For the performance assessment of the geologic radioactive waste disposal system, a biosphere model is suggested. This biosphere model is intended to calculate the annual dose rate to humans caused by contaminated river water for eight pathways and each radionuclide. This model can also be applied to assess the radiological effects of any contaminated aquatic system. To account for the uncertainties on the model parameter values, a certain distribution has been assigned to each model parameter value. Then the Monte Carlo simulation method with the Latin Hypercube sampling technique is used. This technique is compared with the crude Monte Carlo method and optimal sample size is examined by changing the sample size. Also, sensitivity analysis is performed by using the Spearman rank correlation coefficients. It is found that this method is very useful tool to treat uncertainties on the model parameter values and to analyze the biosphere model. A conversion factor is proposed to calculate the annual dose rate to humans arising from a given radionuclide concentration in river water. This conversion factor allows to substitute the biosphere model in a probabilistic performance assessment computer code by one single variable.