Identifying the geochemical evolution and controlling factors of the shallow groundwater in a high fluoride area, Feng County, China

Understanding how groundwater is formed and evolves is critical for water resource exploitation and utilization. In this study, hydrochemistry and stable isotope tracing techniques were adopted to determine the key factors influencing groundwater chemical evolution in Feng County. A total of fourteen wells and five surface water samples were investigated in November 2021. The SD and delta O-18 compositions show that both surface water and groundwater are recharged from atmospheric precipitation. The dominating order of cations and anions in groundwater appears to be Na+ >Mg2+ > Ca2+ > K+ and HCO3- > SO42- > Cl- >NO3- > F-, respectively. The groundwater hydrochemical facies are mainly characterized by HCO3-Ca-Mg and SO4-Cl-Na types. The chemical evolution of groundwater is dominated by water-rock interaction and cation exchange reactions. The major ions in groundwater are mainly controlled by various geogenic processes including halite, gypsum, calcite, dolomite, Glauber's salt, feldspar, and fluorite dissolution/precipitation. Furthermore, the abundant fluoride-bearing sediments, together with low Ca2+, promote the formation of high F- groundwater. Approximately 85.7% and 28.6% of groundwater samples exceeded the permissible limit for F- and NO3- respectively. Apart from geogenic F-, human interventions (i.e., industrial fluoride-containing wastewater discharge and agricultural phosphate fertilizer uses) also regulate the F- enrichment in the shallow groundwater. Nitrate pollution of the groundwater may be attributed to domestic waste and animal feces. Our findings could provide valuable information for the sustainable exploitation of groundwater in the study area and the development of effective management strategies by the authorities.

Automated summary

Understanding groundwater formation and evolution is crucial for water resource exploitation. This study used hydrochemistry and stable isotope tracing techniques to determine the key factors influencing groundwater chemical evolution in Feng County. The results showed that groundwater is primarily recharged from atmospheric precipitation, with dominant cations including Na+ and Mg2+, and dominant anions including HCO3- and SO42-. The groundwater hydrochemical facies are mainly HCO3-Ca-Mg and SO4-Cl-Na types, controlled by water-rock interaction and cation exchange reactions. In addition to natural sources, human activities also contribute to the enrichment of fluoride ions in shallow groundwater.