UNSATURATED ZONE TEMPERATURE INCREASES DUE TO AEROBIC METHANOTROPHIC ACTIVITY

In 1979 a pipeline ruptured near Bemidji, Minnesota, USA, releasing crude oil onto the land surface. In spite of a major cleanup effort, oil infiltrated into the glacial outwash aquifer and is now present in the unsaturated zone and at the water table 6-8 m below the surface. Previous results demonstrated that methane from methanogenic degradation of the subsurface oil diffuses upward where it mixes with oxygen from the surface, supporting an active methanotrophic zone at 2-4 m depth. Aerobic oxidation of the biogenic methane should result in a measureable increase in temperature due to the enthalpy of reaction (CH_4(aq) + 2O_2(aq) -> HCO₃^- + H⁺ + H₂O, ΔH^o = -865.4 kJ/mol CH₄). To test this hypothesis, vertical temperature profiles at four sites were measured in 3.8-cm PVC tubes filled with water in boreholes extending from the surface to the water table (5.5-6.5 m depth). Three sites were located above the oil, and a background site was 65 m away, beyond the influence of the oil contamination. All of the sites were in full sunlight. In addition to temperature, surficial carbon dioxide efflux was measured at each site using flux chambers. Sediment samples were collected for determination of microbial concentrations using DNA quantitation (qPCR with primers in parentheses) of aerobic methanotrophs (pmoA), iron reducers (Geo494F/Geo825R), sulfate reducers (dsrB), and methanogens (mcrA). Oil samples were collected at each oil-contaminated site for non-volatile hydrocarbon analysis by GC/MS.

Temperatures near the water table at the three oil-contaminated sites were about 1°C more than at the background site. At the level of the methanotrophic zone, temperatures were elevated 2-6°C over the background values and these temperature differences continued to the land surface. Carbon dioxide efflux over the oil sites averaged more than twice that at the background site. Methanotroph concentrations were 10-1000 times higher above the oil-contaminated sediments compared with background concentrations. Results support the hypothesis that there is an active methanotrophic community above the oil body that is increasing the temperatures in the unsaturated zone. Future efforts will focus on comparing the magnitude of temperature increases to prokaryote populations in the oil, degradation state of the oil, and carbon dioxide efflux.