This study investigates the impact of natural and anthropogenic factors on the physicochemical composition of groundwater in the Qazvin aquifer. Based on the optimized Gibbs diagram, the concentration of samples at the end of the freshwater interaction path with silicate units results from geochemical evolution due to the dissolution of these geological units and an increase in the Na/(Na+Ca) ratio. The ion exchange mechanism was assessed using bivariate diagrams of Ca+Mg vs. SO4+HCO3 and Schoeller's chloro-alkaline indices CAI-1 and CAI-2. The results indicate that in 68% of the samples, direct ion exchange, and in 32%, reverse ion exchange control the groundwater chemistry. The changes in Ca vs. SO4 indicate that gypsum dissolution alone is not the source of these ions. These changes could be due to ion mobility and transport during pedogenic processes (sulfur biogeochemical cycle) and anthropogenic factors. The study also examined the role of factors such as agricultural input, atmospheric input, soil nitrogen, sewage input, manure input, chemical fertilizers, and the denitrification process in groundwater pollution using NO3/Na vs. Cl/Na and the NO3/Cl vs. Cl diagrams. The results reveal that agricultural and sewage inputs significantly impact the NO3 and Cl content. Furthermore, in some locations, especially in the southeast of the aquifer, the denitrification process causes a decrease in NO3 concentration. These findings can contribute to effective water resource management in this strategic aquifer by understanding the controlling mechanisms of physicochemical composition and identifying potential groundwater pollution sources.