Importance of methanogenesis for organic carbon mineralisation in groundwater contaminated by liquid effluent, South Australia

Groundwater in the vicinity of a former abattoir and cheese factory in south-east South Australia has very high concentrations of dissolved inorganic carbon (DIC), total alkalinity (ΣAlk) and organic and inorganic N due to the disposal of organic-rich effluent down drainage boreholes directly into a limestone aquifer. One of the possible means of reducing potential nitrate contamination of the aquifer is by adding organic carbon as a substrate for inducing bacterial denitrification. A mass balance model of alkalinity, dissolved inorganic carbon (CO₃^2- + HCO₃^- + CO₂) and δ¹³C was used to evaluate this approach and to determine the fate of organic carbon derived from the waste. Isotope and chemical data could be used to distinguish the plume of contaminated groundwater and identify regions which are dominated by methanogenesis or those regions where oxidation reactions dominate. The model shows that methanogenesis is responsible for degradation of the largest fraction (up to 80%) of the organic carbon present. Organic carbon oxidation, rather than methanogenesis, is a more important mechanism for C_org degradation in the less contaminated boreholes farthest from the factory. Calcite dissolution appears to be unimportant, at least on the time scale since disposal of effluent (∼70 a).