INTRINSIC BIODEGRADATION OF DIESEL FUEL IN AN INTERVAL OF SEPARATE PHASE HYDROCARBONS

Emerging acceptance of the limitations of separate phase product recov ery has spawned interest in the intrinsic alteration of residual separ ate phase petroleum products. In this study the geochemical changes in a continuous core through soil containing a separate phase diesel fue l #2 (SPD) in contact with groundwater are investigated. Chemical hete rogeneities are shown to exist which can be attributed to weathering, particularly intrinsic biodegradation. The results show that the aliph atic hydrocarbon content is reduced and the delta(13)C ratio of the al iphatic hydrocarbons increased from top to bottom in the core. Both ch anges are thought to be due to preferential biodegradation of (isotopi cally lighter) n-alkanes. A slight increase in the relative abundance of shorter chain n-alkanes (<n-C-17) was also observed. The distributi on of the dominant aromatic hydrocarbons (C-0-C-3 alkyl-naphthalenes) is remarkably consistent throughout the core, although naphthalene is depleted below the oil-water interface. In spite of low oil saturation (S-0), little or no evidence of biodegradation is noted at the upperm ost boundary of the SPD. However, intrinsic biodegradation is evident approximately 0.3 m above the oil-water interface in spite of higher S -0. The extent of the chemical changes attributable to biodegradation (described above) gradually increases below the oil-water interface, e ventually reaching a maximum at the bottom of the SPD profile (similar to 1.2 m below the interface) where S-0 is again reduced. The relativ ely higher level of biodegradation observed at and below the oil-water interface may be attributed to the reduced S-0 in this zone. An estim ate of the mass reduction in diesel fuel between the uppermost and bot tommost parts of the core is calculated to be 23% (by weight), due pre dominantly to the biodegradation of n-alkanes. (C) 1998 Elsevier Scien ce Ltd. All rights reserved.