Methanogenesis in a shallow sandy aquifer, Rømø, Denmark

The degradation of organic matter and the formation of methane were investigated in a shallow sandy aquifer. The aquifer was found to be anoxic from the water table downward; the upper 2 m contained sulfate and was enriched in Fe(II). Methane was present in the groundwater from 2 to 3 m below the water table in concentrations of up to 0.4 mM. Fermentative metabolic intermediates such as acetate and formate were present at levels of a few micromoles, whereas hydrogen concentrations ranged from 0.1 to 8 nM. Radiotracer methods were used to quantify organic matter degradation rates. In the upper part of the aquifer, rates of acetate oxidation of up to 4 mM/yr were measured in the same zone where sulfate reduction and the reduction of iron oxides takes place. Total methane formation rates range from 0.1 to 4 mM/yr and proceeds through both the pathway of CO2 reduction and acetate fermentation. CO2 reduction was found to be the dominant pathway, although in some cases acetate fermentation contributed up to 50% of the total methane formation rate. High spatial variation, both vertically and horizontally, in methane formation rates are a characteristic feature of this aquifer sediment. Therefore the groundwater methane concentration is not a reliable indicator for the occurrence and intensity of methanogenesis at a detailed scale. Methane stable isotope data yielded values between 280 and 250‰ for d13CCH4 , and the few available measurements for dDCH4 are in the range of 2320 to 2300‰. The usual interpretation of the stable isotope data would then suggest acetate fermentation to be the dominant pathway for methanogenesis, in conflict with the radiotracer data. However, recent evidence suggests the deuterium content of the groundwater to have a dominant effect on the deuterium content of methane rather than the pathway of methane formation. Comparison of the depth distribution of the rates of sulfate reduction and methane formation with the H2 concentration shows that the latter is not a reliable indicator of the predominant terminal electron acceptor process. The free energy of reaction was calculated for different substrates and electron acceptors. The results indicate that the free energy gains are well constrained by bacterial metabolism and are close to the threshold for energy storage. However, for CO2 reduction, the free energy gain is below the energy storage threshold, which suggests methane formation predominantly occurs in microenvironments with higher H2 concentrations. Copyright © 2001 Elsevier Science Ltd