Hydrochemistry and isotopic composition of Na–HCO3-rich groundwaters from the Ballimore region, central New South Wales, Australia

Groundwaters flowing under artesian pressure from the Ballimore region, central New South Wales, Australia, contain up to 95% Na–HCO3. Carbon dioxide effervesces from these groundwaters as they discharge from boreholes. These groundwaters flow from a confined aquifer situated within fractured Permian to Jurassic sediments. Regional geological data and K–Ar dating of outcropping volcanic units indicate that magmatism occurred in this area between 12 and 14 Ma ago. Radiometric and magnetic data supports the interpreted presence of intrusive bodies. X-ray fluorescence (XRF) data shows that the Miocene magmatic units are sodium-rich with the available samples being classified as either syenitic or trachytic. Hydrochemical and isotopic data indicate that these groundwaters are of meteoric origin and that this system is open with respect to CO2 (there is an external source of CO2 feeding this groundwater system causing DIC≫Total Alk). The salinity of the Na–HCO3-rich groundwaters range from 3000 to 8000 mg/l and CO2 (aq) concentrations are up to 1558 mg/l (log PCO2 of 0.6). The pH of these samples range from 6.1 to 6.9, supporting the conclusion that the H2CO3/HCO3 couple control pH buffering. The carbon-13 values (δ13C‰PDB) of −5‰ to +3‰ indicate that the carbon source for these groundwaters is of inorganic origin. It is inferred that this carbon source has a primary δ13C similar to that typically quoted for mantle-derived carbon (−5.9‰). Varying degrees of completeness of equilibrium isotopic fractionation resulting from the hydration/dissociation of CO2 into the groundwater forming HCO3 (CO2–H2CO3–HCO3) leads to the observed range of carbon-13 data. Oxygen-18 (δ18O‰SMOW) and deuterium (δD‰SMOW) isotopic data show a shift to the left of the meteoric water line, supporting an interpretation of CO2 exsolution (gas exchange) and fractionation resulting from incongruent silicate weathering reactions. Dissolved inorganic carbon, carbon-13 and regional geological data suggest that the most plausible explanation for the high concentration of CO2 in these effervescent sodium-bicarbonate-rich groundwaters is the mantle, with this gas initially being delivered to the fractured strata by magmatic activity.