Evolution of Hydrogeochemistry in the Haolebaojinao Watershed of the Ordos Basin, China

The exploitation of groundwater in arid and semi-arid areas, especially in basins where groundwater is intensively exploited, is likely to have adverse effects on the originally fragile ecological environment, which also greatly alter the hydrogeochemical evolution process. Over-abstraction of groundwater in the Haolebaoji watershed of the Ordos Basin located in the semi-arid regions has led to a series of changes in the groundwater system, which has attracted considerable attention from environmental protection organizations. However, the origin and geochemical evolution of groundwater in the Haolebaoji watershed have not been revealed. In this study, the Haolebaoji watershed is selected as the typical study area to investigate hydrogeochemical evolution under the intensive groundwater exploitation. Groundwater samples were collected and tested for major ions and stable isotopes (delta O-18, delta D). Various approaches including the ion proportional relationship diagram, chlor-alkali index, saturation index, Gibbs diagram, and principal factor analysis were used to reveal the hydrogeochemical processes regulating the groundwater geochemistry. The groundwater in the study area is divided into five hydrochemical types according to the Piper diagram. It was found that the chemical composition of groundwater in the study area is mainly controlled by the dissolution of calcite, dolomite, gypsum, and halite. The cation exchange intensity gradually increased with the flow of groundwater from the recharge to the discharge area. Rock weathering plays a controlling role in the formation of groundwater geochemistry, but it is also controlled by evaporative crystallization in some runoff and discharge areas. Groundwater is predominantly recharged by modern local atmospheric precipitation, and deep wells receive water supply during cold periods. The study findings provide important information for the development of sustainable groundwater management strategies for the Ordos Energy Base.