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Temperature Response of Soil Organic Matter Decomposition Rates: Construction and Applications of a Temperature Gradient Block
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Relating subsurface temperature changes to microbial activity at a crude oil-contaminated site
Ean Warren1, Barbara A Bekins1
1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States.
Journal of Contaminant Hydrology
|September 27, 2015
Summary
Heat generated by microbial degradation of crude oil spills is measurable. Aerobic methane oxidation in the unsaturated zone significantly increases temperatures, indicating temperature can be a low-cost indicator of biodegradation activity.
Area of Science:
- Environmental Science
- Geochemistry
- Microbiology
Background:
- Over 30 years of crude oil biodegradation at a Bemidji, Minnesota site has formed a large contaminant plume.
- Microbial activity during biodegradation generates heat, a potentially measurable phenomenon.
Purpose of the Study:
- To investigate the thermal effects of biodegradation at the Bemidji oil spill site.
- To determine if temperature measurements can serve as an indicator of microbial activity and contaminant plume evolution.
Main Methods:
- Installation of thermistors in monitoring wells within both saturated and unsaturated zones.
- Measurement of temperature variations in groundwater and soil above the residual oil body.
- Enthalpy calculations and comparison with CO2 efflux data to identify heat sources.
Main Results:
- A thermal groundwater plume was detected, with temperatures 2.9°C above background near the oil and 1.2°C down gradient.
- Unsaturated zone temperatures were up to 2.7°C higher than background.
- Observed temperature increases correlate with aerobic methane oxidation in the unsaturated zone.
Conclusions:
- Temperature increases are primarily driven by aerobic methane oxidation above the oil body.
- Temperature measurements offer a potentially low-cost method for assessing biodegradation activity.
- Accurate interpretation requires accounting for specific heat-generating reactions and environmental factors like recharge.
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