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This paper aims to assess MODFLOW and MT3D capabilities for simulating the spread of contaminants from a river exhibiting an unusual relationship with an alluvial aquifer, with the groundwater head higher than the river head on one side and lower on the other (flow‐through stream). A series of simulation tests is conducted using a simple hypothetical model so as to characterize and quantify these limitations. Simulation results show that the expected contaminant spread could be achieved with a specific configuration composed of two sets of parameters: (1) modeled object parameters (hydraulic groundwater gradient, hydraulic conductivity values of aquifer and streambed), and (2) modeling parameters (vertical discretization of aquifer, horizontal refinement of stream modeled with River [RIV] package). The influence of these various parameters on simulation results is investigated, and potential complications and errors are identified. Contaminant spread from stream to aquifer is not always reproduced by MT3D due to the RIV package's inability to simulate lateral exchange fluxes between stream and aquifer. This paper identifies the need for a MODFLOW streamflow package allowing lateral stream‐aquifer interactions and streamflow routine calculations. Such developments could be of particular interest for modeling contaminated flow‐through streams.

Near surface disposal of low and intermediate level radioactive waste is probably the best solution for a long term radioactive waste management today. For safety assurance all near surface disposal aspects and evolution scenarios should be reviewed and analyzed including repository design and site selection, because releases of radionuclides from the repository into the environment cause radiation exposure to the public. Mechanisms of 14C, 129I and 137Cs transportation from the repository into the environment, modelled for the hypothetical radioactive waste disposal facility in Lithuania, are investigated and the results of effective dose calculations for the main exposure pathways are discussed in this paper. The RESRAD-OFFSITE code has been used in this study for modelling purposes. It is shown that the radionuclide transport time into the environment, its radioactive contamination and the radiation exposure to the public mostly depends on the mobility and sorption properties of radionuclides and on the appropriate transport pathways from the repository into the environment.