Environmental isotopes to study radionuclide migration mechanisms at the Little Forest legacy site

Abstract

The Little Forest Legacy Site (LFLS) was used by the Australian Atomic Energy Commission (AAEC) to dispose of low level radioactive waste in shallow trenches during the 1960s. Various radionuclides have been detected in environmental samples from around the site including: isotopes of the actinides plutonium, uranium, and americium; fission products 137Cs and 90Sr; and tritium (Payne et al, 2013). A major research project to study the movement of radioactivity and support management of the site has identified "bathtubbing' (where waste trenches fill with water during rainfall events then overflow or slowly drain) as an important mobilisation mechanism. This has led to readily measureable quantities of radionuclides (particularly Pu and Am) to be present in surface soils near the trenches (Payne et al, 2013) and driven the movement of tritium in the subsurface. Environmental isotopes, contaminant distributions and hydrogeochemistry at LFLS are being used to assess the relative importance of different contaminant transport pathways, sources and mechanisms (Hughes et al, 2011 ). Sources of anthropogenic radioactivity include the LFLS waste, fallout from nuclear weapons testing, and adjacent landfills (which contain some artificial radioactive substances). Therefore, interpretation of the radionuclides and stable isotopic distributions is complex, as they originate from various sources and are influenced by a number of natural and human-induced perturbations.

Tritium is a useful marker of subsurface water movement, and the presence of readily measurable tritium in many groundwater samples (and some surface water samples) from the LFLS has enabled the pathways of water movement to be identified (Hughes et al, 2011). In addition, variations in 87Sr/86Sr, measured δ13CDIC values and evolution of δ34S 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 δ2 H and δ13CDIC values in adjacent subsurface water. Migration and mixing processes are indicated by the variation of isotopic ratios (δ2H, Sr and SO4 isotopes) and concentrations with distance from the trenches (3 H, The isotopic signatures enable differentiation of LFLS contaminants from those originating from nearby sources. At one adjacent site, municipal wastes were placed into a quarry from which the underlying shale layer had been excavated during mining, making it possible for associated water to enter the underlying sandstone formation. This has facilitated the mixing of some municipal waste leachates (characterised by elevated 3H, enriched δ2H and δ13CDIC as well as a chemical signature) into the deeper groundwater system. Although subsurface transport from the LFLS is limited by the low transmissivity of the clay-rich soils, thereby retarding the migration of actinides, it is the low-permeability of these clays which is largely responsible (together with infiltration from episodic rainfall) for the actinide contamination in the surface soils.