Volatilization of Trichloroethene from Groundwater in Karst, Mitigating a Human-Health Concern in a Show Cave

Vapor intrusion of Volatile Organic Compounds (VOCs) such as trichloroethene (TCE) is a human health risk as contaminants volatilize into confined spaces and occupants are unknowingly exposed. Many studies have focused on vapor intrusion in buildings, but few have addressed vapor intrusion in commercial caves. Missouri is known as the “Cave State” with thousands of known caves, several of which are commercial caves offering guided tours.

 In 1990, the U.S. Geological Survey (USGS) detected TCE in a spring within a commercial show cave near a Superfund site, subsequently (2002) the Missouri Department of Natural Resources (MDNR) detected TCE in air inside the cave. TCE levels inside the cave became a concern after the U.S. Environmental Protection Agency (USEPA) lowered allowable TCE concentrations in air, resulting in the owner closing the cave for several months during 2016. Collaborative efforts between the USGS, USEPA, MDNR, cave owner, and a potentially responsible party (PRP), investigated TCE transport within the cave system using a network of airflow and temperature monitors and periodic water and air sampling.

Volatilization from groundwater in the karst system beyond the mapped cave extent generates substantial TCE concentrations in cave air. During the summer when outside air concentrations are higher than cave air, convection moves this TCE-contaminated air “downcave” though toured areas to the cave mouth. During the winter when outside temperatures are lower than the cave, flow reverses with fresh air entering the cave mouth dramatically decreasing TCE concentrations inside the cave. In the summer nearly 20% of the TCE flux occurs in the cave air. Concentrations peak during fall and spring when outside temperature approximates cave temperature and airflow stagnates. A mitigation system was designed to reproduce “winter-like” airflow that have historically produced the lowest TCE concentrations in air and allowed the cave to reopen for tours.