Temporal Changes of Groundwater Quality within the Groundwater Depression Cone and Prediction of Confined Groundwater Salinity Using Grey Markov Model in Yinchuan Area of Northwest China

Due to long-term overexploitation, the Yinchuan area of China has formed a groundwater depression cone in the confined aquifer. The formation of the groundwater depression cone will have a great impact on the hydrochemical evolution of groundwater, thus affecting the groundwater quality and associated human health. To study the characteristics and evolution of the depression cone and its impacts on the confined groundwater quality, groundwater level data from 1990 to 2019 and groundwater quality data from 1991 to 2018 were collected and analyzed. Groundwater level charts were generated to delineate the evolution of the groundwater depression cone. Graphical approaches and correlation analysis were applied to reveal the general hydrochemical characteristics of the groundwater within the depression cone and the major factors affecting the hydrochemical evolution of the confined groundwater. Grey Markov model was used to predict the TDS concentration in the confined aquifer in 2019 and 2020. The results show that the groundwater level in the groundwater depression cone center has recovered to a certain extent since 1993 due to existing groundwater protection measures, but the horizontal area of the groundwater depression cone (the area confined by 1100 m groundwater level contour) has increased greatly and it has expanded to 310.51 km 2 by 2019. Groundwater samples from the phreatic aquifer were mainly of SO 4 ·Cl–Ca·Mg and HCO 3 –Ca·Mg types, and the confined groundwater was mainly of SO 4 ·Cl–Na and HCO 3 –Ca·Mg types. Groundwater leakage from phreatic aquifer to the confined aquifer, dissolution and precipitation of minerals and cation exchange are important factors affecting the hydrochemical evolution of confined groundwater, and these factors have different effects in different years. The forecasted values of TDS concentration in 2019 and 2020 are 450.60 and 433.50 mg/L, respectively.