Based on the geo-statistical theory and ArcGIS geo-statistical module, datas of 30 groundwater level observation wells were used to estimate the decline of groundwater level in Beijing piedmont. Seven different interpolation methods (inverse distance weighted interpolation, global polynomial interpolation, local polynomial interpolation, tension spline interpolation, ordinary Kriging interpolation, simple Kriging interpolation and universal Kriging interpolation) were used for interpolating groundwater level between 2001 and 2013. Cross-validation, absolute error and coefficient of determination (R2) was applied to evaluate the accuracy of different methods. The result shows that simple Kriging method gave the best fit. The analysis of spatial and temporal variability suggest that the nugget effects from 2001 to 2013 were increasing, which means the spatial correlation weakened gradually under the influence of human activities. The spatial variability in the middle areas of the alluvial–proluvial fan is relatively higher than area in top and bottom. Since the changes of the land use, groundwater level also has a temporal variation, the average decline rate of groundwater level between 2007 and 2013 increases compared with 2001–2006. Urban development and population growth cause over-exploitation of residential and industrial areas. The decline rate of the groundwater level in residential, industrial and river areas is relatively high, while the decreasing of farmland area and development of water-saving irrigation reduce the quantity of water using by agriculture and decline rate of groundwater level in agricultural area is not significant.

The usage of reclaimed water can efficiently mitigate water crises, but it may cause groundwater pollution. To clearly understand the potential influences of long-term reclaimed water usage, a total of 91 samples of shallow and deep groundwater were collected from a typical reclaimed water use area during the dry and rainy seasons. The results suggest both shallow and deep groundwater are mainly naturally alkaline freshwater, which are composed mainly of Ca-HCO₃, followed by mixed types such as Ca-Na-HCO₃ and Ca-Mg-HCO₃. A seasonal desalination trend was observed in both shallow and deep aquifers due to dilution effects in the rainy season. Groundwater chemical compositions in both shallow and deep aquifers are still dominantly controlled by natural processes such as silicate weathering, minerals dissolution and cation exchange. Human activities are also the factors influencing groundwater chemistry. Urbanization has been found responsible for the deterioration of groundwater quality, especially in shallow aquifers, because of the relative thin aquitard. Reclaimed water usage for agricultural irrigation and landscape purposes has nearly no influences on groundwater quality in rural areas due to thick aquitards. Therefore, reclaimed water usage should be encouraged in arid and semiarid areas with proper hydrogeological condition.

In the karst area of southern China, karst water is important for supporting the sustainable production and home living for the local residents. Consequently, it is of significance to fully understand the water cycle, so as to make full use of water resources. In karst areas, epikarst and conduits are developed, participating in the hydrological cycle actively. For conventional lumped hydrologic models, it is difficult to simulate the hydrological cycle accurately. These models neglect to consider the variation of underlying surface and weather change. Meanwhile, for the original distributed hydrological model, the existence of epikarst and underground conduits as well as inadequate data information also make it difficult to achieve accurate simulation. To this end, the framework combining the advantages of lumped model–reservoir model and distributed hydrologic model–Soil and Water Assessment Tool (SWAT) model is established to simulate the water cycle efficiently in a karst area. Xianghualing karst watershed in southern China was selected as the study area and the improved SWAT model was used to simulate the water cycle. Results show that the indicators of E_NS and R² in the calibration and verification periods are both above 0.8, which is evidently improved in comparison with the original model. The improved SWAT model is verified to have better efficiency in describing the hydrological cycle in a typical karst area.

Ecological water supplement relies on river channels to introduce surface water, to make a reasonable supplement of groundwater, to repair the regional groundwater environment and urban river ecosystem. Evaluating the degree of groundwater restoration after ecological water supplement (by taking appropriate measures) is a critical problem that needs to be solved. Thus, based on the Yongding River ecological water supplement in 2019 and 2020, we analyzed the groundwater monitoring situates in the ecological water supplement region. We established an unstructured groundwater flow numerical model in the study area through the quadtree grids. The model was calibrated with the measured water level. The simulated results could accurately reflect the real groundwater dynamic characteristics, and it showed that the water level rise was concentrated in the 3–6 km range of the Yongding River after the ecological water supplement. In 2019, the calculated ecological water infiltration amount was 101.28 × 10⁶ m³, the affected area was 265.19 km², and the average groundwater level rise in the affected area was 2.10 m. In 2020, the calculated ecological water infiltration amount was 102.64 × 10⁶ m³, the affected area was 506.88 km², and the average groundwater level rise in the affected area was 1.25 m. While the ecological water supplement had a positive impact on groundwater level restoration, the groundwater level around the typical buildings within the study area, including Beijing West Railway Station and Beijing Daxing International Airport, would not be significantly affected.

Hexavalent chromium contamination in groundwater has become a very serious and challenging problem. Identification of the groundwater chemical characteristics of the sites and their control mechanisms for remediation of pollutants is a significant challenge. In this study, a contaminated site in Xinxiang City, Henan Province, was investigated and 92 groundwater samples were collected from the site. Furthermore, the hydrogeochemical characteristics and the distribution patterns of components in the groundwater were analyzed by a combination of multivariate statistical analysis, Piper diagram, Gibbs diagram, ions ratio and hydrogeochemical simulation. The results showed that the HCO₃-Cl-Mg-Ca type, SO₄-HCO₃-Na type, and HCO₃-Mg-Ca-Na type characterize the hydrogeochemical composition of shallow groundwater and HCO₃-Cl-Mg-Ca type, HCO₃-Na-Mg type, and HCO₃-SO₄-Mg-Na-Ca type characterize the hydrogeochemical composition of deep groundwater. Ion ratios and saturation index indicated that the groundwater hydrogeochemical characteristics of the study area are mainly controlled by water–rock action and evaporative crystallization. The dissolution of halite, gypsum and anhydrite, the precipitation of aragonite, calcite and dolomite, and the precipitation of trivalent chromium minerals other than CrCl₃ and the dissolution of hexavalent chromium minerals occurred in groundwater at the site. The minimum value of pH in groundwater at the site is 7.55 and the maximum value is 9.26. The influence of pH on the fugacity state of minerals was further investigated. It was concluded that the saturation index of dolomite, calcite, aragonite and MgCr₂O₄ increases with the increase of pH, indicating that these minerals are more prone to precipitation, and the saturation index of Na₂Cr₂O₇, K₂Cr₂O₇ and CrCl₃ decreases with the increase of pH, implying that Na₂Cr₂O₇, K₂Cr₂O₇ and CrCl₃ are more prone to dissolution. The saturation index of the remaining minerals is less affected by pH changes. The study can provide a scientific basis for groundwater remediation.