Methane Isotope Instrument Validation and Source Identification at Four Corners, New Mexico, United States

Measurements of delta(CH4)-C-13 and CH4 concentration were made at a field site in Four Corners, New Mexico (FC), where we observed large sustained CH4 enhancements (2-8 ppm peaks for hours) during nocturnal inversions. Potential sources of this large CH4 signal at FC include (1) fugitive emissions from coal mining and gas processing that are thermogenic and isotopically C-13 enriched relative to background atmosphere and (2) emissions from agriculture, ruminants, landfills, and coalbed biogenic methane that are C-13 depleted relative to background atmosphere. We analyze our measurements of methane concentration and delta C-13 during spring and summer of 2012 to identify fugitive methane sources. We find CH4 plumes that are both enriched and depleted in C-13 relative to CH4 in background air. Keeling plots show a continuum of delta C-13 source compositions between -40 parts per thousand and -60 parts per thousand that are consistent with thermogenic and biogenic sources. The Picarro Mobile Methane Investigator (PMMI), a mobile delta(CH4)-C-13 instrument platform, was deployed in the spring of 2013 and used to verify the isotopic enrichment of coal bed methane in the region. We combine our results with meteorological data to spatially separate these sources in the Four Corners regions. Using CO and CO2 data, along with meteorological data, we propose that the high methane concentration events ([CH4] > 3.5 ppm) are from both thermogenic and biogenic methane released from coal beds.