Centring Wiannamatta (mother place): the story of the environmental persistence of tri-butyl phosphate within a legacy waste site

Tri-butyl phosphate (TBP) is an organic chelate that was used in the 1960s at the Lucas Heights Laboratories of the Australian Atomic Energy Commission (AAEC) before being disposed in sealed containers in trenches at a nearby waste site across a discrete 8-year period. The first study investigating whether TBP was still present at the site was conducted in 2016, which detected low levels of TBP. The aim of the current study was to assess whether the TBP concentrations had changed and, if so, what environmental factors could have contributed to this outcome. This was achieved using a multidisciplinary and two-ways knowing approach, which combines Indigenous knowledge with multiple modern analytical assessments. In 2023, solid-phase extraction (SPE) of groundwater samples was combined with ultra high-pressure liquid chromatography–time of flight–mass spectrometry (UHPLC-TOF-MS) analysis. The results indicate the TBP present at the site is contained within the main trenched area; all other areas were at or below detection limits. These results are significantly lower than those obtained 7 years earlier using gas chromatography–mass spectrometry (GCMS). However, as the GCMS instrument used previously was less sensitive, these same samples were reanalysed using UHPLC-TOF-MS. Comparing the results from these two instruments provided similar trends. In brief, a small amount of TBP appears to remain stable whether it was stored within the original source containers or released to the waste trenches. The only uncontrolled variable entering or leaving the trenched area was water. Results from two extreme rain events from 2016 show that TBP concentrations fluctuate with the rise and fall of the groundwater, indicating a link between water and TBP’s mobilisation. This illustrates that the original disposal of TBP in sealed containers, if they remain completely separated from the environment, is a disposal method. It seems likely that failure in one or more containers is the point source of TBP. The selected site had the attributes of a stable geology and impermeable clay, which help to contain the contaminants long term. To some extent, this is consistent with Care for Country principles. No further wastes have been added to the main trenched area since 1968 and no further waste will be added. Therefore, TBP should diminish over time due to exposure to natural environmental processes, thus allowing Country to heal.