Chemical characteristics, evolution, and quality of groundwater and processes controlling its fluoride concentration features: case study of a typical high-fluoride areas in the Southwestern Shandong Plain, China

In this study, the groundwater (GW) in the high-fluorine area of the Southwestern Shandong Plain was divided according to the characteristics of high Na% (> 75%), and its water chemistry characteristics and causes were discussed separately, and the hydrochemical process of the formation of high-fluorine GW was determined. Finally, the GW quality of the study area was evaluated. The results proved that silicate hydrolysis can significantly promote the release of F⁻ in fluorine-containing minerals; high %Na can be used as one of the early-warning conditions for judging high-fluoride areas. To this end, 132 GW samples were collected from 66 wells during the dry and wet seasons. The study area was found to have weakly alkaline GW (pH 7.1–8.9) and could be divided into high %Na areas (HNA) and non-HNA. GW exhibited different hydrochemical characteristics between HNA and non-HNA. In non-HNA, total hardness (TH) exceeded 200 mg/L, and total dissolved solids (TDS) ranged from 514.1 to 5246.1 mg/L; in HNA, TH was less than 200 mg/L, TDS ranged from 552.8 to 1298.3 mg/L, and Na⁺ increased with TDS, whereas Ca²⁺ and Mg²⁺ contents were low. The main water type in HNA was HCO₃-Na and in non-HNA was SO₄·Cl-Ca·Mg and SO₄·Cl-Na. The study area is experiencing serious fluoride pollution. GW in HNA is mostly controlled by carbonate and silicate hydrolysis and evaporation, whereas GW in non-HNA is controlled by dolomite dissolution and cation exchange in the main. Moreover, GW in HNA has significantly been altered by albite hydrolysis, which produces Na⁺ and HCO₃⁻ and triggers various reactions promoting the release of F⁻ from fluorine-containing minerals (FCM). Regarding the water quality for irrigation, GW in HNA was found to be less suitable than that in non-HNA. Nevertheless, in terms of the water quality index (WQI), GW is moderate for drinking and poor for irrigation. Therefore, extensive attention should be paid to the exploitation and management of high-sodium GW in the plain area.