Nitrate contamination in groundwater of the Tehran–Karaj aquifer, Iran, poses significant human health risks. Using data from 75 wells over 11 years (2007–2018), researchers mapped nitrate concentrations and non-carcinogenic risks. Nitrate levels ranged from 6 to 150 mg/L, exceeding the WHO standard of 50 mg/L for drinking water. High nitrate concentrations were mainly found in the eastern part of the aquifer, threatening children's health, the most vulnerable group. While the number of wells with elevated nitrate levels for adults decreased over time, the eastern region remained a high-risk area. Improved sanitation infrastructure and reduced agricultural practices helped mitigate nitrate pollution. The study highlights the need for further actions to reduce nitrate levels below 50 mg/L to protect public health. Nitrate contamination is a global issue, with significant health impacts, including methemoglobinemia and cancer risks. The study underscores the importance of monitoring and managing groundwater quality in densely populated areas.Nitrate contamination in groundwater of the Tehran–Karaj aquifer, Iran, poses significant human health risks. Using data from 75 wells over 11 years (2007–2018), researchers mapped nitrate concentrations and non-carcinogenic risks. Nitrate levels ranged from 6 to 150 mg/L, exceeding the WHO standard of 50 mg/L for drinking water. High nitrate concentrations were mainly found in the eastern part of the aquifer, threatening children's health, the most vulnerable group. While the number of wells with elevated nitrate levels for adults decreased over time, the eastern region remained a high-risk area. Improved sanitation infrastructure and reduced agricultural practices helped mitigate nitrate pollution. The study highlights the need for further actions to reduce nitrate levels below 50 mg/L to protect public health. Nitrate contamination is a global issue, with significant health impacts, including methemoglobinemia and cancer risks. The study underscores the importance of monitoring and managing groundwater quality in densely populated areas.