The carbon source for effervescent sodium-bicarbonate groundwater in the Ballimore region, central New South Wales

This paper describes the integrated use of stable isotopic (δ13C) data for delineating the origin of carbon in groundwater systems. Groundwater in the Ballimore region is unusual: the main interests of this study are artesian boreholes that produce effervescent Na-HCO3 (soda) waters. Carbon-dioxide gas is present in concentrations of up to 1g/l, and CO2 partial pressures are one thousand times higher than atmospheric levels. Major-ion geochemical interpretation indicates that the evolution of the effervescent Na-HCO3-rich groundwater in the region relies on fresh water entering the deep cell of the local flow system from Late Jurassic depocentres that are filled with fresh water. This groundwater flows S-SE through the fracture network that constitutes the deep cell of the local groundwater system. As it travels along the flowpath it mixes with CO2(g): decreasing the groundwater pH and making it chemically aggressive. When this water comes into contact with Late Miocene (?), sodium-rich intrusive rocks along flow path, sodic silicates (probably nepheline) weather to form kaolinite, this reaction produces a Na:HCO3 ratio (in mmol/l) of unity. Stable isotopic data from rock samples and groundwater collected throughout the Ballimore region provide support for the interpreted major-ion chemical evolution of effervescent Na-HCO3-rich groundwater. Carbon isotope data helps constrain the source of carbon in this system