Observations of River Solute Concentrations during Ice Formation

It has been observed in some aquatic systems, such as lakes and oceans, that when ice forms solutes are excluded from the ice, increasing their concentrations in the water. This process, known as solute ice exclusion, could be important in rivers and streams where ice can be a significant fraction of the total volume, potentially significantly increasing chemical concentrations in the water. There have been no reported observations of solute dynamics during ice formation in rivers and streams, however, even though the freeze−thaw cycle is a common phenomenon for lotic systems in cold regions. To investigate this process, a variety of solutes were measured during three ice formation events in a mine-waste-polluted river in western Montana. A laboratory study was also conducted to determine how different metal cations partition when ice forms. It was found that solute concentrations significantly increase during the early stages of ice formation. Correlations with ion size and changes in temperature support the idea that ice exclusion was controlling solute dynamics during these periods. These observations could have important implications for predicting the environmental impacts of toxic solutes in rivers and streams subject to seasonal ice formation.