The Izeh Plain in northeast Khuzestan, with a population of over 200,000 people, faces significant nitrate contamination in its groundwater due to its location on an alluvial aquifer, extensive agricultural activities, and wastewater disposal through absorption wells. This study aimed to investigate the hydrogeochemistry of the Izeh aquifer, determine the extent of human impacts on groundwater chemistry, and evaluate the efficiency of the simple and rapid Reference Ion Method (RIM) in identifying the primary sources of nitrate pollution and estimating the relative contribution of each source. Groundwater samples were collected from 19 water wells during both dry and wet seasons. Hydrochemical diagrams, statistical analyses, including Pearson correlation coefficients, factor analysis, and cluster analysis, were applied to identify influencing factors. The results showed that the hydrochemistry of the aquifer is influenced by adjacent carbonate formations, karstic aquifers and mixing with infiltrated wastewater, leading to increased chloride and nitrate concentrations in the groundwater. The results indicated that in the dry season, the largest share of nitrate sources was attributed to sewage and manure, while in the wet season, the contribution from chemical fertilizers increased. Validation of the RIM results with landuse maps demonstrated good agreement between the identified nitrate sources and upstream landuse in the groundwater flow direction. This study confirmed the high capability of the RIM method in identifying nitrate pollution sources and suggested that threshold values for identifying nitrate sources should be determined locally. The findings of this research can assist decision-makers in managing the quality of groundwater resources.