Previous workers have observed that ratios of short-chain aliphatic acid anions have relatively constant slope. For example, Shock (1988) shows a slope of 3:2 for propanoic versus ethanoic acid anions from high temperature (>95°C) oil field waters. A similar plot (Figure 1) for samples from oilfields and shallow groundwater shows that low temperature (<95°C) samples have a slope of 1:1. Based on the concentration and isotopic composition of dissolved carbon and sulfur species, the 1:1 slope is associated with microbial sulfate reduction (BSR). Using data from Shock and Helgeson (1990) and Helgeson et al (1993) equilibrium constants for the reaction:

CH₃CH₂COOH + 3/2O ₂=CH₃COOH + H ⁺ + HCO ₃ ^-

can be calculated. Using dissolved bicarbonate concentrations from oil field waters and landfill leachates at low temperatures (<95°C) and assuming equilibrium with calcite, the fugacity of oxygen can be calculated. The calculated oxygen fugacity suggests that these waters are apparently in redox equilibrium with CO ₂, rather than CH ₄. As well as controlling sulfur species in solution, microbial reactions appear to play a role in determining ratios of organic acids in low temperature waters – and hence influence their oxygen fugacity.

Figure 1. Covariation between ethanoic and propanoic acid anions in Gulf Coast oilfield waters (c.f. Shock, 1988), oilfield waters from Thistle, UKCS (67-93°C) and landfill leachates from the UK (Manning, 1997; <30°C).