Enrichment and release mechanisms of geogenic fluoride in multi-layered clayey sediments of coastal aquifers under groundwater overexploitation

The compression-release of porewater fluoride (F) from clayey sediments under overexploitation has been identified as a primary source of Quaternary high F groundwater, which poses a risk of waterborne fluorosis. However, the formation mechanisms of F in clay porewater during compression remain unknown, restricting the genesis identification and scientific prevention of high F groundwater. The enrichment and release mechanisms of F in clayey sediments from subsidence (Xianxian, XX) and non-subsidence (Huanghua, HH) zones within the Cangzhou coastal plain are analyzed by integrating multiple approaches, including borehole samples collection, mineralogical studies, and hydrochemical and isotopic analysis. The F^- concentrations in clay porewater ranged from 0.77 to 4.20 mg/L (mean: 2.37 mg/L) in the XX zone and from 0.46 to 3.40 mg/L (mean: 1.93 mg/L) in the HH zone, mostly exceeding the permissible limit in drinking water recommended by the Chinese Government (1.0 mg/L). Deep clay porewater generally exhibited higher F^- levels compared to shallow strata. The clay minerals in sediments contained substantial F and facilitated the enrichment of F^- in porewater through a series of water-rock interactions. In the HH zone, porewater F^- primarily originated from the F-bearing minerals dissolution, while the additional F transformation mechanisms, including silicate weathering, desorption, and Fe (III) (hydrogen)oxides reductive dissolution, were detected in deep layers (below -124.8 m). The fate of released F^- was further influenced by ionic interactions such as cation exchange and competitive adsorption in porewater. The enrichment mechanisms of porewater F in the XX zone were similar with those in HH, but the clay compression intensified the dissolution and desorption of F-bearing mineral, resulting in reduced water-soluble F and elevated porewater F^- contents in deep clayey sediments (below -148.7 m). Isotopic signatures (δ²H, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr) exhibited similar compositional characteristics between deep groundwater and clay porewater samples, indicating significant mixing effects. Based on the calculation results from end-member mixing models, the compression-release of clay porewater increased F^- concentration in deep groundwater by 0.54-0.78 mg/L within the XX zone. To the best of our knowledge, this study represents the first comprehensive effort to elucidate the formation mechanisms and quantitatively assess the release impact of fluoride in Quaternary clayey sediments of coastal plains. The findings provide a scientific basis for mitigating high F groundwater caused by overexploitation.