Integrated Human Health Risk Assessment of Microplastics and Heavy Metals in Drinking Water Sources: A Case Study of Nnewi, Anambra State, Nigeria

Abstract

As waste management infrastructure struggles to keep pace with rapid urbanization in developing nations, freshwater systems have increasingly become reservoirs for complex pollutants. This study presents a dual-assessment of microplastic (MP) and heavy metal (HM) contamination in five critical river systems (Miri Eze, Miri Uwaka, Miri Utu Ukpor, Miri Ele Umudim, and Eze River Amuliba) within the industrial hub of Nnewi, Anambra State, Nigeria. Through hydrogen peroxide digestion and FTIR spectroscopy, we identified a ubiquitous presence of microplastics across all ten sampling sites, with concentrations ranging from 4 to 17 particles/L (mean: 8.60 ± 3.89 particles/L). Notably, microbeads dominated the samples (80.2%), suggesting significant input from personal care products and local plastic fragmentation. In parallel, analysis of heavy metals—specifically Lead (Pb), Cadmium (Cd), and Mercury (Hg)—revealed concentrations far exceeding WHO and international safety thresholds (Pb: 0.10–0.37 mg/L vs. WHO limit 0.01 mg/L; Cd: 0.10–0.35 mg/L vs. 0.003 mg/L). Our health risk models indicate a disproportionate burden on the pediatric population; the Estimated Daily Intake (EDI) for children (0.57 particles/kg/day) was more than double that of adults (0.27 particles/kg/day). Furthermore, the high Incremental Life Cancer Risk (ILCR) linked to Cadmium (ranging from 1.2 × 10⁻⁴ to 4.8 × 10⁻⁴ for children) underscores a looming public health crisis. These findings highlight the "carrier effect" whereby microplastics may facilitate the transport of carcinogens into the human food chain, and underscore the urgent need for integrated regulatory frameworks to address the synergistic toxicity of microplastics and heavy metals in Nigerian drinking water