Impact assessment of buried low-level wastes under unsaturated flow conditions

The environmental impact assessment of a shallow land burial site for LLW depends on a realistic assessment of the actual and worst-case infiltration rate for water, the retention and drainage capacity of the trench backfill and underlying soil, the leach rate under unsaturated flow conditions, and the ion exchange properties of the soil. Additional factors of importance are the pH of the water after equilibration with the soil, the existence of suspended particulates, the presence of any chelating reagents in the waste, the residual moisture content of the soil, and any tendency to crack formation. Extensive modeling has been done to simulate 1D and 3D dispersion effects in waste sites. The presence of interfaces and interstices can affect flow patterns and reduce the source term, or conversely increase it by causing ponding. The installation of trench floor drainage can significantly reduce the source term but only for soils with a low residual moisture value. For areas of low or lightly intermittent rain fall conventional models tend to overestimate impact by assuming flooding of the site and saturated flow conditions. To be meaningful a model must incorporate highly site-specific parameters and the drainage conditions of the site must be planned carefully