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.