Petroleum Hydrocarbons And Chlorinated Hydrocarbons Differ In Their Potential For Vapor Intrusion

Response issued draft vapor intrusion guidance (EPA, 2002), which specifically states that it is not recommended for Subtitle I underground storage tank (UST) sites. EPA's Office of Underground Storage Tanks (OUST) is thus currently developing guidance to address petroleum vapor intrusion (PVI) at UST sites. In September 2009, OUST assembled experts in the field of vapor intrusion and petroleum releases from EPA, state regulatory agencies, private consultants, and industry groups to provide technical and practitioner input for EPA to consider in developing the UST PVI guidance. This information paper 1 describes how petroleum compounds behave differently in the subsurface from other volatile chemicals, in particular chlorinated hydrocarbons (CHCs), and how these behaviors can be considered when evaluating the potential for vapor intrusion at sites contaminated by leaking Subtitle I USTs or other sources of petroleum hydrocarbons (PHCs). 2 PHCs typically degrade biologically in groundwater as well as in unsaturated soil zones. In many cases, this aerobic 3 biodegradation is substantial and can limit the potential for PVI. In contrast, biodegradation of CHCs is anaerobic (under anoxic conditions), which is generally slower than aerobic biodegradation of PHCs. This limited biodegradability is to some degree responsible for the greater observed prevalence of chlorinated solvent vapor intrusion (CVI) when compared with PVI. During the 1980s and 1990s, a better understanding of PHC biodegradation in groundwater led to the development of monitored natural attenuation, a remediation approach that involves no external inputs and has now been used successfully to address groundwater contamination at many leaking UST sites (Wilson et al., EPA recognizes that analogous aerobic biodegradation processes are active in the unsaturated zone and that these processes can limit the potential for PVI. 1 This information paper is intended to communicate the overall concepts of petroleum vapor intrusion. It is not intended to be interpreted as either a technical guidance document or statement of regulatory policy. 2 Petroleum hydrocarbons are chemical compounds made up of hydrogen and carbon that are constituents of petroleum and various refined products of petroleum, including automotive gasoline, diesel fuel, lubricating oils, and the like. 3 Aerobic means that the process requires oxygen. In contrast, anaerobic means the process does not require oxygen. Anoxic refers to the absence of oxygen.