Lithium and strontium isotope dynamics in a carbonate island aquifer, Rottnest Island, Western Australia

Water-rock interactions in aquifer systems are a key control on water quality but remain poorly understood. Lithium (Li) isotopes are useful for understanding water-rock interactions, but there are few data available for groundwater aquifers. Here we present a Li isotope dataset for rainfall and groundwater samples from a carbonate island aquifer system: Rottnest Island, Western Australia. This dataset was complemented by strontium (Sr) isotope and major and trace element data for groundwaters, and leaching experiments on bedrock samples. The delta Li-7 values and Sr-87/Sr-86 ratios of fresh groundwaters ranged from +23 to +36 parts per thousand and 0.709167 to 0.709198, respectively. Mass balance calculations indicated that silicate weathering supplied similar to 60 and 70% of dissolved Li and Sr in fresh groundwaters, respectively, with the remainder provided by atmospheric input, and carbonate weathering: for major cations, the majority of calcium and sodium (Na) are supplied by carbonate weathering and atmospheric input, respectively. The estimated low proportion of Sr produced by carbonate weathering was surprising in a carbonate aquifer, and the 87Srj86Sr data indicated that the silicate Sr source had low Rb/Sr and Sr-87/Sr-86 ratios. There was an increase in the maximum delta Li-7 values in fresh groundwaters (+36 parts per thousand) relative to the maximum value in rainfall and seawater (ca. +31 parts per thousand). As clay minerals are undersaturated in fresh groundwaters, this increase may be explained by Li isotope fractionation associated with ion-exchange reactions on days and iron(oxy)hydroxides. In the more saline groundwaters, the minimum delta Li-7 values decreased with depth to +14.5 parts per thousand, suggesting increased silicate mineral dissolution in the deeper aquifer. These results reveal the importance of water-rock interactions in a coastal carbonate aquifer, and demonstrate the usefulness of Li isotopes for tracing weathering reactions in an environmental setting where traditional weathering tracers, such as sodium and Sr isotopes, are less appropriate. Crown Copyright (C) 2020 Published by Elsevier B.V. All rights reserved.