In a flow system from the recharge to the corresponding discharge area, there are usually systematic changes in the hydrochemistry of groundwater. Although it is well known that silicates are the main minerals in sandstones, silicates and minor carbonates in sandstones both consume CO2 and control of hydrochemistry of groundwater. However, it is difficult to differentiate the contribution of each type of minerals. Considering that 87Sr/86Sr ratios of water is mainly controlled by different sources, Sr isotopes of groundwater could be used to evaluate the contents of Sr derived from silicates and carbonates with different Sr isotopic compositions. In this study, it is assumed that due to the relatively abundant CO2 in the shallow part of the aquifer, the contents and isotopes of Sr in shallow groundwater are determined by rainwater influenced by silicates weathering and carbonate dissolution. From recharge to discharge area, due to the decreased dissolved CO2 in a closed system, the isotopes of Sr in deep groundwater are determined by shallow groundwater influenced by carbonate dissolution. The contribution of carbonates to Sr isotopes is found to be less than that of silicates in the recharge area, but outweigh that of silicates in the discharge area. The different ratios of carbonate to silicates partly explain the difference of Mg isotopes in shallow and deep groundwater in the same study area (Zhang et al., 2018).
Zhang H., Jiang X.W., Wan L., Ke S., Liu S.A., Han G.L., Guo H.M. and Dong A.G. (2018) Fractionation of Mg isotopes by clay formation and calcite precipitation in groundwater with long residence times in a sandstone aquifer, Ordos Basin, China. Geochimica et Cosmochimica Acta. 237, 261-274.