Decontamination of Soil Contaminated at the Surface with <i>Bacillus anthracis</i> (Anthrax) Surrogate Spores Using Steam Vapor

In the event of a wide-area release of <i>Bacillus anthracis</i> spores, soils and other outdoor materials will likely become contaminated with the biological agent. Soils may also become contaminated with <i>B. anthracis</i> when livestock or wildlife succumb to anthrax disease. This study was conducted to assess the <i>in situ</i> remediation of soil using steam vapor to inactivate a <i>B. anthracis</i> spore surrogate (<i>Bacillus atrophaeus</i>) inoculated into soil samples. Tests were conducted using small columns (~0.04 m³ of soil) filled with either loam, clay, or a sandy soil. Following steam treatment, the <i>B. atrophaeus</i> spores were recovered from the test and positive control soil samples via liquid extraction and this liquid was subsequently dilution plated to quantify viable spores in terms of colony-forming units. Decontamination efficacy was assessed as a function of soil type, soil depth, soil moisture, soil temperature, and steam exposure time. Results showed that spore inactivation improved with increasing steam exposure time and diminished with depth. The clay soil generally exhibited the highest soil temperatures and correspondingly showed the highest inactivation of spores. Adding moisture to the soil prior to the steam treatment increased heat transfer within the soil column, and sealing the columns to mitigate steam leakage increased spore inactivation. The results showed that a steam mass of 40-50 kg applied per square meter of soil surface was sufficient to inactivate bacterial spores to depths between 7 and 10 cm. With bacterial spores on the soil column surface, a contact time of 15 min with the steam vapor at 99°C was sufficient for complete inactivation. These findings provide a foundation for estimating costs and time requirements for applying steam to the soil surface, and further confirmatory testing at field-scale is suggested.