Groundwater-flow-system characterization with hydrogeochemistry: a case in the lakes discharge area of the Ordos Plateau, China

Understanding the pattern of regional groundwater circulation is essential for sustainable management of groundwater resources and ecosystems protection. A large-scale basin may develop nested groundwater flow systems including local, intermediate and regional flow systems. Hydrogeochemical tracing may be an effective methodology to identify different groundwater flow systems, considering its routine application in field investigation. This study uses wavy-topography-driven regional groundwater flow in the groundwater-fed lakes area of the northern Ordos Plateau, China, as an example to test the effectiveness of a hydrogeochemical method for groundwater-flow-system characterization. Samples of groundwater from wells with different depths and lake water were collected and analyzed. Hierarchical cluster analysis was conducted using the pH, electrical conductivity, and major ions as the input, which leads to three clusters with distinct geochemical compositions. Considering the hydrochemical characteristics, wells depths, and sampling locations, different groundwater flow systems were identified. Geochemical evolution was affected by processes such as leaching, cation exchange, evaporation and human activities. The relationship between D and O-18 indicates that the shallow and deep groundwater were recharged by atmospheric precipitation during the modern time and a past colder period, respectively. The groundwater geochemistry is closely related to groundwater circulation depth within different flow systems, indicated by comparison of geochemical processes among the three clusters. This work highlights a hydrochemical method that can identify nested groundwater flow systems in the lakes discharge area of this large-scale basin and provides a better understanding of the hydrogeochemical evolution from the processes involved in relation to groundwater flow systems.