Relative Contributions of Hypoxia and Natural Gas Production and Transport to Methane Emissions from Lake Erie

Abstract:

The central basin of Lake Erie exhibits seasonally hypoxic bottom waters, which contribute to biological methane (CH₄) production. Leaks from Canadian natural gas wells in rocks underlying Lake Erie are a potential contributor to CH₄ emissions. Previous work shows that Lake Erie is a positive source of CH₄ to the atmosphere in late summer, with some evidence for enhanced CH₄ emissions from regions with natural gas wells. This study updates past CH₄ emission studies of Lake Erie, utilizing stable isotopic composition to determine the distribution of CH₄ sources. d¹³C-CH₄ ratios were measured using the University of Cincinnati’s isotope ratio mass spectrometer. Samples collected in late summer of 2013, during the period of widespread hypoxia in the central basin, indicate d¹³C-CH₄ enrichment in locations near gas wells. This is consistent with previous studies showing that thermogenic CH₄ is enriched in ¹³C relative to biological CH₄. The d¹³C-CH₄ in water at hypoxic sites is -58.6 ‰, while locations near gas wells average d¹³C of -42.5 ‰. Concentrations of CH₄ in bottom waters were similar in both hypoxic and natural gas extraction regions. This preliminary data will be further assisted by isotopic data obtained during spring and summer of 2014. In addition, recent samples gathered early summer 2015 are currently under analysis, and fieldwork scheduled for late summer and fall 2015 will further build on this data set. Ongoing work includes additional measurements of hydrogen isotopic composition of methane and CH₄ efflux throughout the year.