Health risk assessment of nitrate and fluoride in groundwater across 10 Nigerian states

Nitrate and Fluoride are both useful and deadly at varying concentrations, hence the need for continuous monitoring. The cancer risk associated with nitrate is yet to be investigated in Nigerian waters. This study assessed the human health risks associated with groundwater usage by adults, teenagers, children, and infants across Lagos, Ogun, Oyo, Osun, Ekiti, Ondo, Imo, Ebonyi, Delta, and the Federal Capital Territory through oral and dermal exposures. A total of 623 groundwater samples (537 hand-dug wells and 86 boreholes) were analyzed for nitrate and fluoride using standard procedures. Fluoride concentrations ranged from 1.25 ± 0.07 to 8.47 ± 3.19 mg/L, with most samples exceeding national and international water quality guidelines of 1.5 mg/L. Nitrate levels were generally within safe limits (2.50 ± 0.14-13.37 ± 0.48 mg/L), except in Delta communities (Kurutie, Kunukunuma, Okerenkoko), where values exceeded 50 mg/L, suggesting contamination from anthropogenic activities. Risk assessment showed oral ingestion as the primary exposure pathway while dermal risk was negligible. The Hazard Quotient for fluoride via ingestion (HQ > 1) indicated significant non-cancer risks for all age groups (0.846-8.997) while for nitrate it was negligible in all communities except for Kurutie (1.827-2.690), Kunukunuma (1.645-2.423), and Okerenkoko (1.639-2.413). The Mean Cancer Risk (MCR) values for nitrate exceeded the USEPA threshold (1 × 10^-6) across all age groups, with Delta State hotspots reaching 10^-3. Findings demonstrate that groundwater in the region poses both non-cancer and cancer risks, underscoring the urgent need for intervention strategies such as defluoridation, denitrification, and safe alternative provision.