Geochemical evidence for annual variability of fluoride levels in deep groundwater on land subsidence plain in Cangzhou, Hebei Province, China

Ecological water supplementation effectively controls the land subsidence rate and protects the groundwater environment. However, the annual variability and evolutionary mechanisms of F- concentration in deep groundwater during ecological water supplementation are poorly understood. In total, 92 deep groundwater samples from 2012 to 2021 were collected and analyzed in the Cangzhou land subsidence area. The results show that the range of F- concentrations in deep groundwater was 0.10-7.83 mg/L during 2012-2021, with 44.08 % of samples exceeding China's national standards (1.00 mg/L) for drinking water. The F- concentration in deep groundwater increased steadily during 2012-2017 and exhibited a notable decrease during 2018-2021. Halite dissolution was more effective after ecological water supplementation, inducing a decrease in F- concentration by dilution effect in deep groundwater. Meanwhile, stronger cation exchange facilitated F-desorption into the deep groundwater after ecological water supplementation. This research will aid in improving our understanding of the influence of ecological water supplementation on groundwater quality and provide new insights for the sustainable management of deep groundwater resources.