An 8-year record of gas geochemistry and isotopic composition of methane during baseline sampling at a groundwater observation well in Alberta (Canada)

Humez, P.; Mayer, B.; Nightingale, M.; Ing, J.; Becker, V.; Jones, Don; Lam, Vien

An 8-year record of gas geochemistry and isotopic composition of methane during baseline sampling at a groundwater observation well in Alberta (Canada) | 8 ans d’enregistrement de données de géochimie des gaz et de la composition isotopique du méthane durant les campagnes de surveillance de base d’un puits d’observation des eaux souterraines en Alberta (Canada) Un registro de 8 años de la geoquímica de gases y la composición isotópica del metano durante el muestreo de la línea de base en un pozo de observación de agua subterránea en Alberta (Canadá) (加拿大)亚伯达省地下水观测井基线采样期间甲烷地球化学和同位素组分的8年记录 Um registro de 8 anos da composição isotópica e geoquímica do metano, para a determinação das concentrações de base em um poço de observação em Alberta (Canadá)

2016

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 μg/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. δ¹³CCH₄ and δ²HCH₄ 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 δ¹³CCH₄ and δ²HCH₄ 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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