Migration of transuranic elements in groundwater from the near-surface radioactive waste site

► Plutonium isotopes spatial distribution in unconfined aquifer near shallow waste dump. ► We found that plutonium in aquifer was originated from the dump. ► Pu mobility in waste and aquifer is characterized by in situ partition coefficients. ► Pu in groundwater is associated mainly with low molecular weight species (<1 kDa). Field experiments and laboratory studies were performed to investigate migration processes of plutonium isotopes from a near-surface radioactive waste trench to the underlying sandy aquifer at the Red Forest waste dump in the Chernobyl zone. The objectives of these experiments were to characterize the spatial distribution and possible migration mechanisms of plutonium in the aquifer. During 2002–2007 experimental investigations were carried out and spatial distributions of plutonium isotopes ( 239 , 240 Pu, 238 Pu), 90 Sr and major ions in the aquifer in the direction of the groundwater flow were obtained. Specific activities of radionuclides in groundwater depended on the location of the piezometer and varied in the range of 1–360 mBq kg −1 for 239 , 240 Pu, 0.5–180 mBq kg −1 for 238 Pu and n–n·10 4 Bq kg −1 for 90 Sr. It was found that the spatial features of the distributions of plutonium and strontium specific activities in the upper eolian aquifer were similar, i.e. there was a correlation between the positions of the activity maxima of the radionuclides. The Pu isotopes plume in the aquifer spreads about 15 m downstream of the radionuclides source. Characterization of the initial radionuclide composition of the waste showed that all plutonium in the aquifer originated from the trench. The ratio of plutonium isotopes ( 239 , 240 Pu/ 238 Pu) at the sampling time was the same in waste material and in groundwater samples. In situ ultrafiltration of several groundwater samples was carried out. The size fractionation data obtained suggest that a significant part of plutonium (50–98%) in the groundwater sampled close to the source from the upper part of the aquifer is associated with a very low molecular weight fraction (<1 kDa).