Spatial distribution and temporal variation of methane, ethane and propane background levels in shallow aquifers – A case study from Lower Saxony (Germany)

Study region: This survey was conducted in Lower Saxony/Northern Germany, the major German hydrocarbon province. Study focus: The study aimed at improving our understanding of background concentrations for dissolved hydrocarbon gases in shallow groundwater. More than 1700 groundwater wells were sampled, determining the concentrations of dissolved methane, ethane and propane. New hydrological insights for the region: Methane was detected in all wells with 34% 1 ml/l (with 80% < 0.1 ml/l). A decreasing regional trend of methane concentrations from the North West towards South East was observed. Highest concentrations are encountered in marshes and lowlands although numerous exceptions are noted. Methane is prevalently of biogenic origin, as demonstrated by its isotopic composition. Generally, high methane concentrations are linked to high DOC, low sulfate and oxygen concentrations. Methane oxidation appears to be locally important. Ethane was detected in 30% of all wells, propane in 12.5%. The occurrence is interpreted as microbial/geogenic background related to methanogenesis or organic matter degradation during diagenesis. Low present day ethane and propane concentrations make them the preferred monitoring parameter with respect to possible anthropogenic induced input of thermogenic gas. Sampling of identical wells between 2014 and 2017 demonstrate that methane concentrations and carbon isotopic composition remain stable in general, although if it comes down to individual wells, significant fluctuations were noticed. Keywords: Dissolved hydrocarbon gases, Baseline monitoring, Temporal variation, Lower Saxony, Isotope