Formation and health risk assessment of trihalomethanes in coastal wells impacted by seawater intrusion

The formation of trihalomethanes (THMs) in coastal wells impacted by seawater intrusion (SWI) is relatively understudied. Coastal wells, unlike typical groundwater sources, frequently exhibit elevated levels of chloride and bromide ions, potentially influencing the formation and speciation of THMs. The current study investigated the THM formation in coastal well water from a location affected by SWI. Samples were chlorinated with 2.5 mg L⁻¹ and 5 mg L⁻¹ chlorine doses to replicate field conditions. The THM concentration in the samples exceeded the maximum permissible limit for drinking water. The well with the highest chloride-to-bromide (Cl/Br) ratio (Cl/Br = 645) exhibited the highest total THM concentration (503.2 μg L⁻¹). Samples with high bromide concentration had more brominated THMs over the 5-day reaction period. The bromine substitution factors for wells after 24 h with a chlorine dose of 2.5 mg L⁻¹ were 1.89, 2.21, and 2.78, corresponding to bromide concentrations of 0.098, 0.109, and 1.275 mM, respectively. The average cancer risk associated with the well water was estimated to be 20.9 × 10⁻⁰⁶ through dermal contact and 6.84 × 10⁻⁰⁴ through inhalation. The study area had an estimated average cancer risk of 705 cases per million population. The study area's common diseases and cancer incidence data for the past four decades indicated a decreasing trend for waterborne diseases and a steep increase in cancers. While several factors may contribute to increasing cancer cases, our study highlights chlorinated coastal well water as an additional potential cancer risk agent.