Identification of sources and processes in a low-level radioactive waste site adjacent to landfills: groundwater hydrogeochemistry and isotopes
Creators: name: D. I. Cendón
name: C. E. Hughes
name: J. J. Harrison
name: S. I. Hankin
name: M. P. Johansen
name: T. E. Payne
name: H. Wong
name: B. Rowling
name: M. Vine
name: K. Wilsher
name: A. Guinea
name: S. Thiruvoth
Publisher: name: Taylor & Francis
Year: 2015
Subjects: subject: Science Policy
subject: Biological Sciences
subject: Ecology
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: Chemistry
subject: Earth and Environmental Sciences
subject: Evolutionary Biology

description: Multiple tracer-element and isotope approaches were applied at a 1960s-era low-level radioactive waste burial site located in the Lucas Heights area on the southwest urban fringe of Sydney, Australia. The site is situated among other municipal and industrial (solid and liquid) waste disposal sites causing potential mixing of leachates. Local rainfall contains marine-derived major ion ratios that are modified during infiltration depending on waste interactions. The local geology favours the retention of contaminants by ion-exchange processes within the clay-rich soils and the shale layer underlying the burial site. Local soils experience periodic infiltration and wetting fronts that can fully saturate the waste trenches (bathtub effect) while surrounding soils are mostly unsaturated with discontinuous perched lenses. Within the trenches, the degradation of organic matter results in localised methanogenesis, as suggested by enriched δ<sup>2</sup>H and δ<sup>13</sup>C<sub>DIC</sub> values in adjacent subsurface water<sub>.</sub> Movement of contaminants from the trenches is indicated by Na<sup>+</sup>, Br<sup>−</sup> and I<sup>−</sup> concentrations, variations in <sup>87</sup>Sr/<sup>86</sup>Sr, enriched δ<sup>13</sup>C<sub>DIC</sub> values and evolution of δ<sup>34</sup>S of dissolved sulfate in perched water bodies above the shale. Although transport is limited by the low transmissivity of the clay-rich soils, migration and mixing processes are indicated by the variation of concentrations with distance from the trenches, disappearance of δ<sup>2</sup>H enrichments, mixing with other sources of Sr and sulfate isotope fractionations. The depth distribution of waste-derived contaminants (specifically <sup>3</sup>H and Be) between the perched water surrounding the trenches, and the underlying shale and sandstone layers, indicates limited downward transport of contaminants. Past removal of the shale layer in an adjacent site, Harrington's Quarry, has facilitated the mixing of some municipal waste leachates (characterised by circum-neutral pH, high alkalinity, low sulfate, high <sup>3</sup>H, high Be, enriched δ<sup>2</sup>H and δ<sup>13</sup>C<sub>DIC</sub>) into the underlying groundwater system as suggested by high TDS, Cl<sup>−</sup>/Br<sup>−</sup> ratios, Be and <sup>3</sup>H found in deeper wells. This study demonstrates the applicability of using trace elements, stable- and radio-isotopes to document the existing geochemistry and the contaminant transport from the waste trenches. The multiple tracer approach addresses the complexities of transport at the site and differentiates various municipal, industrial and radioactive waste sources.