HYDROLOGICAL EVALUATION FOR THE DISPOSAL OF LOW-LEVEL WASTE USING ONSITE LYSIMETER AND VARIABLY SATURATED FLOW MODEL

Cementitious and glass waste forms are being evaluated for disposal in engineered facilities as a strategy for the long-term immobilization of radioactive and hazardous contaminants. Contaminant release is largely influenced by evolving hydrological and physical conditions within the disposal environment. This study investigates the hydrological behavior of cementitious and glass waste forms and surrounding backfill through field experiments and numerical modeling. Field lysimeter experiments containing waste forms are ongoing at the Hanford Site, WA, to measure water movement, water balance, and solute transport. The lysimeters are filled with representative backfill materials from the Integrated Disposal Facility site. Variably saturated flow (VSF) models were developed to simulate water movement within the lysimeters using site-specific soil water retention properties and showed generally good agreement with measured water content profiles. Particle-tracking analysis suggests that pan lysimeters installed below the waste forms capture a substantial fraction of percolating water that did not directly contact the waste forms. Water originating beyond the 0.3-m diffusion distance accounted for approximately 28% of collected percolation, although irrigation uniformity may influence this dilution and percolation chemistry. The lysimeter tests approximate predicted high-saturation perimeter areas of the disposal facility, providing an upper-bound assessment of water-waste interaction. These findings support lysimeter data interpretation, field-scale extrapolation, and boundary-condition development for contaminant transport modeling.