Quantifying MTBE biodegradation in the Vandenberg Air Force Base ethanol release study using stable carbon isotopes.

Area of Science:

• Environmental Science
• Geochemistry
• Microbiology

Background:

• Methyl tert-butyl ether (MTBE) is a common groundwater contaminant.
• Ethanol is often used as a fuel oxygenate and can be released into the environment.
• Understanding MTBE biodegradation pathways is crucial for effective remediation strategies.

Purpose of the Study:

• To assess the impact of ethanol on MTBE biodegradation using compound-specific isotope analysis (CSIA).
• To quantify MTBE biodegradation rates under field conditions with and without ethanol addition.
• To investigate the biodegradation of tert-butyl alcohol (TBA), a common MTBE degradation product.

Main Methods:

• Two side-by-side field releases of groundwater were conducted, one with ethanol and one without.
• Compound-specific isotope analysis (CSIA) of carbon isotopes (δ13C) in MTBE and TBA was employed.
• MTBE biodegradation rates were calculated using isotopic enrichment factors and compared to mass-discharge data.

Main Results:

• A significant enrichment in 13C of MTBE (40.6 per thousand) was observed in the "With ethanol lane", indicating substantial MTBE biodegradation.
• MTBE biodegradation rates in the "With ethanol lane" ranged from 12.0 to 20.3 year(-1), consistent with previous estimates.
• Substantial amounts of TBA were produced, but its δ13C values remained uniform, suggesting limited TBA biodegradation during the experiment.

Conclusions:

• Ethanol addition significantly enhances MTBE biodegradation in groundwater.
• CSIA is a reliable method for assessing MTBE biodegradation and determining biodegradation rates.
• While MTBE biodegrades in the presence of ethanol, the co-produced TBA appears to be persistent under the studied conditions.