Fate and Transport of Fuel Components Below Slightly Leaking Underground Storage Tanks

Fuel leaks from underground storage tanks (USTs) and piping have been a major source of groundwater contamination. In the U.S. and Europe, regulations requiring upgrading of USTs to meet specific standards have significantly reduced instances of fuel contamination. Leak detection is primarily dependent on physical measurement systems that are generally capable of detecting leak rates as small as 0.2 L/h. Fuel leaks that are smaller than this detection threshold may remain undetected for long periods of time, posing a risk of contamination to shallow groundwater resources in sensitive areas. This risk was evaluated by modeling fate and transport of fuel components from small UST leaks under a variety of subsurface conditions and assuming that secondary containment does not exist. It was found that small leaks do have the potential to impact shallow groundwater, particularly if subsurface conditions are not conducive to natural attenuation processes. This may explain situations where groundwater contamination has been found below service stations in virgin areas that have upgraded leak detection systems. Modeling indicates that passive venting of tank and piping backfill could virtually eliminate the volatile components of fuel resulting from small leaks. Monitoring the tank and piping backfill for persistent gasoline vapor under very low vapor extraction conditions may be the best way to detect small chronic fuel leaks. Routine monitoring of shallow groundwater should be a component of a leak detection program, particularly in high-risk areas.