Groundwater hydrochemical characteristics and processes along flow paths in the North China Plain

• Deep groundwaters are depleted in D, 18O and Ca2+ and NO3- relative to shallow ones.
• Na+ and Cl− increased, while Ca2+ and Mg2+ decreased along flow paths.
• Carbonate precipitation, gypsum and halite dissolution, ion exchange were dominant.
• Possible contamination of NO3- in shallow groundwater of alluvial fan areas.
• Evaporation occurred in shallow groundwater while sulfate reduction in deep one.

The North China Plain is one of the biggest plains in China, where municipal, agricultural and industrial water supplies are highly dependent on groundwater resources. It is crucial to investigate water chemistry and hydrogeochemical processes related to hydrogeologic settings for sustainable utilization of groundwater resources. Two hydrochemical profiles proximately along the groundwater flow paths were selected for hydrogeochemical study. Major components and 2H and 18O isotopes were analyzed in groundwater samples from the profiles. The study area was divided into three zones, including strong runoff-alluvial/pluvial fans in the piedmont area (Zone I), slow runoff-alluvial/lacustrine plain in the central area (Zone II), and discharge-alluvial/marine plain in the coastal area (Zone III). Major components of groundwater samples showed obvious zonation patterns from Zone I to Zone III. Total dissolved solid (TDS) concentrations gradually increased, and the hydrochemical type changed from HCO3–SO4–Ca–Mg and HCO3–Cl–Ca–Mg types to HCO3–SO4–Na–Ca, SO4–Cl–Na–Ca and SO4–Cl–Na types from Zone I to Zone III. Abrupt increases in concentrations of Na+, Cl− and SO42- in deep groundwater were observed around the depression cones, which indicated that overexploitation resulted in water quality deterioration. Calcite and dolomite precipitation occurred in Zone I of deep groundwater systems and shallow groundwater systems. Cation exchange was believed to take place along the entire flow paths. Gypsum tended to dissolve in groundwater systems. The depletion in D and 18O isotopes in deep groundwater was related to the recharge from precipitation in paleo-climate conditions in glacial or interglacial periods, indicating that renewal groundwater was very limited. Efficient strategies must be taken to preserve the valued water resources for sustainable development.