Abstract  Between 1955 and 1973, an estimated 920,000 kg of carbon tetrachloride (CT) were discharged to the soil in the 200 West Area of the Hanford Site as part of the plutonium production process. Of this amount, some CT reached the groundwater more than 70 m below the ground surface and formed a plume of 10 km2 extent. Remediation of the CT plume is underway using a pump-and-treat approach to reduce concentrations of CT and other contaminants such as chloroform (CF) and radionuclides in the aquifer over a projected 25-year operational period. Following the period of pump-and-treat operation, monitored natural attenuation will be the continuing remedy to reach groundwater remediation goals. Multiple mechanisms of natural attenuation can reduce CT and CF concentrations over time during pumpand-treat and the subsequent monitored natural attenuation remedy phases. Of these attenuation mechanisms, hydrolysis reactions are expected to contribute as a degradation process for CT and CF. This report documents the results of long-term experiments to quantify the homogeneous and heterogeneous hydrolysis rates for CT and CF needed to estimate the contribution of hydrolysis to natural attenuation of the contaminant plumes under the subsurface conditions at the Hanford Site (e.g., temperature and sediment mineralogy). Experiments were conducted across a temperature range of 20° – 93°C for periods as long as 6 years, and the Arrhenius equation was used to estimate activation energies and calculate half-lives for typical Hanford groundwater conditions (temperature of 16°C and pH of 7.75). A half-life of 630 years for hydrolysis for CT under these conditions was estimated, where CT hydrolysis was unaffected by contact with sterilized, oxidized minerals or Hanford sediment within the sensitivity of the experiments. In contrast to CT, hydrolysis of CF was generally slower and very sensitive to pH due to the presence of both neutral and base-catalyzed hydrolysis pathways. A half-life of 3400 years was estimated for hydrolysis of CF in homogeneous solution at 16°C and pH 7.75. Experiments with CF in suspensions of Hanford sediment or smectite, the dominant clay mineral in Hanford sediment, equilibrated to an initial pH of 7.2, yielded calculated half-lives of 1700 years and 190 years, respectively, at 16°C. Experiments for CF with three other mineral phases at the same pH (muscovite mica, albite feldspar, and kaolinite) showed no change from the homogeneous solution results (i.e., a half-life of 3400 years). The strong influence of Hanford sediment on CF hydrolysis was attributed to the presence of smectite and its ability to adsorb protons, thereby buffering the solution pH at a higher level than would otherwise occur.

Abstract  This document presents background information and justification for the Integrated Risk Information System (IRIS) Summary of the hazard and dose-response assessment of carbon tetrachloride. IRIS Summaries may include oral reference dose (RfD) and inhalation reference concentration (RfC) values for chronic and other exposure durations, and a carcinogenicity assessment. The RfD and RfC, if derived, provide quantitative information for use in risk assessments for health effects known or assumed to be produced through a nonlinear (presumed threshold) mode of action. The RfD (expressed in units of mg/kg-day) is defined as an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. The inhalation RfC (expressed in units of mg/m3) is analogous to the oral RfD, but provides a continuous inhalation exposure estimate. The inhalation RfC considers toxic effects for both the respiratory system (portal of entry) and for effects peripheral to the respiratory system (extrarespiratory or systemic effects).