Geochemical and environmental processes affecting radionuclide migration from a formerly used seepage trench

Abstract

The disposal of liquid waste, containing about 0.3 million curies (10 7 GBq) of fission nuclides, activation products, actinides and transuranics, in a shallow land seepage trench from 1962 to 1966, provided a field opportunity for investigating the chemical, geological, and hydrological processes which affect contaminant migration in soils and weathered bedrock. Gamma-log profiles of wells near the trench indicate that the waste liquids seeped along discrete layers parallel to bedding and along the strikes of faults and folds. Most of the radioactivity measured in the groundwaters consisted of 3H, 99Tc, 60Co, and 233U. The mobility of 99Tc, 60Co, and 233U has been attributed to low molecular weight anionic complexing. Concentrations of 90Sr and 137Cs in the groundwaters were extremely low because of the chemical treatments and precautions taken to establish and maintain an alkaline environment near the trench, which allows for 90Sr sorption and precipitation, and because of the strong tendency for 137Cs to be selectively sorbed by illite. Plutonium isotopic ratios indicate that much of the plutonium contamination near the trench results from the migration of 242Cm and 244Cm and their subsequent decay to 238Pu and 240Pu. Radionuclide concentrations in the groundwaters near the north end of the trench undergo seasonal variations, with the highest activities occurring in the spring and after prolonged rainfall. This suggests that contamination may be leached from the trench or from the relict waste migration layers when the groundwater level rises to saturate these zones or when precipitation infiltrates into the trench or along these relict migration layers during drainage. Suspected transport pathways from the trench to a nearby seep area appear to be associated with fault zones and limbs of a plunging limestone fold.