Isotopic and hydrogeochemical study of radionuclide and landfill leachate migration in a contaminant disposal precinct in Sydney´s Southwest

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National Centre for Groundwater Research and Training
During the 1960s the Australian Atomic Energy Commission disposed of low level radioactive waste in shallow trenches at the Little Forest Legacy Site (LFLS). Radionuclides included small amounts of plutonium, uranium, fission products Cs-137 and Sr-90, and tritium. Adjacent sites were used for disposal of night soil, municipal waste and industrial liquid waste. Contaminant distributions, groundwater levels, environmental isotopes and hydrogeochemistry were measured at LFLS to assess the relative importance of different contaminant transport pathways, sources and mechanisms and support management of the site. Sources include the LFLS waste, fallout from nuclear weapons testing, and adjacent landfills which contain some artificial radioactive substances. Tritium is a conservative tracer of subsurface water movement, and has been used to identify potential contaminant pathways at the LFLS. In addition, variations in Sr-87/Sr-86, δC-13(DIC) and evolution of δS-34 have been used to help identify processes occurring at the site. Within the trenches, the degradation of organic matter results in localised methanogenesis, as suggested by enriched δH-2 and δC-13(DIC) values in adjacent subsurface water. The isotopic signatures enable differentiation of LFLS contaminants from those originating from nearby sources including an adjacent quarry used for municipal waste which contributes leachate (characterised by elevated tritium, enriched δH-2 and δC-13(DIC) as well as a chemical signature) into the underlying groundwater system. The study has shown “bathtubbing” (overflow of waste trenches during rainfall events) to be an important mobilisation mechanism, contributing to tritium movement at the site and localised migration of radionuclides (particularly actinides Pu and Am) from trenches to surface soils. However sub-surface migration of actinides from the LFLS trenches has been retarded by low transmissivity clay-rich soils. The value of hydrogeochemical and isotopic tracers in understanding hydrological and contaminant transport processes is demonstrated by this work and will contribute to future management of the precinct.
Isotopes, Contamination, Radioisotopes, Sanitary landfills, Plutonium, Uranium
Hughes, C., Cendón, D., Harrison, J., Payne,T., and Johansen, M. (2015). Isotopic and hydrogeochemical study of radionuclide and landfill leachate migration in a contaminant disposal precinct in Sydney´s Southwest. Paper presented at the Australian Groundwater Conference, 3-5 November 2015, The Shine Dome, Canberra, Australia, (pp. 125). Retrieved from: