The 18 main physicochemical parameters were analyzed in 8 surface water samples and 67 groundwater samples from a typical riverside alluvial aquifer (RAA), which is the main water source in northeast China and is mainly characterized by nitrogen pollution. By integrating hydrochemical methods and multivariate statistical methods (hierarchical cluster analysis (HCA), principal component analysis (PCA), and factor analysis (FA)), the hydrogeochemistry of this RAA was investigated in the present study to reveal the spatial characteristics of groundwater chemistry and estimate the major controlling factors and their contribution to the spatial variations of groundwater chemistry. The results indicated that the groundwater chemistry had high spatial variability in the study area and was characterized by exceeding concentrations of nitrogen, Fe, and Mn. According to the dominant chemical composition, the HCA classified these samples into four chemically distinct groups (groups 1–4). The groundwater quality deteriorated from group 1 to group 4, which was induced by anthropogenic pollution (agricultural activities and polluted river water) and diverse hydrogeochemical processes. PCA and FA were used to evaluate and verify the contribution of these processes to groundwater chemistry. Four factors that explained 76.52% of the total variance of these distinct groups were identified: (1) Cl⁻ and/or SO₄²⁻ salt input, mineral dissolution, and industrial and domestic sewage discharges; (2) nitrate generation processes; (3) the original geological environment based on water-rock interaction; (4) ammonia input from industrial and domestic sewage, and agricultural fertilizers. The results of the present study provide a valuable scientific basis for better understanding the characteristics of groundwater hydrochemical and hydrogeochemical processes, and it can be considered as a reference that contributes to the management of RAAs and relevant policy-making for similar regions.