The interaction of surface water (SW) and groundwater (GW) is becoming more and more complex under the effects of climate change and human activity. It is of great significance to fully understand the characteristics of regional SW–GW circulation to reveal the water circulation system and the effect of its evolution mechanism to improve the rational allocation of water resources, especially in arid and semi-arid areas. In this paper, Yinchuan Plain is selected as the study area, where the SW–GW interaction is intensive. Three typical profiles are selected to build two-dimensional hydrogeological structure models, using an integrated approach involving field investigation, numerical simulation, hydrogeochemistry and isotope analysis. The SW–GW transformation characteristics are analyzed with these models, showing that geological structure controls the SW–GW interaction in Yinchuan Plain. The SW–GW flow system presents a multi-level nested system including local, intermediate and regional flow systems. The runoff intensity and renewal rate of different flow systems are evidently different, motivating evolution of the hydro-chemical field; human activities (well mining, agricultural irrigation, ditch drainage, etc.) change the local water flow system with a certain impacting width and depth, resulting in a variation of the hydrological and hydro-chemical fields. This study presents the efficacy of an integrated approach combining numerical simulation, hydrogeochemistry and isotope data, as well as an analysis for the determination of GW-SW interactions in Yinchuan Plain.

Groundwater in the Yinchuan Plain (China) is a critical domestic resource that is also used for agricultural irrigation and to maintain ecological environments, among other purposes. Recent research has shown that ineffective planning of water resources, along with large-scale groundwater pumping (mining) has led to ecological problems. To further analyze the characteristics of the regional groundwater flow patterns, and guide the development and utilization of water resources, potentiometric, hydrochemical, and isotopic data were collected along a 60-km transect that crosses the middle Yinchuan Plain. The data were used to develop a two-dimensional conceptual model of the sources, flow patterns, and geochemical evolution of groundwater from the Helan Mountains in the west across the Yellow River in the east. An important component of the model is that in the process of groundwater flow from west to east, the flow direction changes due to the influence of a thick fine-grained sandy-clay unit and long-term groundwater pumping. Local lakes and the shallow groundwater system are recharged by water from the Yellow River. Geochemically, water within the proluvial deposits exhibits relatively low concentrations of total dissolved solids. Further east, the water gradually becomes brackish. The geochemical composition of the shallow groundwater beneath the fluviolacustrine plain west of the Yellow River is also controlled by evaporation, precipitation and cation exchange processes. In other areas, groundwater chemistry is mainly controlled by water-rock interactions and cation exchange. This study enhances understanding of groundwater flow in the region, and provides information critical to water resources development and management.

In this study, the focus was placed on the entire Hetao Basin that could fall into four hydrogeological units, i.e., Houtao Plain, Sanhuhe Plain, Hubao Plain, and South Bank Plain of Yellow River, which are under different geological and environmental conditions. To systematically investigate the hydrochemical characteristic and spatial distribution of high As groundwater (As >10 μg/L), 974 samples were collected from shallow groundwater. As indicated from the results, high As groundwater had an extensive distribution, and its spatial distribution in the four hydrogeological units exhibited significant variability. Three concentrated distribution areas were reported with high As groundwater, which were all in the discharge areas of groundwater, and the arsenic contents in groundwater was found to exceed 50 μg/L. The hydrochemical types of high As groundwater in the HT Plain and the SHH Plain consisted of HCO3 SO4·Cl by anions and Na by cations, while those in the other two plains included HCO3 by anions as well as Na·Mg·Ca by cations. According to the pH values, the groundwater was weakly alkaline in the areas with high As groundwater, and arsenic primarily existed as arsenite. Furthermore, high As groundwater in the Hetao Basin was characterized by high contents of Fe (mean value of 2.77 mg/L) and HCO3- (mean value of 460 mg/L) and a low relative concentration of SO42- (average value of 310 mg/L). This study did not identify any significant correlation in groundwater arsenic and other ions (e.g., Fe2+, Fe3+, HCO3-, SO42-, NO2- and NO3-) in the entire Hetao Basin and a wide range of hydrogeological units. The results remained unchanged after the four hydrogeological units were analyzed. The special sedimentary environment evolution of Hetao basin was found as the prerequisite for the formation of high arsenic groundwater. Furthermore, groundwater runoff conditions and hydrogeochemical processes in the basin were indicated as the factors controlling the formation of high arsenic groundwater.