The use of biodegradation signatures and biomarkers to differentiate spills of petroleum hydrocarbon liquids in the subsurface and estimate natural mass loss

The history of diesel fuel spills into a sand aquifer was investigated using biodegradation signatures and biomarker compounds of the light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. Samples of the weathered LNAPL diesel from 28 monitoring wells, along with LNAPL from cores, in a 2 ha plume were characterised by GC–MS techniques. The relative extent of biodegradation was assessed using ratios of partially degraded and resistant compounds including n-alkanes (n-C17), pristane and phytane. Biomarkers cadalene and methyldibenzothiophenes were also used to differentiate two different crude oil sources (Middle Eastern and South-east Asian). Age relationships were also investigated from the LNAPL composition. Both the extent and distribution of biodegradation and biomarkers identified a recent spill into the aquifer and traced its spread within the existing LNAPL plume. Evidence of an older, discrete and more degraded spill was also identified within the larger plume. Importantly, these analyses also identified a loss of as much as 80% of spilled diesel mass through biodegradation and other natural attenuation processes. Oxidised hydrocarbon material immediately above the LNAPL in the profile and reduced pristane:phytane ratios at the top of the LNAPL profile, provide supporting evidence of aerobic biodegradation.