Evaluating the occurrence of methane in groundwater in southeastern New Brunswick

Abstract

Concerns over possible impacts from unconventional oil and natural gas (ONG) resource development prompted two regional domestic well sampling programs (476 wells) in southeastern New Brunswick, Canada. The regional programs presented a unique opportunity to better understand the factors affecting the occurrence of methane in shallow, <125 m depth, groundwater and this is the principal objective of this thesis. Recent developments in robust multivariate statistical methods were used to overcome issues with non-Gaussian geochemical data and support the development of a conceptual model to explain trends in the regional groundwater chemistry. Combined spatial, statistical and chemical analyses show that, while trace or low levels of methane, <1 mg/L, are common in New Brunswick groundwater, elevated concentrations, >1 mg/L, are associated with Horton Group bedrock, the host of ONG resources in the province. The near-surface structural juxtaposition of the Horton Group with other rock units of the Maritimes Basin may also lead to naturally high methane concentrations in groundwater from other units. Temporal variability in methane concentrations was predominantly observed in water from wells that have multiple, relatively low-yield, hydrogeochemical zones. The elevated methane concentrations correlate to zones with Na-HCO₃ type water. Biogenic and thermogenic isotopic signatures of methane were observed throughout the study areas. There is inherent uncertainty when inferring the sources of hydrocarbon gasses in shallow aquifers from δ¹³C and δ²H values; oxidation of biogenic methane and biogenic overprinting of thermogenic methane appear common and complicate interpretation. The effect of oxidation on isotopic compositions is apparent when C₁ concentrations were ≤1 mg/L. In some samples with C₁ concentrations >5 mg/L, the isotopic signature suggests a biogenic origin. However, molecular ratios <75 and the presence of >C₃ fractions, indicate a thermogenic origin; a thermogenic isotopic signature has been overprinted by biological activity. Reactive transport modeling was used to demonstrate that diffusive transport may produce secondary fractionations that could lead to isotopic signatures that overlap with those resulting from microbial oxidation and mixing from different sources. These results highlight the importance of acquiring multiple lines of evidence, such as evaluation of higher weight hydrocarbons (>C₃), isotopic analyses, inorganic geochemistry and local geologic knowledge, to provide an accurate assessment of sources of methane in groundwater.