Weathering of a subarctic oil spill over 25 years: the Caribou-Poker Creeks Research Watershed experiment

A small experimental oil spill conducted in an open black spruce forest within the Caribou-Poker Creeks Research Watershed (CPCRW), 48 km north of Fairbanks, AK, in the winter of 1976 was designed to examine the effects of crude oil spills in permafrost terrain. No clean-up was attempted, and the site now provides an opportunity to follow the natural weathering of spilled oil under these conditions. In summer 2001, more than 25 years after the spill, we sampled soils from the spill plot and a nearby reference plot to determine how the oil had weathered, and to assess microbial populations and activity. All samples collected from the oiled plot contained substantial amounts of methylene chloride extractable oil, between 4% and 66% by weight. Using 17α(H)21β(H)hopane as a conserved internal marker within the oil, we determined that while some heavily oiled samples were almost unchanged since the spill, others had lost more than 80% of their initial hydrocarbon. Evaporation, biodegradation and photooxidation all seem to have played important roles in this process, but to varying degrees in different samples. Assays of culturable populations of total heterotrophs and crude oil emulsifiers, and mineralization potentials for hexadecane and phenanthrene, indicate that the microbial population in the oiled soils has remained acclimated to degrade hydrocarbons. We conclude that natural weathering processes will eventually lead to the removal of much of the hydrocarbon from these heavily oiled subarctic soils; however, the combination of low rates of nutrient turnover, a short thaw season, and high hydrocarbon concentrations will result in the persistence of oil residue for many more decades. Finding an environmentally appropriate cleanup technology for sites like this remains an important challenge for future research.