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In inland arid regions, high fluoride concentrations are frequently reported in groundwater which is an important source of drinking water. Investigating its distribution, enrichment and controlling factors could provide insights for better understanding the geochemistry of high-fluoride groundwater, and are critical foundation to ensure the safety of water supply in inland arid regions. To delineate the distribution of high-fluoride groundwater, a comprehensive hydrogeochemical investigation has been conducted in a typical piedmont proluvial fan in Aksu area, Xinjiang, China. The relationship between fluoride concentration and various geochemical parameters has been analyzed for identifying the controlling processes of groundwater fluoride enrichment in this region. The results are: ①Fluoride concentrations in groundwater range from 0.8 to 6.1 mg/L, and 83% of the samples exceed the maximum limit of 1.0 mg/L set by the sanitary standards for drinking water (GB 5749-2006). ②The fluoride concentration was found increasing along groundwater flow paths, and low-fluoride groundwater (ρ(F-)≤1.0 mg/L) mainly distributed in recharge areas by the north of National Highway 314 while high-fluoride groundwater (ρ(F-)>1.0 mg/L) mainly occurred in runoff and discharge areas by the south of National Highway 314. ③The groundwater with high and low fluoride concentration is classified as Cl·HCO3-Na type and Cl·SO4-Na type, respectively, indicating the dominant anion of high-fluoride groundwater is biased towards HCO-3. ④The pH range of groundwater is 7.98.9 with an average value of 8.4, demonstrating a weakly alkaline environment. The groundwater F- content is positively correlated with the pH value. Anion exchange between OH- in groundwater and F- on mineral surface might contribute to the enrichment of fluoride, since there is a certain amount of exchangeable F- in black mica, fluorapatite and other minerals in surrounding Upper Pleistocene sediments. ⑤The F- concentration is negatively correlated with the Ca2+ concentration in groundwater. Considering that calcium fluoride (CaF2) is the main fluorine-containing mineral in the nature and the major source of fluoride in groundwater, the negative correlation between ρ(F-) and ρ(Ca2+) indicates the removal of Ca2+ and Mg2+ in high-fluoride groundwater via cation exchange, adsorption and/or carbonate precipitation. The F- content and Mg2+ concentration of groundwater are also in negative correlation and are highly similar to that between ρ(F-) and ρ(Ca2+), which also evidences the removal of Ca2+ and Mg2+. ⑥The chlor-alkalinity index (CAI) are negative for the majority of groundwater samples, and both CAI-1 and CAI-2 are negatively related with ρ(F-), indicating the exchange between Ca2+, Mg2+ and Na+ in high-fluoride groundwater. There is a great positive correlation between F- content and SAR value, and the average SAR value of high-fluoride groundwater (5.71) is much higher than that of low-fluoride groundwater (1.67), which further proves a strong alternation of Ca2+ and Mg2+ with Na+ in the aquifer and this exchange process plays a significant role in fluoride enrichment. ⑦All groundwater samples are undersaturated with respect to fluorite. An obvious positive correlation between the saturation index (SI) of fluorite and the F- content demonstrates that the continuous dissolution of fluorine-containing minerals (mainly fluorite) is the primary contribution to the accumulation of fluoride in groundwater in the study area. In contrast, the calcite is supersaturated in all groundwater samples, which suggests that the precipitation of calcite may promote the dissolution of CaF2, leading to the increase of F- content in groundwater. ⑧ The δ18O value is between -11.20‰ and -10.67‰ with an average value of -10.94‰ in low-fluoride groundwater, and between -11.65‰ and -11.21‰ with an average value of -11.49‰ in high-fluoride groundwater. Low-fluoride groundwater is more enriched with δ18O than high-fluoride groundwater. Furthermore, the groundwater δ18O value is negatively correlated with F- content when ρ(F-)≤3.0 mg/L, while it remains the same when ρ(F-)≥4.8 mg/L. The above pattern suggests that evaporation contributes little to the enrichment of fluoride in groundwater. ⑨The ρ(F-)/ρ(Cl-) ratio in groundwater is positively correlated with ρ(F-), which also evidences that the dissolution of fluorine-containing minerals leads to the enrichment of fluoride in groundwater, rather than the evaporation. In addition, Gibbs diagram shows that all groundwater chemistry are controlled by water-rock interaction, with little influence of evaporation. In conclusion, our results indicates that high fluoride concentrations in groundwater are controlled by fluorite dissolution and anion exchange between OH- in groundwater and F- in minerals, whereas the influence of evaporation is negligible. And the fluorite dissolution is promoted by the Ca2+ depletion due to calcite precipitation and cation exchange between Na+ absorbed on mineral surface and Ca2+ in groundwater. |
