Methanogenesis in a shallow sandy aquifer, Romo, Denmark

The degradation of organic matter and the formation of methane were investi gated in a shallow sandy aquifer. The aquifer was found to be anoxic from t he water table downward; the upper 2 m contained sulfate and was enriched i n Fe(II). Methane was present in the groundwater from 2 to 3 m below the wa ter table in concentrations of up to 0.4 mM. Fermentative metabolic interme diates such as acetate and formate were present at levels of a few micromol es, whereas hydrogen concentrations ranged from 0.1 to 8 nM. Radiotracer me thods 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 measu red in the same zone where sulfate reduction and the reduction of iron oxid es 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 fermentatio n. CO2 reduction was found to be the dominant pathway, although in some cas es acetate fermentation contributed up to 50% of the total methane formatio n rate. High spatial variation, both vertically and horizontally, in methan e fort-nation 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 -80 and -50%(omicron) fo r delta C-13(CH4), and the few available measurements for deltaD(CH4) are i n the range of -320 to -300%(omicron). The usual interpretation of the stab le 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 pathw ay of methane formation. Comparison of the depth distribution of the rates of sulfate reduction and methane formation with the H-2 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 ele ctron acceptors. The results indicate that the free energy gains are well c onstrained by bacterial metabolism and are close to the threshold for energ y storage. However, for CO2 reduction, the free energy gain is below the en ergy storage threshold, which suggests methane formation predominantly occu rs in microenvironments with higher H-2 concentrations. Copyright (C) 2001 Elsevier Science Ltd.