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

Journal of Geophysical Research: Biogeosciences

By:
ORCID iD , ORCID iD , ORCID iD , ORCID iD , , and ORCID iD
https://doi.org/10.1002/2017JG004026

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Abstract

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.

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Additional publication details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Origin of methane and sources of high concentrations in Los Angeles groundwater
Series title:
Journal of Geophysical Research: Biogeosciences
DOI:
10.1002/2017JG004026
Volume:
123
Issue:
3
Year Published:
2018
Language:
English
Publisher:
American Geophysical Union
Contributing office(s):
California Water Science Center
Description:
14 p.
First page:
818
Last page:
831
Country:
United States
State:
California
City:
Los Angeles
Other Geospatial:
Los Angeles Basin