ORIGINS OF METHANE AND H2S IN SOIL VAPORS AT THE BELMONT LEARNING CENTER IN LOS ANGELES: A CASE OF ANOXIC METHANE OXIDATION IN A TERRESTRIAL ENVIRONMENT?

The property of the Belmont Learning Center, a major new school development in South Central Los Angeles, is located on the southern rim of the Los Angeles City oil field which produced in the early 20th century heavy oils with very low gas oil rations (~50 cf/barrel oil). During the development of the property methane and H2S was detected in shallow borings. The concern was, that these gases migrate directly from the LA City oil field and pose a permanent danger to the school that could possibly not be mitigated. Analyses on the gases allow a better understanding on their origin. Methane carbon and hydrogen isotope analyses suggest that methane from 40 to 50 feet deep borings is similar to gases of the LA City oil field (del13C ~ -43‰; delD ~ -175‰). However, with decreasing depth methane isotope signatures change systematically to more positive values (del13C ~ -20 to -30‰; delD ~ 0 to +50‰). This isotope change can be related to the oxygen depletion in the fumes suggesting a oxygen-consuming removal process. We conclude that the methane is derived from incorrectly abandoned oil wells and is removed by microbial processes. This is supported by the observation that methane is observed mostly in the area of the old oil wells. The spatial distribution of H2S is similar to that of methane, suggesting a similar source. However, the LA City oil field reservoir, as all LA oil fields, very likely does not contain free H2S. The measured del 34 S of water-dissolved sulfate (del34S ~ +52‰) and the associated sulfide(del34S ~ +14‰) strongly suggests an origin of the H2S through sulfate reduction in the groundwater. The close spatial association of methane and H2S could indicate that the two are linked and that the methane-consuming and H2S-generating microbial consortia are co-dependent in their metabolism similar to the observations in marine sediments. Our data allow the conclusion, that the methane is exsolving from oil leakages of abandoned wells, whereas, the H2S is of shallow origin and most probaby linked to both the sulfate reduction and methane oxidation processes. We will discuss alternatives to the generally inferred direct reduction of sulfate by methanotrophic consortia.