EXPLOSIVE AND TOXIC VAPORS IN THE VADOSE ZONES OF KARST FLOW SYSTEMS: CASE STUDIES AND ONGOING CONCERNS

Karst aquifers often have high porosity and permeability that can allow rapid movement not only of water and contaminants, but also air and other gasses in the unsaturated zone. These may sometimes include explosive and or toxic vapors. While the physics of airflow has been studied in explorable caves and shown to be driven by forces resulting from pressure and density variations, gasses may also move through the typically extensive network of smaller, unexplorable spaces to permeate regions of the vadose zone. Serious environmental problems—including ecological, health related, and economic—have resulted from contamination of underground air in the unsaturated zones of karst systems.

For example, a tragic accident took place in 1966 when an explorer’s carbide lamp flame set off a “blinding explosion” in Howard’s Waterfall Cave in Georgia leaving one explorer dead and another critically burned. Two would-be rescuers also died from resulting carbon monoxide. The fumes had come from a leaking gasoline station some 800 m away. Another cave explorer was killed and two injured from a similar explosion in New Mexico’s Cable Cave in 1951. Fumes from leaking gasoline also permeated an extensive area of the unsaturated zone above Bowling Green Kentucky’s Lost River Cave in the 1980’s causing the USEPA to declare a Health Advisory associated with evacuation of homes, businesses and schools “on the grounds that explosive levels of fumes existed and that detectable levels of benzene, toluene, and xylene exceeded standards for ‘non-occupational settings.’” Economic losses occurred when Missouri’s tourist cave Meramac Caverns was closed in 2016 due to health and safety concerns from Trichloroethylene (TCE) fumes that had come laterally through the vadose zone for more than six kilometers from a Superfund site. The tours were reopened only after extensive intervention to control air flow.

The various gasses in these cases moved through the unsaturated zones in response to natural gradients and associated forces. In contrast, large proposed high-pressure natural gas pipelines that would cross highly karstified areas of the Appalachian Mountains are designed to operate at nearly 100 times atmospheric pressures, and little is known about the potential impacts of gas in karst unsaturated zones under such forced pressure gradients.