CRUDE-OIL IN A SHALLOW SAND AND GRAVEL AQUIFER .2. ORGANIC GEOCHEMISTRY

Crude oil spilled from a pipeline break in a remote area of north-cent ral Minnesota has contaminated a shallow glacial outwash aquifer. Part of the oil was sprayed over a large area to the west of the pipeline and part of it accumulated in an oil body that floats at the water tab le to the east of the point of discharge. Total dissolved organic carb on (TDOC) concentrations in shallow groundwater collected in the oil s pray area reach 16 mg/l. This is nearly an order of magnitude higher t han the TDOC concentrations of native groundwater (approximately 2-3 m g/l). The additional TDOC derives from the partial degradation of petr oleum residues deposited at the land surface and transported to the aq uifer by vertical recharge. In the vicinity of the oil body, TDOC conc entrations in groundwater are 48 mg/l, 58% of the TDOC being composed of non-volatile organic C. The majority of the volatile DOC (63%) is a mixture of low-molecular-weight saturated, aromatic and alicyclic hyd rocarbons derived from the oil. Downgradient from the oil body along t he direction of groundwater flow, concentrations of all measured const ituents of the TDOC pool decrease. Concentrations begin to decline mos t rapidly, however, in the zone where dissolved O2 concentrations begi n to increase, approximately 50 m downgradient from the leading edge o f the oil. Within the anoxic zone near the oil body, removal rates of isomeric monoaromatic hydrocarbons vary widely. This indicates that th e removal processes are mediated mainly by microbiological activity. M olecular and spectroscopic characterization of the TDOC and its spatia l and temporal variation provide evidence of the importance of biogeoc hemical processes in attenuating petroleum contaminants in this pertur bed subsurface environment.