Reference biospheres for post-closure performance assessment: inter-comparison of SHETRAN simulations and BIOMASS...

Example Reference Biosphere 2B (ERB2B) is a hypothetical river catchment, described in the IAEA-sponsored BIOMASS study on biosphere aspects of post-closure radiological safety assessments for repositories for solid radioactive wastes. In ERB2B, a radioactively contaminated aquifer interacts with the soils and sediments of the river catchment. A 'semi-distributed', lumped-parameter model (SDLP) was set up for the site as part of the BIOMASS study. In the model, empirically derived transfer functions are used to reduce the complexity of real hydrological transport systems to readily calculable mass-balance accounting routines. In this work, a physically based, spatially distributed modelling system SHETRAN was set up for the site and comparison made with the existing SDLP model. The work has shown that, using standard soil properties in SHETRAN, the soil rapidly saturates and much of the hydrologically effective rainfall (precipitation less evapotranspiration) is lost as saturation-excess surface runoff. This is contrary to the assumptions in the SDLP model. The difficulty arose from the original formulation of catchment characteristics in BIOMASS. Specifically, there was a large water volume entering the soils from precipitation together with an upward flux of groundwater across the lower boundary of a substantial part of the catchment. This water had to be lost from the catchment in some way and the thinness of the soil zone precluded dominance of subsurface, lateral flow over surface runoff. Increasing the saturated conductivity from 1 to 20 m d(-1) reduced the surface flows to similar values to those assumed in the SDLP model (this could also have been achieved by increasing the soil depth). Even with the high saturated conductivity there were still major differences between the two representations. In the woodland on the upper slopes of the valley, the SHETRAN simulation was slightly wetter than the SDLP model, whereas in the shrubland and marshland near the river it was drier than the SDLP model. In the SDLP model, subsurface lateral flows are ignored if there is surface flow, and deep subsurface flows are ignored if there are shallow subsurface flows. In the SDLP model, there is a major assumed change in flow regime between summer and winter. This is not the case in the SHETRAN simulation. Overall, this work illustrates the problems of using 'semi-distributed', lumped-parameter models without prior calibration against a physically based model and the potential for implying unexpected and possibly implausible hydrological characteristics through the specification of flows without considering whether they could occur for realistic soil depths and properties. As there is a need for application of such SDLP models, particularly when undertaking probabilistic calculations, it is suggested that, in future, explicit hydrological modelling should be undertaken first, so that a physically realistic representation can be produced as a basis for assessment studies of the migration of radionuclides or other contaminants.