Age Dating Heating-Oil Releases, Part 2: Assessing Weathering and Release Time Frames Through Chemistry, Geology and Site History

The age of heating-oil releases from underground storage tanks (USTs) can be estimated by investigating the petroleum chemistry, the on-site environmental conditions, the extent of contaminant migration, the condition of the UST, site history and several additional factors. The heating-oil chemistry, and in particular the degree of weathering, can be assessed through an examination of: compound depletion, the carbon range, the n-alkane depletion and the size of the unresolved complex mixture (UCM). Each release site can be assigned a weathering regime, such as very aggressive, aggressive, moderate, weak and very weak. These designations are based on the on-site geological, hydrological and biological conditions. A matrix was constructed which compares the weathering regimes to the stages of petroleum weathering and provides potential age ranges. The matrix is a method to lead the investigator towards the correct age, but is not the final say on that age. The age ranges then need to be compared to site-specific characteristics, such as the UST age, the UST condition, the extent of contaminant migration and other extenuating factors. The error range for this method is estimated at ± 2 years; however, there are numerous limitations to the methods and instances will arise where the age can only be constrained.