Groundwater contamination by nitrates (NO3-) poses a major environmental and public health risk worldwide. This study investigates nitrate pollution in the Mitidja aquifer (Algeria) using the Nitrate Pollution Index (NPI) and non-carcinogenic risk assessment through the hazard quotient (HQ) method proposed by the US Environmental Protection Agency (USEPA). Forty-nine groundwater samples were collected in 2019-2026 during the wet season and 23 during the dry season. Nitrate concentrations were analyzed using the Gaussian mixture model (GMM) for cluster classification, while non-carcinogenic risks were estimated for adults and children through Monte Carlo simulations (10 000 iterations) to quantify parameter uncertainty. Results reveal that 42.3% of wet season and 43.5% of dry season samples exceed the World Health Organization (WHO) guideline of 50 mg/L. GMM identified two clusters per season, distinguishing moderately and highly contaminated zones. Based on NPI, 50% of wet season samples and 47.5% of dry season samples were classified as "clean to low pollution," while 38.5% and 43.4%, respectively, indicated "high to very high pollution." HQ values ranged from 0.06 to 7.18 (children) and from 0.02 to 3.08 (adults) in the wet season and from 0 to 7.81 (children) and from 0 to 3.34 (adults) in the dry season. Probabilistic results showed 25th-95th percentile HQ values of 0-8.95 for children and 0-2.94 for adults in the wet season, and 0-8.34 and 0.027-2.72, respectively, in the dry season. Sensitivity analysis confirmed children's higher vulnerability to nitrate exposure. These results highlight the urgent need for groundwater protection and remediation strategies in the Mitidja region.