Evaluation of distribution coefficient of vadose zone soil from proposed near-surface disposal facility at Kalpakkam, India

Near-surface disposal facility for storing low and intermediate levels of radioactive waste is proposed to be built in the premises of the Madras Atomic Power Station, Kalpakkam, India. The waste inventory of proposed near-surface disposal facility (NSDF) at Kalpakkam would predominantly contain radioactive strontium and cesium ions, and major portion of the NSDF would be located in the 3.2 m thick unsaturated soil zone. As distribution coefficient (K-d) has an important bearing on contaminant transport, the paper examines how strontium retention varies with surface coverage in column experiments, and compares cumulative strontium retention using column and batch techniques. Results of column experiments show that strontium ion retention by the vadose zone soil for individual pore volumes (K-dpvi) is maximum at immediate surface coverage (between 0.002 and 0.007 C/C-0). The maximum K-dpvi values at immediate surface coverage are strongly influenced by variations in cation composition of the permeants. The maximum K-d values steeply reduce (by 99 %) to near equilibrium values at 0.1-0.29 C/C-0 for all permeant compositions, suggesting that preferred adsorption site for strontium retention is exhausted at low surface coverage. The K-d values from batch experiments are smaller than cumulative K-d values of column experiments owing to much larger solution volume to soil solid ratio of batch experiments. Retardation factors based on cumulative K-d values of column experiments and those based on batch K-d values are comparable