Groundwater quality assessment in urban parts of Vellore city, India: Focusing on Nitrogen Species contamination and its health risk analysis

Nitrogen contamination in groundwater is a global issue, particularly in urban areas of developing nations, where identifying pollution sources is critical for sustainable groundwater management. This study undertook a comprehensive investigation of nitrogenous compounds, hydrogeochemical parameters, and seasonal variations in groundwater in Vellore City, Tamil Nadu, with a focus on nitrate contamination and its associated health risks. A total of 256 groundwater samples were systematically collected from Vellore city, Tamil Nadu, with 32 samples collected during each season dry (January and May) and wet (September and November) over the period 2022–2023. The results shown the anion dominance followed the order Cl > HCO 3 > SO 4 > NO 3 , and cation dominance was Na > Ca > Mg > K. During the wet season of 2022, 93 % of groundwater samples exceeded the BIS nitrate limits, while 84 % in the dry season. Additionally, 100 % of samples from both seasons exceeded the EPA limit for Total Kjeldahl Nitrogen (TKN). A strong correlation between TKN and nitrate, especially during wet seasons, suggested shared pollution sources. The Piper diagram indicated a mixed Ca-Mg-Cl water type in 62.5 % of dry season samples and 56.25 % of wet season samples. Water Quality Index (WQI) classified 68 % of samples as "poor" quality. Health risk assessments revealed significant risks for all demographics, with children being most vulnerable, indicated by high Hazard Quotient (HQ) and Hazard Index (HI) values exceeding 5. Further the Nitrate Pollution Index (NPI) classified 68.75 % of dry season samples and 81.25 % of wet season samples as having “very significant” pollution. The study stresses the need for improved waste management, Sewage Treatment Plants (STPs), and better sanitation to mitigate nitrogen pollution in Vellore city. • 256 groundwater samples were collected in wet and dry seasons of 2022 and 2023, with 32 samples per season. • Dominant ions observed were Cl > HCO₃ > SO₄> NO₃> F and Na > Mg > Ca> K in both seasons. • TKN and NO₃ exceeded limits in 90% of samples, indicating severe nitrogen contamination. • WQI showed 68% of samples as poor quality; Ca-Mg-Cl type dominated water chemistry. • Hazard Index showed health risks to children and women, NPI indicated very high nitrate pollution.