Methane from the plume center has a 13C content of -54 ± 2 per mil. Here, carbon isotopic compositions of dissolved inorganic carbon (DIC) are high (> 10 per mil), typical of methanogenesis. Methane concentration decreases and 13C content increases to as high as -30 per mil around the margins of the plume, strong evidence for microbial methane oxidation. Low dissolved oxygen levels (< 5 µM) in all but the shallowest wells suggest this process is mediated by anaerobes or microaerophiles. DIC 13C contents in margin areas also show evidence of methane oxidation, but remain high because DIC contributed by methane oxidation is overwhelmed by the high DIC content of the leachate plume.
Two different modes of methane oxidation are suggested by distinct 13C enrichment factors calculated from d13C-concentration trends. We observe a large value (-13 per mil) within the distal portion of the central plume, but a smaller value (-4 per mil) at the upper and lower margins of the plume nearer the landfill. We hypothesize that these fractionation differences reflect differences in methane oxidation rate or mechanism associated with electron acceptor availability. High concentration gradients at the upper and lower margins of the plume result in more rapid oxidation rates and less isotopic fractionation relative to the distal portion of the plume. Future microbiological studies will explore the possibility that different oxidation processes (microaerophilic versus anaerobic) operate at different parts of the plume. Understanding these differences is important for predicting the spatial distribution of bioremediation in landfill-contaminated aquifers.
Jason Masoner