Soil and sediment samples from the Sydney basin were measured to ascertain fallout radionuclide activity concentrations and atom ratios. Caesium-137 (137Cs) was measured using gamma spectroscopy, and plutonium isotopes (239Pu and 240Pu) were quantified using accelerator mass spectrometry (AMS).
Fallout radionuclide activity concentrations were variable ranging from 0.6 to 26.1 Bq/kg for 137Cs and 0.02–0.52 Bq/kg for 239+240Pu. Radionuclides in creek sediment samples were an order of magnitude lower than in soils. 137Cs and 239+240Pu activity concentration in soils were well correlated (r2 = 0.80) although some deviation was observed in samples collected at higher elevations. Soil ratios of 137Cs/239+240Pu (decay corrected to 1/1/2014) ranged from 11.5 to 52.1 (average = 37.0 ± 12.4) and showed more variability than previous studies.
240Pu/239Pu atom ratios ranged from 0.117 to 0.165 with an average of 0.146 (±0.013) and an error weighted mean of 0.138 (±0.001). These ratios are lower than a previously reported ratio for Sydney, and lower than the global average. However, these ratios are similar to those reported for other sites within Australia that are located away from former weapons testing sites and indicate that atom ratio measurements from other parts of the world are unlikely to be applicable to the Australian context.