Biodegradation of crude oil into nonvolatile organic acids in a contaminated aquifer near Bemidji, Minnesota

As the result of a pipeline burst, a body of light aliphatic crude oil floa ts atop the groundwater in a shallow sand and gravel aquifer in a remote ar ea outside Bemidji, Minnesota. Biodegradation has resulted in the formation of a plume of DOC downgradient from the oil body. Groundwater has also bee n contaminated in an area known as the spray zone, from vertical infiltrati on of DOC resulting from biodegradation of crude oil in the overlying unsat urated zone. The majority of DOC in the contaminated groundwater is in the form of nonvolatile organic acids (NVOA's)which represent the partial oxida tion products of the crude oil constituents. The NVOA's have ken classified into three fractions according to their isolation on XAD resins: hydrophob ic neutrals (HPO-N), hydrophobic acids (HPO-A) and hydrophilic acids (HPI-A ). These fractions of NVOA's were isolated from a well downgradient from th e oil body (well 530; DOC = 21 mg C/l), from a well in the spray zone (well 603; DOC = 15 mg C/l) and from an uncontaminated well upgradient of the oi l body where the naturally occurring DOC is 2.9 mg C/l (well 310). The thre e sets of NVOA's were characterized by elemental analyses, molecular weight determinations, C-14 ages and liquid phase H-1 and C-13 NMR. The crude oil and the saturate, aromatic, resin and asphaltene fractions of the crude oi l were similarly analyzed by elemental analysis and NMR. The NVOA's from th e contaminated wells were clearly distinguishable from the naturally occurr ing groundwater DOC. Based upon molecular weights, sulfur contents, aro mat icities and the presence of methyl groups bonded to aromatic rings, the cha racterization data suggests that the NVOA's originate from the C-18 or grea ter alkylaromatic, naphthenoaromatic and sulfur-containing constituents of the crude oil, including possibly the resins and asphaltenes. (C) 1998 Else vier Science Ltd. All rights reserved.