Radionuclide transport in the Neogene aquifer system located in the environment of the Boom clay

Document available in extended abstract form only. In the framework the Belgian research program on the long term management of high-level and/or long-lived radioactive waste coordinated by ONDRAF/NIRAS, the Boom Clay is considered as a reference host rock for the geological disposal of high-level radioactive waste in NE-Belgium (Campine area). In the frame of the performance assessments of a disposal system located in the Boom Clay Formation, the transport of radionuclides diffusing through the clay barrier into the aquifers located above is modelled. The transport model for the Neogene aquifer is based on a series of groundwater flow models simulating the aquifer systems in the surroundings of the Boom Clay. This series of groundwater models include the regional north-eastern Belgium model simulating flow both above and below the Boom Clay, the recently updated deep-aquifer pumping model, simulating transient flow in the over-exploited aquifers below the Boom Clay and finally the catchment-scale Neogene aquifer model, simulating flow in the aquifer system above the Boom Clay. The Neogene aquifer system consists of two main aquifers. The Pliocene aquifer is located at the top, separated from the underlying Miocene aquifer by the Kasterlee Clay aquitard. The Miocene aquifer consists of three hydrostratigraphic units: the Diest, Berchem and Voort Formations; with the last two having a lower hydraulic conductivity than the Diest unit. The transport model for the Neogene aquifer represents a fraction of the catchment-scale Neogene aquifer model. It stretches from the local divide between the Grote and Kleine Nete Rivers up to the Kleine Nete River, representing the main model sink. The boundary conditions and the sources/sinks in the Pliocene aquifer are defined mostly by the surface water features, such as the rivers, brooks, lakes and canals. In the partially confined Miocene aquifer, the effect of the surface water features is dampened and the heads at the model boundaries are imposed from the resulting heads in the catchment-scale Neogene aquifer model. In this way, a regional flow pattern is modelled in the Miocene aquifer. A constant radionuclide source flux is defined at the bottom of the model, coinciding with the top of the Boom Clay. A square source of 1x1 km is assumed, corresponding to a hypothetical repository footprint at the reference Mol site. The radionuclide decay is neglected, since only long-lived radionuclides are expected to leach out of the Boom Clay, whereas the steady-state in the Neogene aquifer occurs within 20 000 years. In a reference simulation, only the advection-dispersion including diffusion are assumed. Including the latter process is inevitable to simulate the transport in the lowest parts of the Neogene aquifer system (Berchem and Voort Formations), where the combination of the low hydraulic gradient associated with the catchment divide and a relatively low hydraulic conductivity result in very low groundwater velocities and related low Peclet numbers. The transport modelling results provide the spatial spreading of the steady-state radionuclide concentrations in the Neogene aquifer. Three types of biosphere entry points are assumed, the rivers, the well and the soil. In the used conservative approach (neither decay, nor sorption is assumed), the entire radionuclide flux enters the rivers at steady-state. The model then identifies the influenced river sections. In case of the well, the concentration distribution in the Diest sands is evaluated. The well is then assumed to be located at the most adverse location. The soil recipient corresponds to wet pastures located close to the Kleine Nete river, characterized by shallow groundwater levels and an upwards contaminant flux. The maximum concentration in the top-most layer (non-river modelling cells) is then taken forward as representative of the concentrations in the soil. The sensitivity of the used conceptual model and parameters on the estimated transfer factors (river flux, well and soil concentration ratios to the source flux) was assessed. The most important parameters influencing the results are the aquifer hydraulic conductivities and the model recharge, whereas the latter is especially important in view of a climate change. The transport modelling in the Neogene aquifers provided the necessary estimates of the important safety indicators characterizing the transfer of the radionuclides between the disposal system in the Boom Clay and the biosphere. The sensitivity analyses point out the robustness of the model and the aquifer role. (authors)