Journal
HYDROGEOLOGY JOURNAL
Volume 24, Issue 1, Pages 109-122Publisher
SPRINGER
DOI: 10.1007/s10040-015-1319-1
Keywords
Groundwater monitoring; Stable isotopes; Methane; Canada; Sampling technique
Categories
Funding
- Natural Sciences and Engineering Research Council (NSERC) of Canada
- Alberta Innovates Energy & Environment Solutions (AI-EES), Alberta Environment
- University of Calgary's Eyes High postdoctoral fellow program
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Variability in baseline groundwater methane concentrations and isotopic compositions was assessed while comparing free and dissolved gas sampling approaches for a groundwater monitoring well in Alberta (Canada) over an 8-year period. Methane concentrations in dissolved gas samples (n= 12) were on average 4,380 +/- 2,452 mu g/L, yielding a coefficient of variation (CV) >50 %. Methane concentrations in free gas samples (n= 12) were on average 228,756 +/- 62, 498 ppm by volume, yielding a CV of 27 %. Quantification of combined sampling, sample handling and analytical uncertainties was assessed via triplicate sampling (CV of 19 % and 12 % for free gas and dissolved gas methane concentrations, respectively). Free and dissolved gas samples yielded comparable methane concentration patterns and there was evidence that sampling operations and pumping rates had a marked influence on the obtained methane concentrations in free gas. delta C-13(CH4) and delta H-2(CH4) values of methane were essentially constant (-78.6 +/- 1.3 and -300 +/- 3%, respectively) throughout the observation period, suggesting that methane was derived from the same biogenic source irrespective of methane concentration variations. The isotopic composition of methane constitutes a robust and highly valuable baseline parameter and increasing delta C-13(CH4) and delta H-2(CH4) values during repeat sampling may indicate influx of thermogenic methane. Careful sampling and analytical procedures with identical and repeatable approaches are required in baseline-monitoring programs to generate methane concentration and isotope data for groundwater that can be reliably compared to repeat measurements once potential impact from oil and gas development, for example, may occur.
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