Oral Presentation Australasian Groundwater Conference 2017

Evolution of chemical and isotopic composition of inorganic carbon in the unsaturated and saturated zones of a semi-arid zone environment. (#124)

Karina T Meredith 1 , L F Han 2 , Suzanne E Hollins 3 , Dioni I Cendón 3 , Geraldine E Jacobsen 3 , Andy Baker 4
  1. Environment - NSTLI, ANSTO, Sydney, NSW, Australia
  2. Nanjing Hydraulic Research Institute, Nanjing, China
  3. ANSTO, Kirrawee DC, NSW, Australia
  4. BEES, UNSW, Syndey, NSW, Australia

Estimating groundwater age is important for any groundwater resource assessment and radiocarbon (14C) dating can provide this information. However a thorough investigation of the water in the soil-plant-atmosphere continuum leading to the evolution of dissolved inorganic carbon (DIC) in groundwater is required for interpretation in a water-limited environment. In this study we trace the evolution of DIC through the unsaturated and saturated zones after prolonged drought and post-flooding of a major river system, the Darling River. In doing so, we quantified the contribution of carbon from various processes influencing the 14C content of DIC in groundwater. None of the simple 14C adjustment models could be applied for age estimation. Therefore, we used a combination of a graphical method and mass-balance calculations. It was found that the saline groundwaters evolved via carbon exchange between DIC- carbon dioxide gas (CO2(g)) in the unsaturated zone and DIC-carbonate minerals in the saturated zone with water-sediment reactions driving ion exchange on clay minerals facilitating carbonate dissolution. This study shows the problems associated with using radiocarbon dating in a semi-arid zone or water-limited environment and the required carbon measurements needed to reduce this uncertainty.

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