Behaviors of lithium and its isotopes in groundwater with different concentrations of dissolved CO2

Lithium (Li) isotopes have shown large potential in tracing weathering in various water bodies, but there is limited study on Li isotopes in subsurface conditions where CO2 has been largely consumed. In this study, we use a thick sandstone aquifer in the Ordos Basin, NW China, as a natural setting to investigate the behaviors of Li isotopes in hydrogeochemical conditions with different concentrations of dissolved CO2. For young groundwater in the recharge area (group R) where CO2 is abundant (mean P-CO2 = 10(-2.5) atm), clay formation accompanying with weathering leads to the enrichment of Li-7 in groundwater. The four deep samples in the recharge area have uniform Li/Na ratios (with a mean of 2.52 lmol/mmol) and delta Li-7 (with a mean of 25.0 parts per thousand), corresponding to a mean Li removal rate of 81.2% compared with the sandstone leachate. For groundwater in the shallow part of the discharge area (group D1), Li was firstly removed by clay formation during weathering in the recharge area and was later removed by physisorption when CO2 becomes much lower (mean P-CO2 = 10(-3.1) atm). Different degrees of weathering lead to a wide range of delta Li-7 varying from 19.7 parts per thousand in the deepest well to 33.0 parts per thousand in the shallowest well. The proportion of Li removal caused by physisorption is found to increase with groundwater age. After the stage of Li removal by adsorption, Li was released in the deeper part of the discharge area (group D2), and the positive correlation of delta Li-7 versus Li/Na is explained by a ternary mixing model. The endmember of water brought by cation exchange is inferred to have a heavier delta Li-7 than sandstone leachate, demonstrating that cation exchange could cause an enrichment of Li-7 in water. This study enhances our understanding of the controlling factors of Li isotopes in deep groundwater with low dissolved CO2, which have implications for the application of Li isotopes in subsurface water. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the behavior of lithium isotopes in deep groundwater and their relationship with dissolved CO2 concentrations. The findings highlight the influence of weathering and physical adsorption on lithium enrichment or removal in groundwater. Understanding these processes can contribute to the application of lithium isotopes in subsurface water.