Methanogenesis and secondary processes in a fractured claystone aquifer influenced by a former landfill site: Implications for the interpretation of dissolved gases

Abstract

Similar to marine and/or freshwater sediments, dissolved methane and longer chained hydrocarbons are ubiquitous in all natural groundwater environments including highly productive porous aquifers. The isotopic and chemical characteristics of dissolved gases in groundwater related to municipal landfills commonly differ from other non-polluted environments. In the survey presented in this study we investigated the geochemical and isotopic variations of dissolved hydrocarbons in a unique study area with different well defined ecological micro-habitats influenced by a former landfill for industrial and special waste. The dissolved methane concentrations have generally remained stable over the last decade but vary by several orders of magnitude in different zones of the area. The dominant methanogenic pathways can be clearly distinguished in the different ecosystems. Methyl-type fermentation methanogenesis prevails in the shallow groundwater zone (5–15 m depth) in- and outside the landfill. Anaerobic methane oxidation coupled to sulfate reduction occurs in the shallow downstream area within a former leachate plume. At larger depths CO2 reduction is the predominant methane formation pathway except within the physically isolated landfill site. The high ethane concentrations in a small part of the study area are potentially related to the degradation of volatile organic compounds. The background ethane concentrations are higher in the deeper groundwater zones, indicating the coupling to the CO2 reduction methanogenic pathway. The propane concentrations in most wells are below the detection limit, except within the waste site.Secondary methane oxidation, which frequently occurs in the groundwater in all environments can alter the gas composition mimicing the thermogenic gas signature in the standard Bernard-diagram. The isotopic fractionation between co-existing methane and formation water and carbon dioxide, respectively, differs from that suggested in standard interpretational diagrams (e.g. Bernard-Diagram and Whiticar-plots). Thus, the standard evaluation based on methane hydrogen and carbon isotopes is also susceptible to an erroneous interpretation as non-biogenic dissolved gases in other groundwater environments.