Radionuclide distributions and migration pathways at a legacy trench disposal site

This paper examines the distributions of several anthropogenic radionuclides (^239+240Pu, ²⁴¹Am, ¹³⁷Cs, ⁹⁰Sr, ⁶⁰Co and ³H) at a legacy trench disposal site in eastern Australia. We compare the results to previously published data for Pu and tritium at the site. Plutonium has previously been shown to reach the surface by a bath-tubbing mechanism, following filling of the former trenches with water during intense rainfall events. This has led to some movement of Pu away from the trenched area, and we also provide evidence of elevated Pu concentrations in shallow subsurface layers above the trenched area. The distribution of ²⁴¹Am is similar to Pu, and this is attributed to the similar chemistry of these actinides and the likely in-situ generation of ²⁴¹Am from its parent ²⁴¹Pu. Concentrations of ¹³⁷Cs are mostly low in surface soils immediately above the trenches. However, similar to the actinides, there is evidence of elevated ¹³⁷Cs and ⁹⁰Sr concentrations in shallow subsurface layers above the trenched area. While the subsurface radionuclide peaks suggest a mechanism of subsurface transport, their interpretation is complicated by the presence of soil layers added following disposals and during the subsequent years. The distribution of ⁹⁰Sr and ¹³⁷Cs at the ground surface shows some elevated levels immediately above the trenches which were filled during the final 24 months of disposal operations. This is in agreement with disposal records, which indicate that greater amounts of fission products were disposed in this period. The surface distribution of ^239+240Pu is also consistent with the disposal documents. Although there is extensive evidence of a mobile tritium plume in groundwater, migration of the other radionuclides by this pathway is limited. The data highlight the importance of taking into account multiple pathways for the mobilisation of key radioactive contaminants at legacy waste trench sites.