Uranium contamination of fluvial systems. Mechanisms and processes. Part III : diurnal and event-related fluctuations of stream chemistry. Pitfalls from mining affected streams in South Africa, Germany and Australia

Abstract

Once dissolved uranium from tailings deposits has entered the stream downstream-transport depends not only on the flow conditions in the streams but also on the rate of immobilisation. The latter comprises all hydrochemical processes in which dissolved uranium turns into solid phases, which can accumulate in the stream channel. It includes adsorption onto sediments and suspended solids, as well as precipitation or co-precipitation as insoluble compounds. While immobilisation removes dissolved uranium from the water column, it simultaneously leads to the contamination of the sediments in which the radionuclide is retained. The rate of immobilisation of metals in aquatic environments is strongly controlled by their speciation. This in turn largely depends on the pH and redox-potential of the system. Independently from the climatic region in all studied streams, pronounced diurnal fluctuations of both parameters were found. However, the amplitude of the daily oscillation and the covered range of the pH differs remarkably. While photosynthesis based shifts of the calciumcarbonate-carbondioxide-equilibrium are the main cause of the fluctuations, also impacts of the geology, the season and meteorological parameters were found. With changes of the pH of up to two logarithmic units per day, these processes are likely to have profound effects on the rate of immobilisation and thus the downstream transport of dissolved uranium. Interactions and dependencies between the fluctuations of various parameters are analysed and their implications for the uranium behaviour in the system is discussed. Apart from diurnal fluctuations, also event-related changes of the stream chemistry, like sudden drops of the pH in response to acid rain fall, were observed. Their impacts of the remobilization of uranium and other heavy metals from sediments, which might resulting in short duration peaks of pollution, are also analysed.