The origin of methane in an Eastern Ontario aquifer

Abstract

Data from a comprehensive isotopic study is presented for a bedrock interface aquifer affected by Pleistocene post-glacial marine transgression in Eastern Ontario, and hosting groundwaters with anomalously high dissolved methane concentrations surpassing Province of Ontario recommended guidelines for drinking water, in an effort to constrain both the enrichment mechanisms and sources of methane. Stable isotopic signatures (δ13C & δ2H) of collected methane gas samples indicate a mixture of microbial methane gases generated via CO2 reduction and acetoclastic methanogenic pathways. Radiocarbon data for methane and dissolved inorganic/organic carbon indicate that the dominant sources of carbon in the aquifer are marine organic matter dating from both the Champlain Sea incursion (10–12 ka BP) and ancient terrestrial organic matter from an Early-Wisconsinian interstadial period. The prevalence of ancient organic matter sources is particularly evident in the methane pool. During early stages of methanogenesis, CO2 derived from the preferential decomposition of labile marine organic matter is the dominant substrate for methanogenesis, whereas a substrate transition to acetate derived from the fermentation of refractory ancient terrestrial organic matter is observed during later stages of methanogenesis. Both sources of organic matter (marine, terrestrial) are present in the glaciomarine sediment matrix associated with Champlain Sea incursion. Contributions of thermogenic methane from the underlying Billings shale are possible, and merit further investigation.