Natural background levels (NBLs) and threshold values (TVs) are two important parameters for identifying and quantifying groundwater pollution. Given this, this study investigated the spatiotemporal changes and affecting factors of NBLs and TVs for the phreatic groundwater in different environmental units divided as per the geomorphological units, the geological structures, the catchment conditions, and the types of the aquifer media in the Greater Xi'an area of China. The groundwater environmental background values in each unit were determined using the Grubbs method. The NBLs and TVs of K+, Na+, Mg2+, Ca2+, SO42−, Cl−, HCO3−, and TDS were then estimated and discussed with respect to their changes associated natural and anthropogenic processes. The results show that all ions have exceeded their standard limits to different degrees. The water quality in 2020 was worse than that in 2015. The NBLs of the groundwater hydrochemical components in the northern Wei River zone were higher compared with those in the southern Wei River zone during the study period due to the differences in geological formations, rainfall, evaporation and ground surface slope. In agricultural areas, agricultural activities may lead to the percolation of surface pollutants into the deeper parts of the aquifer with the irrigation water, causing the NBLs in the alluvial plain area being generally higher. In addition, urbanization also affects the hydrochemical compositions of groundwater. The high NBLs of TDS, Mg2+, SO42−, HCO3− in the area near Weinan City may be due to the over-exploitation of groundwater. The spatiotemporal evaluation of NBLs lays a foundation for determining the scope and sources of groundwater pollution, and may support the effective groundwater development, utilization, and protection.
