Assessment of chemical inhalation risks to burrowing mammals

Background/Question/Methods

Since the onset of plutonium production at the US Department of Energy’s Hanford Site (Washington, USA), several hundred cubic meters of carbon tetrachloride (CCl4) have been discharged to the soil column, resulting in a dispersed CCl4 vapor plume in the subsurface. Inhalation of volatile organic chemicals could be an important exposure pathway for Hanford's burrowing animals. Safety thresholds for inhalation of volatile chemicals were developed using relevant toxicity information and exposure modeling based on the site’s predominant small mammal, the Great Basin pocket mouse, Perognathus parvus.  Historical levels of CCl4 in soil pore gas exceeded these inhalation thresholds but pore gas represents an ecologically unrealistic exposure medium.  To meet basal metabolic demands, fossorial mammals construct burrows to maximize exchange of subsurface air with the atmosphere above, thereby facilitating oxygen replenishment and consequently diluting gasses that might otherwise build up in the burrow.  Consequently, the inhalation exposure pathway was evaluated with the use of artificial burrows deployed at locations that had elevated levels of CCl4 in soil gas. Artificial burrows were designed on the basis of information available for Hanford Site fossorial wildlife. After installation, the artificial burrow atmosphere was sampled and analyzed for CCl4 and its degradation products: chloroform,methylene chloride, and chloromethane. Sampling was repeated on 3 occasions to capture varying atmospheric conditions affecting exposure concentrations.

Results/Conclusions

CCl4 was detected in the artificial burrows, and maximum exposures were observed during relatively low barometric conditions. The highest CCl4 detections were still well below the inhalation-based safety thresholds and CCl4 degradation products were never detected. This study shows that artificial burrows are an efficient method for obtaining relevant exposure data and illustrates the utility of directly measuring the medium for exposure under ecologically realistic conditions.