Strontium isotope and multi-tracer insights into groundwater mixing and fluoride mobilization in the Luanhe River Delta, Northern China

Abstract

Understanding groundwater mixing processes and geochemical behavior is an urgent priority for elucidating the mechanisms responsible for fluoride (F⁻) enrichment in groundwater. This study collected 54 groundwater samples and 8 surface water samples from the coastal Luanhe Delta and analyzed their chemical compositions, ⁸⁷Sr/⁸⁶Sr ratio, ¹⁴C, as well as ¹⁸O and D isotopes. The results revealed substantial mixing between surface water and groundwater, as well as among groundwater from different aquifers, primarily induced by intensive groundwater extraction. Using the IsoSource mixing model, which incorporated ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values, vertical leakage was quantitatively identified as the primary recharge mechanism for shallow and deep groundwaters. This process of vertical mixing also enhanced the potential of high F⁻ groundwater dispersing across different aquifers. Furthermore, ternary mixing models using ⁸⁷Sr/⁸⁶Sr ratios, Ca/Sr and Mg/Ca molar ratios indicated that the chemistry of shallow groundwater was predominantly controlled by evaporite dissolution, while that of deep groundwater was mainly influenced by calcite dissolution. The observed relationship between ⁸⁷Sr/⁸⁶Sr ratios and Na/Sr molar ratios indicated that cation exchange occurred in deep groundwater, whereby this process promoted the dissolution of F-bearing minerals through the consumption of dissolved Ca²⁺. Additionally, under high pH conditions, the competitive adsorption between OH⁻ and F⁻ further enhanced the mobilization of F⁻. This study demonstrated that ⁸⁷Sr/⁸⁶Sr ratio served as a reliable tracer for unraveling groundwater evolution and F⁻ enrichment processes in complex coastal aquifer systems.