PROCESSES AFFECTING THE FATE OF MONOAROMATIC HYDROCARBONS IN AN AQUIFER CONTAMINATED BY CRUDE-OIL

Crude oil spilled from a subsurface pipeline in northcentral Minnesota has dissolved in the groundwater, resulting in the formation of a plu me of aliphatic, aromatic, and alicyclic hydrocarbons, Comparison of p aired oil and groundwater samples collected along the central axis of the residual oil body shows that the trailing edge of the oil is deple ted in the more soluble aromatic hydrocarbons tag, benzene, toluene, e tc.) when compared with the leading edge. At the same time, concentrat ions of monoaromatic hydrocarbons in groundwater beneath the oil incre ase as the water moves toward the leading edge of the ail. Immediately downgradient from the leading edge of the oil body, certain aromatic hydrocarbons (e.g., benzene) are found at concentrations near those ex pected of a system at equilibrium, and the concentrations exhibit litt le variation over time (approximate to 8-20%). Other compounds (e.g., toluene) appear to be undersaturated, and their concentrations show co nsiderably more temporal variation (approximate to 20-130%). The forme r are persistent within the anoxic zone downgradient from the oil, whe reas concentrations of the latter decrease rapidly. Together, these ob servations suggest that the volatile hydrocarbon composition of the an oxic groundwater near the oil body is controlled by a balance between dissolution and removal rates with only the most persistent compounds reaching saturation, Examination of the distributions of homologous se ries and isomeric assemblages of alkylbenzenes reveals that microbial degradation is the dominant process controlling the fate of these comp ounds once groundwater moves away from the oil. For all but the mast p ersistent compounds, the distal boundary of the plume al the water tab le extends no more than 10-15 m downgradient from the oxic/anoxic tran sition zone. Thus, transport of the monoaromatic hydrocarbons is limit ed by redox conditions that are tightly coupled to biological degradat ion processes.