migrated primarily in a vertical direction below the disposal site and where some CT DNAPL likely migrated across the water table into the regional aquifer. Results also show that the lower permeability Cold Creek unit retained more CT DNAPL within the vadose zone than other hydrologic units during the infiltration and redistribution process. Due to the relatively high vapor pressure of the CT, the resulting vapor plumes are extensive and influenced by density‐driven advection. Any continued migration of CT from the vadose zone to the groundwater is likely through interaction of vapor phase CT with the groundwater and not through continued DNAPL migration. Additional simulations assessed the impacts of soil vapor extraction (SVE) as a remediation method. These simulations showed rapid CT removal associated with the assumed local equilibrium of CT between the phases. Additional efforts are needed to enhance the understanding of rate‐limited volatilization to improve simulation of the SVE process and to provide a basis for refining the design and operation of SVE systems.

V'DISCLAIMER This report was prepared as an account of work .... ; j_'_ _., sponsored by an agency of the United States Goverr._ent. Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors or their-_ employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party's use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessari ly constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors. The views and opinions o," authors = expressed herein do not necessarily state or reflect those of the United States Government or any a_encv thereof.

This report presents a refined ge.hydrologic and geochemical conceptual model of the hcst site for the Volatile Organic Compounds-Arid Integrated Demonstration (VOC-Arid lD) and 200 West Area Carbon Tetrachloride (CC14)Expedited Response • Action (ERA), based on the results from fiscal year 1992 site characterization activities. The ERA was initiated in December 1990 to minimize or stabilize CCI4 migration within the unsaturated (vadose) zone in the vicinity of three CC14 disposal sites in the 200 West Area (216-Z-lA tile field, 216-Z-9 trench, and 216-Z-18 crib). Implementation of this ERA was based on concerns that CC14 residing in the soils was continuing to spread to the groundwater and, if left unchecked, would significantly increase the area of groundwater contamination. A soil-vapor-extraction system began operating at the site in February 1992.