This paper is the result of hydrogeological survey engineering.
Objective The Luanhe River Delta has naturally occurring high fluoride groundwater. Identifying the spatial distribution characteristics of fluoride in groundwater and analyzing its enrichment mechanism is beneficial for ensuring the safety of local residents' water supply.
Methods This study collected a total of 96 shallow and 190 deep groundwater samples on-site and conducted a systematic analysis of the hydrochemical characteristics of groundwater and the hydrogeochemical processes related to the formation of high fluoride groundwater.
Results Results show that 8% of the shallow groundwater samples and 21% of the deep groundwater samples in the Luan River Delta contain fluoride content higher than 1.0 mg/L of Chinese drinking water standards and groundwater quality standards. Shallow high fluoride groundwater is locally distributed in a small area, while deep high fluoride water is mainly concentrated in deep aquifers dominated by HCO3-Na·Ca type water. The calculation result of PHREEQC saturation index indicates that fluorite in groundwater is unsaturated, and fluorite mineral dissolution is the main source of groundwater F-.
Conlusions A comparative analysis of the water chemistry characteristics of shallow and deep groundwater with high F- suggested that evaporation and concentration have a significant impact on the enrichment of F- in shallow groundwater, while the enrichment of fluoride in deep groundwater is mainly controlled by desorption, competitive adsorption and cation exchange. In addition, seawater intrusion has little effect on F- enrichment in groundwater. Therefore, this study recommends that high fluoride groundwater in the shallow and deep aquifers should be treated separately using electrochemical methods and coagulation-precipitation or adsorption methods, respectively, to reduce fluoride levels.
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