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With the exception of brine systems and crystalline-rock aquifers, groundwater Sr-87/Sr-86 ratios rarely differ by more than +/- 0.01 from the Phanerozoic seawater value of 0.70923. Groundwater Sr-87/Sr-86 ratios from Central Australia (0.725620.76248) are the highest ever recorded for a low salinity, sedimentary aquifer. We propose a model for the evolution of these Sr-87/Sr-86 ratios and demonstrate how the lowest Sr-87/Sr-86 ratios reflect water-rock interactions with predominantly carbonate minerals, while the highest Sr-87/Sr-86 ratios are ultimately controlled by weathering of old primary silicate minerals. Detailed analysis of aquifer core material identified albite and microcline as the most prevalent primary silicates and where these minerals were abundant, relatively high Rb concentrations were measured. High Rb combined with long time scales since deposition has produced very high Sr-87/Sr-86 ratios in the rocks and subsequently the groundwater. Therefore, strontium isotopes can provide further constraints for chemical models of and zone groundwater systems, particularly when silicate hydrolysis is believed to significantly contribute to dissolved solute compositions. Crown Copyright (C) 2003 Published by Elsevier Science B.V. All rights reserved.