In order to perform radiological impact assessments of radioactive waste disposals, a compartmental model taking into account generic data for biosphere transfer parameters values is usually used. To improve its performance assessments calculations, Andra (French national radioactive waste management agency) decided to adapt this type of biosphere model to its sites conditions. On one hand, specific models have been developed such as model for 36Cl based on isotopic dilution and on local stable chlorine contents. On the other hand, biosphere transfer factors values are issued from experimental studies adapted to Andra context: soil characteristics, climate, agricultural habits and species. These specific data are useful only if main factors controlling mobility have been determined previously for each radionuclide, which implicates good knowledge of environmental radionuclides behaviour. This site-specific approach allows to reduce uncertainty and range of variation of impact calculations and ensure a consistent model. It helps also to achieve scientific community approval by showing mastery of radionuclides environmental processes and public understanding by focusing demonstration on real local conditions. The methodology is fully presented. Two examples for uranium and chlorine and the limits of this approach are given.