Heavy Metals Contamination in Mmili Ocha River Sediments: Seasonal Variation, Ecological Risk, and Human Health Concerns

Abstract

This study assessed the concentration, contamination status, ecological risk, source relationships, and human
health implications of heavy metals in the sediments of the Mmili Ocha River during the rainy and dry seasons.
Sediment samples were collected from three sampling stations during the rainy and dry seasons and analyzed for
cadmium (Cd), arsenic (As), iron (Fe), zinc (Zn), chromium (Cr), lead (Pb), and mercury (Hg) using atomic
absorption spectroscopy (AAS). The mean concentrations of the metals were: Fe (2332.50 ± 333.18 mg/kg), Zn
(59.83 ± 22.20 mg/kg), Pb (21.98 ± 5.57 mg/kg), Cr (2.24 ± 0.60 mg/kg), Cd (0.69 ± 0.21 mg/kg), As (0.43 ±
0.21 mg/kg), and Hg (0.15 ± 0.01 mg/kg). Pearson correlation analysis revealed strong positive associations
among Cd, As, Fe, Zn, Cr, and Pb (r = 0.790–0.999) in the rainy season and among As, Fe, Zn, Cr, Pb, and Hg
(r = 0.723–0.988) in the dry season, indicating common anthropogenic sources and similar geochemical
behavior. Geo-accumulation Index (Igeo) classified most metals as unpolluted, while Enrichment Factor (EF)
indicated significant to extremely high enrichment for Cd and Pb, suggesting anthropogenic influence.
Contamination Factor (CF) and Degree of Contamination (Cd) indicated low to moderate contamination,
whereas Pollution Load Index (PLI) values (<1) suggested no overall sediment pollution. The Potential
Ecological Risk Index (PERI) ranged from 120.94 to 188.04, indicating low to moderate ecological risk, with
Cd and Hg contributing most to the ecological risk. Human health risk assessment identified ingestion as the
dominant exposure pathway, with Total Hazard Index (THI) values of 32.26 for children and 3.46 for adults,
indicating potential non-carcinogenic risks, particularly among children. The findings indicate increasing
anthropogenic pressure on the river sediments, emphasizing the need for continuous environmental monitoring
and improved waste management practices to safeguard aquatic ecosystems and protect public health.