Evolution of groundwater fluoride in land subsidence areas after groundwater cessation: A case study at Cangzhou

Abstract

Objective The South-to-North Water Transfer Project (SNWTP) has improved water use in northern China and further reduced deep groundwater extraction in the North China Plain. However, its impact on the evolution of regional groundwater hydrochemistry is still unknown. Methods In this paper, Cangzhou, which has experienced severe land subsidence due to groundwater overexploitation and has received water resources from the SNWTP since 2015, was selected as the study area to investigate the effects of the SNWTP on groundwater chemistry. In 2017 and 2021, groundwater samples from Ⅲ and Ⅳ confined aquifers were collected to determine the hydrochemical characteristics. Moreover, the average annual land subsidence of the regional layer was further evaluated by using SBAS-InSAR to identify the correlation between variations in land deformation and changes in groundwater chemistry. Results The results showed that in comparison with those before the SNWTP, the groundwater fluoride concentration was slightly decreased, and the area of the high value zone was reduced after the SNWTP. The groundwater environment characterized by high pH, TDS and HCO3- concentrations and low Ca2+ concentrations favors fluoride enrichment in groundwater. The water chemistry type did not change, and the salinity concentrations in groundwater increased after the SNWTP. Groundwater receives more dissolution of halite and fluorite. Meanwhile, the amount and rate of land subsidence slowed after the SNWTP. A small degree of land uplift was even observed in the southeastern part of the region. The inhibition of land subsidence constrains clay compaction and the release of high fluoride porewater trapped in the clay layer. As a result, aquifers Ⅲ and Ⅳ received more effective lateral recharge, thereby causing a slight decrease in groundwater fluoride. However, it needs to be noted that the longer hydraulic residence time and stronger water-rock interaction after the SNWTP interaction promote the dissolution of sediment evaporites into groundwater, leading to an increase in groundwater salinity. Conclusion The results of the study provide scientific support for drinking water safety and water resource management in Cangzhou.

Keywords

• groundwater
• fluorine
• south-to-north water transfer
• groundwater cessation
• land subsidence