Unveiling origin and enrichment of fluoride in the Daihai lake basin, China, using a hybrid hydrochemical and multi-isotopic method

• Fluoride enrichment and migration were revealed using multiple isotopes. • Water-rock interaction contributed to fluoride enrichment in groundwater. • Evaporation and competitive adsorption promoted fluoride enrichment in lake water. • Lacustrine groundwater discharge drove fluoride from groundwater into lake water. • Results provide a new insight for water resources management in semi-arid regions. Excessive fluoride in lakes threatens lake ecosystem balance. However, the fluoride origin and enrichment associated with lacustrine groundwater discharge (LGD) in closed inland lakes are not fully understood. Based on multiple isotopes (δD, δ 18 O water , δ 34 S, δ 18 O sulfate , 87 Sr/ 86 Sr and 222 Rn), this study investigated the enrichment of fluoride and the impact of LGD on the migration of fluoride in a typical closed inland lake (Daihai Lake) in the semi-arid region of China. The results showed that 65.9 % of the water samples had pH values that exceeded the permissible range with alkalinity. In addition, groundwater (with an average TDS of 359 mg/L) and river water (with an average TDS of 364 mg/L) were freshwater. However, the lake water (with an average TDS of 16,474 mg/L) and pore water (with an average TDS of 17,287 mg/L) belonged to saline water. In this case, there were significant differences in fluoride concentrations in different water bodies: lake water > pore water > groundwater > river water. The fluoride in groundwater and river water primarily originated from fluorite dissolution, which was promoted by silicate weathering, gypsum dissolution, sulfide oxidation and cation exchange. In contrast, fluoride enrichment in the lake was promoted by evaporation, competitive adsorption and gypsum dissolution. Notably, the LGD process promoted the migration of fluoride from groundwater to lake water. In this case, the annual average fluxes of fluoride from groundwater into the lake was 7.66 × 10 4 g/d, respectively, which posed significant threats to the lake ecosystem. This study can enhance the ability to identify the genesis of lake water quality degradation in semi-arid regions.