Sr isotopes in natural waters: Applications to source characterisation and water-rock interaction in contrasting landscapes

Strontium isotopes are now routinely measured in hydrochemical studies to determine sources and mixing relationships. It is often assumed that in contrast to the commonly used stable isotopes of O, H, C and S, ⁸⁷Sr/ ⁸⁶Sr is not fractionated by low temperature processes such as dissolution and precipitation. However, recent modelling and laboratory studies have enhanced our understanding of how Sr isotopes behave in low temperature environments and challenged early assumptions about Sr behaviour. High ⁸⁷Sr/ ⁸⁶Sr may for example be caused by the preferential release of ⁸⁷Sr from sites previously occupied by ⁸⁷Rb due to contrasting ionic radii. Furthermore, variations in the weathering rates and mechanisms of different minerals can often make determining the weathering component of environments difficult. Nevertheless, Sr isotopes are useful, when combined with other hydrochemical data, to constrain models of water-rock interaction and mixing as well as geochemical processes such as ion-exchange. This paper presents a summary of recent work by the authors in constraining the sources of waters and weathering processes in surface catchments and aquifers.