Hydrochemical evolution characteristics and mechanism of groundwater funnel areas under artificial governance in Hengshui City, North China

The largest groundwater funnel area is located in the North China Plain (NCP) due to long-term and large-scale overexploitation of groundwater from 1970s. Comprehensive control and management measures have been implemented both in national-scale and local-scale since 2015. This paper presented the evolution of hydrochemical characteristics and its mechanism before (2014) and during the artificial governance (2015–2020) in the center of the NCP funnels (Hengshui City) based on water level, water chemical (including concentrations of K+, Na+, Ca2+, Mg2+, Cl−, SO42−, HCO3−, TH and TDS) data. By using Piper diagrams, ion ratio relationships, geostatistical analysis, Gibbs diagrams, chlor-alkali index and inverse hydrogeochemical simulation (PHREEQC), the following conclusions can be drawn: Temporally, the dominant cation and anion were Na+ and HCO3− both in shallow groundwater (SG) and deep groundwater (DG), respectively. Hydrochemical types both in SG and DG tend to be diverse, owing to the increase of SO42− and Cl− proportion after groundwater overdraft management. Spatially, ion concentrations (except for HCO3−) of discharge areas distributed higher due to shorter hydraulic retention time and flow path of recharge area. The impact of groundwater overdraft management on distribution characteristics of DG was lower than that in SG. Compared with 2014 (before artificial management), SG and DG hydrochemical components had stronger correlation relationships with groundwater level. Hydrochemistry field was influenced by groundwater depth fluctuation and the rates of it at varying degree in recharge and discharge areas. Leakage from SG to DG can be deduced by similar alteration of ion concentrations. In SG, the major water chemistry was controlled by evaporation and water–rock interaction, while dissolution of silicate and evaporate companied with ion exchange dominating the groundwater chemistry in DG from 2014 to 2020. Both in SG and DG simulation paths, dissolution/precipitation of mineral phases and artificial governance influence the hydrochemistry. DG also caused by SG leakage. During artificial management, human activities are a factor that cannot be ignored. Ion concentrations increased with the water–rock interaction enhanced, due to the rate of groundwater depth change decreasing.