A reactive biogeochemical model and a heat transport model were used together at a crude oil-contaminated aquifer near Bemidji, Minnesota to help elucidate whether temperature data can be used as an inexpensive proxy for monitoring contaminant biodegradation. The site was contaminated by a crude-oil pipeline rupture in 1979 and the natural attenuation processes have been studied since 1983. Biodegradation of the crude oil contaminants creates a methanogenic zone near the source and aerobic zones on the plume fringes; Fe(III) oxyhydroxides are the main terminal electron acceptor in the plume center. The fate of the reduced carbon from the crude oil source and electron acceptors including dissolved oxygen, CO<sub>2</sub>, and Fe(III) were modeled using PHT3D, a finite-difference, reactive multicomponent transport model. The iron-reduction rate is constrained by concentrations of bioavailable Fe(III) from sediments collected during 1988-2007.<br/><br/>Heat production and transport in the subsurface were simulated using SUTRA, a finite-element, groundwater flow and energy transport model. The model results demonstrate that heat from microbial activity is required to account for the increase in temperature observed in the unsaturated zone and groundwater at the site. The enthalpy for the aerobic degradation of hexadecane, a compound representative of crude oil, is -10,615 kJ whereas the enthalpy for iron reduction of hexadecane is -7,358 kJ. Both values are orders of magnitude larger than the enthalpy of methanogenic degradation (-14 kJ), indicating methanogenesis is not a significant source of heat. In this presentation, heating rates are estimated from iron-reduction rates in the PHT3D model and compared to heating rates required in the SUTRA model to reproduce observed aquifer temperatures.

Comparing exothermic reaction rates using geochemical and heat-flow modeling of a contaminated aquifer

251st National Meeting (2016)

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