Applied Geochemistry, Vol. 6, pp. 533--542, 1991 0883-2927/91 $3.00 + .00 Printed in Great Britain © 1991 Pergamon Press pie Importance of methanogenesis for organic carbon mineralisation in groundwater contaminated by liquid effluent, South Australia ANDREW L. HERCZEG, STUART B. RICHARDSON* and PETER J. DILLON Centre for Groundwater Studies and CSIRO Division of Water Resources, Private Bag No. 2, Glen Osmond, South Australia 5064, Australia (Received 2 October 1990; accepted in revised form 25 April 1991) Abstract 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 (~Alk) 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 (CO2- + HCO 3 + CO2) and 613C 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 Corg 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). INTRODUCTION GROUNDWATER in the southern part of south-east South Australia is a major water resource for agricul- tural and domestic use. In recent years studies have shown that this groundwater is under threat from nitrate contamination. Thirty percent of boreholes in a 1000 km 2 area within the region had nitrate levels above the 10 mg/1-N guideline set by the World Health Organisation for human consumption (DILLON, 1988). High nitrate concentrations in groundwater in this region are due to animal wastes and organic material from leguminous crops leaching through the un- saturated zone to the groundwater. Further input of nitrates to groundwater occurs through localised dis- posal of cheese factory and abattoir effluent and piggery, dairy and sale-yard wastes. Near a cheese factory and abattoir at Yahl, South Australia, RICHARDSON (1990) used geophysical methods to delineate a contaminant plume. Six observation boreholes were drilled based on the results of that survey. Groundwater sampling revealed high con- centrations of ammonium, nitrate, and total alka- linity (RICHARDSON, 1990). This paper discusses the distribution of the dis- solved inorganic carbon (CO 2 + HCO 3 - + CO~-) and the associated 13C concentrations. By comparing the 613C values obtained for the polluted ground- *Also at Flinders University of South Australia. Present address: South Australian Department of Agriculture, 25 Grenfell Street, Adelaide, SA 5000, Australia. water samples with the 613C values for background, an attempt is made to distinguish the polluted from non-polluted water. Further, given the large vari- ations of 613C values of potential sources of added dissolved inorganic carbon (see below), mass balance equations can be used to quantify the contributions from each source. The present day isotopic signature of the DIC pool in the groundwater reflects the sum of all chemical and physical processes that have occurred since the factories began operations. There- fore, the measurement of 13C abundances may help us to understand the long-term fate of organic carbon in these types of environments. An understanding of carbon cycling in this system is important because strategies for remediation of plumes contaminated with nitrogen may involve adding organic carbon as a substrate for bacterially mediated denitrification. However, anoxification of phreatic aquifers by high organic carbon.loads may present a hazard by pro- duction of large quantities of methane (cf. RONEN et al., 1987). Other studies using 13C in groundwaters have fo- cused on deep groundwater systems in attempting to determine the origin of methane (BARKERand FRITZ, 1981; CHAP~LLE et al., 1988; GROSSUAN et al., 1989). Also, studies on interstitial waters in lakes and re- stricted marine basins have found a large component of methanogenesis with high Corg fluxes (e.g. NISSEN- BAUM et al., 1972; TU~ER and FRITZ, 1982; H~RCZ~G, 1988). BAEDECKER and BACK (1979a,b) and KF.HEW and PASSERO(1990) used reaction stoichiometry and 13C to model geochemical processes occurring in groundwaters near landfills. 533