Impact of dynamic geochemical conditions on plutonium and americium mobility at a legacy trench disposal site

Low-level radioactive wastes were disposed by burial in closely-spaced unlined trenches in clay-rich surface layers at the Little Forest Site on the southern periphery of Sydney, Australia, between 1960 and 1968. A previous paper [1] gave a general description of the site and described the distribution of plutonium in soils and groundwaters. A process known as the ‘bath-tub’ effect, in which the former trenches become filled to the ground surface with water during intense periodic rainfall events, was implicated in the mobilisation and dispersion of actinides in the surface soils.
A follow-up study investigated the chemical speciation of Pu and Am in water obtained from a sampler within one of the former trenches. The presence of readily detected amounts of Pu and Am provided a unique opportunity to study their aqueous speciation in this environment. The predominant oxidation state of dissolved Pu species was found to be Pu(IV), however the pH-Eh conditions of the groundwater were very close to the transition with Pu (III), suggesting that small changes in chemical conditions could alter its oxidation state [2]. Large proportions of both Pu and Am were associated with mobile colloids in the submicron size range. However, it was clear from ongoing studies at the site that the distribution of dissolved, colloidal and particulate forms of actinides was very variable, and sensitively dependent on the preceding conditions. These included the interval elapsed since the most recent rainfall event at the time of sampling, the initial state of water-saturation of the site when significant rainfall occurred, the intensity of rainfall, and the length of time between rainfall events.
The geochemical form of plutonium was found to be strongly affected by the changes in the redox state and groundwater chemistry during influxes of rainfall-derived water. The cycling of iron, including its precipitation in response to redox changes, is of particular importance, given its potential role as an actinide-sorbing phase. Following rainfall events, the chemical forms of Pu and Am transition from a particle-associated phase immediately after the initial rainwater pulse, to a progressively more soluble form as reducing conditions become re-established.
The formation of strongly sorbing Fe-oxyhydroxide particles is therefore a key process in the cycling of actinides in the system. Simulations in the laboratory using trench-derived water samples showed that increasing concentrations of dissolved silicate progressively retarded Fe(II) oxidation kinetics in the relevant pH range. Furthermore, with increasing Si, the primary Fe(III) oxidation product transitioned from lepidocrocite to a ferrihydrite / silica-ferrihydrite composite [3]. It was inferred that the presence of silicate restricts Fe-polymerization and consequently inhibits Fe(III) solid-phase particle growth. The presence of silicate may therefore significantly retard Fe(II) oxidation rates and facilitate transport of trace concentrations of plutonium and americium, which would otherwise adsorb to the Fe(III) oxide resulting from Fe(II) oxidation.
Microbial analyses demonstrated that oxygen-laden rainwater rapidly altered the redox balance in the trench water, strongly impacting the speciation of radionuclides and microbial functioning. When water levels were lowest, more reducing conditions prevailed, with the microbial community exhibiting a higher representation of dissimilatory sulfate reduction and methanogenesis pathways [4]. When water levels are low in the trenches, the Fe is predominantly present in soluble reduced forms.
In summary, the behaviour of actinides at the site is strongly influenced by a number of hydrological, geochemical and microbial factors and has a strong cyclic variation triggered by episodic rainfall events. Our results have important implications for the ongoing management of this site and are of relevance to other near-surface environmental systems in which redox cycling occurs. In the case of near-surface legacy radioactive waste sites, consideration should be given to the major consequential impact of these fluctuations on the mobility of disposed actinides and other radionuclides.
[1] Payne, T.E., Harrison, J.J., Hughes, C.E., Johansen, M.P., Thiruvoth, S., Wilsher, K.L., Cendón, D.I., Hankin, S.I., Rowling, B. and Zawadzki, A. (2013). Trench ‘Bathtubbing’ and Surface Plutonium Contamination at a Legacy Radioactive Waste Site. Environmental Science & Technology, 47: 13284-13293.
[2] Ikeda-Ohno, A., Harrison, J.J., Thiruvoth, S., Wilsher, K., Wong, H.K.Y., Johansen, M.P., Waite, T.D. and Payne, T.E. (2014). Solution Speciation of Plutonium and Americium at an Australian Legacy Radioactive Waste Disposal Site. Environmental Science & Technology, 48: 10045-10053.
[3] Kinsela, A.S., Jones, A.M., Bligh, M.W., Pham, A.N., Collins, R.N., Harrison, J.J., Wilsher, K.L., Payne, T.E., and Waite, T.D (2016). Influence of Dissolved Silicate on Rates of Fe(II) Oxidation Environmental Science & Technology. 50: 11663−11671
[4] Vazquez-Campos, X, Kinsela, A.S., Bligh, M.W., Harrison, J.J., Payne, T.E., and Waite, T.D., Response of microbial community function to fluctuating geochemical conditions within a legacy radioactive waste trench environment. Submitted to Applied and Environmental Microbiology, 2017.