Processes Affecting the Fate of Monoaromatic Hydrocarbons in an Aquifer Contaminated by Crude Oil

Abstract

Crude oil spilled from a subsurface pipeline in northcentral Minnesota has dissolved in the groundwater, resulting in the formation of a plume of aliphatic, aromatic, and alicyclic hydrocarbons. Comparison of paired oil and groundwater samples collected along the central axis of the residual oil body shows that the trailing edge of the oil is depleted in the more soluble aromatic hydrocarbons (e.g., benzene, toluene, etc.) when compared with the leading edge. At the same time, concentrations of monoaromatic hydrocarbons in groundwater beneath the oil increase as the water moves toward the leading edge of the oil. Immediately downgradient from the leading edge of the oil body, certain aromatic hydrocarbons (e.g., benzene) are found at concentrations near those expected of a system at equilibrium, and the concentrations exhibit little variation over time (≈8-20%). Other compounds (e.g., toluene) appear to be undersaturated, and their concentrations show considerably more temporal variation (≈20-130%). The former are persistent within the anoxic zone downgradient from the oil, whereas concentrations of the latter decrease rapidly. Together, these observations suggest that the volatile hydrocarbon composition of the anoxic 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 series and isomeric assemblages of alkylbenzenes reveals that microbial degradation is the dominant process controlling the fate of these compounds once groundwater moves away from the oil. For all but the most persistent compounds, the distal boundary of the plume at the water table extends no more than 10-15 m downgradient from the oxic/anoxic transition zone. Thus, transport of the monoaromatic hydrocarbons is limited by redox conditions that are tightly coupled to biological degradation processes.

Inverse modeling of BTEX dissolution and biodegradation at the Bemidji, MN crude-oil spill site

“…Observations of benzene, ethylbenzene, and m,p-xylene concentrations at some locations in the field were significantly higher than the effective solubilities predicted using the equilibrium data of Eganhouse et al (1996). This suggests that the equilibrium experiments may have underestimated effective solubilities because the oil samples had already undergone significant alteration in the field.…”

“…The plume was represented by two operationally defined degradable fractions-volatile dissolved organic carbon (VDOC) and nonvolatile dissolved organic carbon (NVDOC). However, evidence suggests that individual hydrocarbon compounds degrade at different rates (Eganhouse et al, 1993a(Eganhouse et al, , 1996. Also, the model used a simple source representation that did not account for dissolution of hydrocarbons from the oil body, and trial-and-error was used to calibrate the model.…”

“…In 1987, Eganhouse et al (1996) collected three oil samples from locations 35 m upgradient of the center of the oil body, at the center of the oil body (Well 421), and 25 m downgradient from the center of the oil body. The elevation and oil saturation of the samples was not determined at the time of collection.…”

“…The equilibrium water phase monoaromatic hydrocarbon concentrations obtained from oil -water equilibration experiments were used to determine the fraction of each compound in the oil. This varied from the upgradient to the downgradient part of the oil body, with the highest values generally being in the least weathered core of the oil body (Eganhouse et al, 1996).…”

“…For the purposes of modeling, and because the original oil composition was unknown, the highest measured concentration of each compound in the equilibration experiments of Eganhouse et al (1996) was assumed to be the effective solubility of the component. The effective solubilities used in the model were: benzene 10.6 mg/l; toluene 4.21 mg/l; ethylbenzene 0.524 mg/l; m,p-xylene 1.26 mg/l; and o-xylene 0.713 mg/l.…”