Evaluating the influencing factors of groundwater evolution in rapidly urbanizing areas using long-term evidence

Groundwater ionic components evolution can simultaneously characterize the process of natural environment and anthropogenic activities on groundwater. In order to reveal the long-term evolution mechanism and influencing factors of urban groundwater in the context of rapid urbanization and industrialization. Studies carried out in Lanzhou, a region in Northwest China known for interconnection of anthropogenic activities and groundwater systems, sought to explore the mechanisms influencing the evolution of groundwater ionic components within the last 20 years. Hydrochemical analysis methods and integrated groundwater ion source analysis methods were utilized to isolate natural and anthropogenic factors in the evolution of groundwater ions, and subsequently combined with LULC to analyze the correlation between the type and intensity of land changes on groundwater ion components. The results show that, the ionic composition of groundwater was dominated by ion exchange and mineral weathering, river recharge, industrial activities, urban activities, and agricultural activities in 26%, 10%, 23%, 25%, and 16% of the sample sites respectively in 2002. However, in 2022, the proportion of sites dominated by these five categories is 20%, 18%, 12%, 40%, and 10%, respectively. Further combining the yearly trends in LULC, it is concluded that the decrease of cultivated land area and the significant increase of urban construction land in Lanzhou are closely related to the groundwater evolution process in Lanzhou in the past 20 years. Today's agricultural activities no longer dominate the ionic fraction of groundwater in Lanzhou, but increased population density and urban sewage discharge due to rapid and intensive urbanization are the main factors influencing the ionic fraction of groundwater.