Modelling the radionuclide balance in some water bodies of central Italy

Abstract

This article illustrates the quantitative analysis of the radionuclide behaviour in some lakes of central Italy and in their outlets. The migration rates and the effective transport velocities of the radionuclides to sediment were estimated. In the examined lakes (Albano, Bolsena, Bracciano, Nemi and Vico), the concentrations of 90Sr and 137Cs show a marked exponential time decay due to the radioactive decay, the radionuclide removal by the outlets and the radionuclide migration from water to bottom sediments. It was estimated that the 137Cs average migration rate to sediment is 1.2 × 10 −9 s −1 corresponding to an effective migration velocity of 5.8 × 10 −8 m s −1. The radionuclide concentrations in watercourses Marta (outlet of Lake Bolsena) and Rio Vicano (outlet of Lake Vico) were fitted by exponential (decay) functions of the distance from the lake. The ‘half-distance’ of the above process (‘half-distance’ = the distance for the radionuclide concentration reduces to one half) is 12.6 km and 3.9 km for 137Cs in river Marta and Rio Vicano, respectively. The half-distances for 90Sr in the above watercourses are, respectively, 30 km and 5.8 km. The half-distances were quantitatively related to the effects of water dilution and to the radionuclide contribution from catchment. The analysed data offered the opportunity of calculating that the 90Sr contribution from catchment to the examined watercourses is in the order of 10 −3–10 −4 Bqs −1m −1 (radionuclide flowing per second and per metre of river).