Lithium in Typical Coastal Drinking Water Systems: Occurrence, Drivers, and Exposure Risk

Despite growing concern about the increasing use of lithium (Li) and its potential health effects, the factors controlling its occurrence in coastal drinking water remain poorly understood. Here, we analyzed 326 tap water and groundwater samples from coastal regions of China to characterize the distribution, sources, and potential human health risks of Li. Median Li concentrations were 0.75 μg/L in tap water and 3.24 μg/L in groundwater, with 3.6 and 16.9% of samples, respectively, exceeding health-based screening levels (10 μg/L). Spatial and correlation analyses indicated that localized Li enrichment in coastal drinking water is mainly linked to tidal intrusion and geologic origin rather than widespread seawater intrusion. Conventional coagulation–sedimentation–filtration technologies were largely ineffective for Li removal, allowing its persistence in treated water. Health risk assessments revealed that Li exposure is generally within safe limits; however, localized hotspots exceeded the safety threshold. Considering multimetal(loid) coexposure, rural residents showed higher risks, with 20% of sites surpassing the total hazard quotient threshold. Li accounted for 15.8–17.6% of total exposure risks, representing an important contributor. These findings identify the determinants of Li concentrations in coastal drinking water systems and provide an empirical basis to inform exposure risk assessment and mitigation strategies in global coastal regions.