Role of the vadose zone in mitigating strontium transport at the near-surface disposal facility (NSDF) in Kalpakkam, India

A proposal has been advanced to construct a near-surface disposal facility for storing low- and intermediate-level radioactive waste on the premises of the Madras Atomic Power Station, Kalpakkam, located 80 km south of Chennai, India. The NSDF comprises a series of reinforced concrete trenches (RCTs), with major portions located in the unsaturated (vadose) soil zone. This zone is an important barrier against contaminant transport, as discontinuity in water-filled voids hinders solute transport to the underlying groundwater table. The hydraulic properties of the vadose zone, including soil–water characteristic curve (SWCC) and unsaturated permeability coefficient (kunsat), are important parameters in developing transport models for moisture and contaminants. As geotechnical characterization of the vadose zone soil at the proposed NSDF location has not been performed, the present study establishes hydraulic properties from field and laboratory measurements of soil samples. A one-dimensional model has been created for predicting strontium migration through the vadose zone for a range of volumetric content (θ; 0.16–0.44), based on the computed groundwater velocities and measured Kd value. The results indicate that the presence of a 3-m vadose zone below the NSDF at Kalpakkam could result in a period of greater than 1000 years to attain strontium breakthrough concentration in groundwater, while the absence of the vadose zone would lead to strontium breakthrough concentration in 1–10 years.