Enrichment mechanism of fluoride in different aquifer systems of the Aksu area located in the Tarim Basin, Northwest China

In inland arid areas, the fluoride (F-) content of groundwater, which is an important source of drinking water, is often reported to exceed the standard. However, systematic comparative studies on the enrichment mechanism of F-in groundwater from different aquifer systems in the same area are lacking. In the present study, hydro-geochemistry, environmental isotopes, and hydrogeochemical reverse simulations were jointly employed to explore the distribution characteristics and enrichment mechanisms of F-in different aquifer systems of Aksu, Xinjiang. Our results showed that low-fluoride ((F-)<1.5 mg/L) groundwater was mainly distributed in the recharge area, whereas the high-fluoride ((F-)>1.5 mg/L) groundwater was mainly distributed in the runoff and discharge areas. The main enrichment mechanism of F-in the natural area of the Akotai shallow aquifer system may be the dissolution of fluorinated minerals (fluorite and biotite). The main enrichment mechanism of F-in the natural area of the Yi-A shallow aquifer system may be the dissolution of fluorinated minerals (fluorite) with cation exchange as the driving mechanism. The main enrichment mechanism of F-in the natural areas of deep aquifer system may also be the dissolution of fluorinated minerals (fluorite), with cation exchange and calcite precipitation as the driving mechanisms. The exchange between hydroxide (OH-) in groundwater and F-on the mineral surface may also contribute to the enrichment of F-in the natural areas of the Yi-A shallow aquifer system and the deep aquifer system. Recharge from irrigation water reduced the F-content of Akotai shallow groundwater in the irrigated area (Duolang Reservoir-Alar area) but did not reduce the F-content of the Yi-A shallow groundwater in the irrigated area (Wutuan area). The recharge from the deep groundwater originating from the direction of Awat outside the study area reduced the F-content of the deep groundwater in the Duolang Reservoir-Alar area. Evaporation had a weak effect on the enrichment of F-in groundwater. The results of the present study can provide a basis for the rational development of local water resources and would be helpful in understanding the common F-enrichment mechanisms in similar inland arid basins globally.

Automated summary

This study investigates and compares the enrichment mechanisms of fluoride (F-) in groundwater from different aquifer systems in Aksu, Xinjiang, using hydro-geochemistry, environmental isotopes, and hydrogeochemical reverse simulations. The results show that low-fluoride groundwater is mainly distributed in the recharge area, while high-fluoride groundwater is mainly distributed in the runoff and discharge areas. The main enrichment mechanisms of F- in different aquifer systems involve the dissolution of fluorinated minerals (fluorite and biotite) and cation exchange. Recharge from irrigation water and deep groundwater can influence the F-content of groundwater. Evaporation has a weak effect on F-enrichment. This study provides insights into the rational development of local water resources and contributes to the understanding of common F-enrichment mechanisms in similar arid basins globally.