[Ion Concentrations and Their Spatial Variability in Underground Water and Surface Water in Typical Terrestrial...

The water chemistry data monitored during 2010-2015 by 33 terrestrial ecological stations from the Chinese Ecosystem Research Network (CERN) and the National Ecosystem Research Network of China (CNERN) were used to characterize ion concentrations and their spatial variability in underground water, still surface water, and flowing surface water from typical terrestrial ecosystems. The results showed the presence of mass-based concentrations of major anions, including HCO₃^- > SO₄^2- > Cl^- > CO₃^2-. Among them, HCO₃^- and SO₄^2- were dominant, and their sums accounted for 71.7%, 75.3%, and 74.9% of the total anions in underground water, still surface water, and flowing surface water, respectively. Cations were mainly Ca²⁺ and Na⁺, and their sums accounted for 69.7%, 64.8%, and 68.9% of the total cations in underground water, still surface water, and flowing surface water, respectively. The ion concentration and ion ratio in the underground water, still surface water, and flowing surface water differed largely among the studied regions. The hydrochemical type varied regionally, e.g., Na-Mg-SO₄-Cl type, usually with high content of salinity, was found in the underground water of ecological systems in the Northwest arid and semiarid areas and in the East Huanghuaihai Plain; Ca-SO₄-HCO₃ type in underground water and Ca-HCO₃-SO₄ type in surface water were found in hilly areas with subtropical red soil; Na-Ca-HCO₃-Cl type was present in underground water of south hilly areas with subtropical latosolic red soil; and Ca-HCO₃ and Ca-Mg-HCO₃ types were found in other ecological systems. Hydrochemical types had low inter-annual variation for both underground water and surface water.