Natural Source Zone Depletion of Petroleum Hydrocarbons at the Water Table

SPEAKER: Barbara Bekins

DATE AND TIME:  Friday, 8 April 2022 • 2:00 pm (HST)

To develop an appropriate mitigation strategy for groundwater contamination from long-term petroleum hydrocarbon spill sites, it is critical to understand how the mass and composition of the source changes over time. This presentation will focus on the research conducted at the site of a 1979 oil spill in northern Minnesota. To estimate the total mass lost, oil samples were collected in 2019 and 2020 and were analyzed for conservative biomarker concentrations, and then compared to a spilled oil reference sample. After 40 years, an estimated 50% of the total oil mass had been lost with values ranging from 22% to 57%, depending on the location. The analyses of the volatile concentration data showed that the losses of oil compounds were controlled by the compound’s susceptibility to degradation under methanogenic conditions and solubility. To study what happened to the compounds lost from the oil, the following were used: (1) carbon dioxide efflux, (2) groundwater concentration data, and (3) modeling. An estimated 86% of the carbon lost, exited the surface as carbon dioxide. The dissolved organic carbon (DOC) migrated to the saturated zone as hydrocarbons (22%), and partially transformed hydrocarbon products (78%). The concentrations of DOC decreased exponentially within 150 m from the source of contamination, indicating further biodegradation. The results on the nature and biological effects of the partial transformation products will be presented.

Dr. Barbara Bekins is a research hydrologist with the U.S. Geological Survey, Water Mission Area, located in Menlo Park, CA. She has studied the fate and transport of crude oil contaminants in groundwater for over 25 years. Barbara serves as research coordinator for the USGS crude oil spill study site near Bemidji, Minnesota.

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