Estimating methane production rates in bogs and landfills by deuterium enrichment of pore water

Raised bogs and municipal waste landfills harbor large populations of methanogens within their domed deposits of anoxic organic matter. Although the methane emissions from these sites have been estimated by various methods, limited data exist on the activity of the methanogens at depth. We therefore analyzed the stable isotopic signature of the pore waters in two raised hogs from northern Minnesota to identify depth intervals in the peat profile where methanogenic metabolism occurs. Methanogenesis enriched the deuterium (H-2) content of the deep peat pore waters by as much as +11 parts per thousand (Vienna Standard Mean Sea Water), which compares to a much greater enrichment factor of +70 parts per thousand in leachate from New York City's Fresh Kills landfill. The bog pore waters were isotopically dated by tritium (H-3) to be about 35 years old at 1.5 m depth, whereas the landfill leachate was estimated as similar to 17 years old from Darcy flow calculations. According to an isotopic mass balance the observed deuterium enrichment indicates that about 1.2 g of CH4 m(-3) d(-1) were produced within the deeper peat, compared to about 2.8 g CH4 m(-3) d(-1) in the landfill. The values for methane production in the bog peat are substantially higher than the flux rates measured at the surface of the bogs or at the landfill, indicating that deeper methane production may be much higher than was previously assumed.