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Origin of methane and sources of high concentrations in Los Angeles groundwater

June 20, 2018

In 2014, samples from 37 monitoring wells at 17 locations, within or near oil fields, and one site >5 km from oil fields, in the Los Angeles Basin, California, were analyzed for dissolved hydrocarbon gas isotopes and abundances. The wells sample a variety of depths of an aquifer system composed of unconsolidated and semiconsolidated sediments under various conditions of confinement. Concentrations of methane in groundwater samples ranged from 0.002 to 150 mg/L—some of the highest concentrations reported in a densely populated urban area. The δ13C and δ2H of the methane ranged from −80.8 to −45.5 per mil (‰) and −249.8 to −134.9‰, respectively, and, along with oxidation‐reduction processes, helped to identify the origin of methane as microbial methanogenesis and CO2 reduction as its main formation pathway. The distribution of methane concentrations and isotopes is consistent with the high concentrations of methane in Los Angeles Basin groundwater originating from relatively shallow microbial production in anoxic or suboxic conditions. Source of the methane is the aquifer sediments rather than the upward migration or leakage of thermogenic methane associated with oil fields in the basin.

Publication Year 2018
Title Origin of methane and sources of high concentrations in Los Angeles groundwater
DOI 10.1002/2017JG004026
Authors Justin T. Kulongoski, Peter B. McMahon, Michael Land, Michael Wright, Theodore Johnson, Matthew K. Landon
Publication Type Article
Publication Subtype Journal Article
Series Title Journal of Geophysical Research: Biogeosciences
Index ID 70197806
Record Source USGS Publications Warehouse
USGS Organization California Water Science Center