Trench ‘Bathtubbing’ and Surface Plutonium Contamination at a Legacy Radioactive Waste Site

Abstract

Radioactive waste containing a few grams of plutonium (Pu) was disposed between 1960 and 1968 in trenches at the Little Forest Burial Ground (LFBG), near Sydney, Australia. A water sampling point installed in a former trench has enabled the radionuclide content of trench water and the response of the water level to rainfall to be studied. The trench water contains readily measurable Pu activity (∼12 Bq/L of 239+240Pu in 0.45 μm-filtered water), and there is an associated contamination of Pu in surface soils. The highest 239+240Pu soil activity was 829 Bq/kg in a shallow sample (0–1 cm depth) near the trench sampling point. Away from the trenches, the elevated concentrations of Pu in surface soils extend for tens of meters down-slope. The broader contamination may be partly attributable to dispersion events in the first decade after disposal, after which a layer of soil was added above the trenched area. Since this time, further Pu contamination has occurred near the trench-sampler within this added layer. The water level in the trench-sampler responds quickly to rainfall and intermittently reaches the surface, hence the Pu dispersion is attributed to saturation and overflow of the trenches during extreme rainfall events, referred to as the ‘bathtub’ effect.

Response of microbial community function to fluctuating geochemical conditions within a legacy radioactive waste trench environment

“…This pipe provides the only point of access into the legacy trenches and was opportunistically installed during the partial collapse of the trench surface. Further details of the trench sampler have been previously described in the literature (10, 29). Briefly, a peristaltic pump operating under low flow rate was used to purge the borehole until chemical parameters, particularly pH and oxidation/reduction potential (ORP), became stable (Supplementary File 1).…”

“…This was also the case for Australia’s only nuclear (research) reactor at Lucas Heights. Known as the Little Forest Legacy Site (LFLS), radioactive materials, including minor amounts of 239+240 Pu and 241 Am were placed in narrow (0.6 m), 3 m deep, unlined trenches from 1960 to 1968 (10–12). Large volumes of contaminated non-radioactive materials and equipment were also disposed of in these trenches (10).…”

“…The LFLS trenches were excavated within undisturbed geological matrices of red-brown and grey clay (primarily kaolinite and illite/smectite) derived from the underlying weathered shale, interspersed with minor phases of hematite and goethite (11, 13). The site was chosen in part due to the low hydraulic conductivities of these materials (~ 9 to 66 mm/day) in order to isolate the LLRW from waters associated with the local hydrology cycle (10–12). However, an unintended consequence of this (also experienced at other disposal sites) has been that periodic intense rainfall and prolonged dry conditions can facilitate complete saturation and desaturation of the more permeable waste material via surface infiltration and evapo(transpi)ration/leakage mechanisms, respectively.…”

Measurement of fallout radionuclides, 239,240Pu and 137Cs, in soil and creek sediment: Sydney Basin, Australia

“…The values have been below minimum detectable activity (MDA), measured by alpha spectrometry, except for a possible trace reported in 1985 (Giles et al, 1988). Although not associated with atmospheric releases, 239þ240 Pu has been detected in the soils at LFLS associated with soilewater transport (Payne et al, 2013).…”

“…Near the LHSTC is an associated legacy low level waste site, the Little Forest Legacy Site (LFLS), formerly known as the Little Forest Burial Ground (LFBG), where small amounts of plutonium and other radionuclides were disposed of in trenches in the 1960s (Payne, 2012;Payne et al, 2013). Environmental monitoring of the LHSTC and LFLS has been undertaken since 1959 (Giles and Stockdale, 1966) and 1966 (Cook et al, 1969) respectively.…”