Seasonal and spatial variations of physicochemical parameters and heavy metals in surface water of interconnected Nigeria lagoons experiencing distinct anthropogenic disturbances

Abstract

Lagoon systems in Nigeria serve as essential ecosystems for fisheries and local livelihoods, yet, they face increasing threats from industrial discharges, agricultural runoff, and urban pollution, which may degrade water quality and pose health risks to coastal communities. This study comparatively assesses how varying anthropogenic activities influence the physicochemical properties and heavy metal levels in surface waters of Nigeria's interconnected coastal lagoons and also evaluates the contamination indices, pollution sources, and human health risks. The physicochemical parameters were; pH (6.86-6.97), dissolved oxygen (4.39-5.91 mg/L), total suspended solids (20.81-21.74 mg/L), electrical conductivity (26.28-217.11 uS/cm), and total dissolved solids (61.82 mg/L) and were within the recommended limits, except for TDS. The distribution trends and average levels of heavy metals were as follows: Zn (1.98-4.60 mg/L) > Fe (0.59-3.79 mg/L) > Cu (0.18-3.10 mg/L) > Cr (0.02-0.785 mg/L) > Pb (0.30-0.50 mg/L) > Cd (0.15-0.41 mg/L) > As (0.05-0.06 mg/L) > Ni (0.02-0.06 mg/L). Heavy metal concentrations were highest during the dry season, and the spatial analysis revealed that the concentrations of Cd, Pb, and As were highest at stations LE6 and LE9 in Lekki Lagoon, indicating areas primarily influenced by agriculture, tourism, and dredging activities. In Lagos Lagoon, stations LA1, LA3, and LA5 presented the highest levels, mostly contaminated with Cd, Cr, and Pb, corresponding with locations dominated by industrial wastewater discharge, crude oil processing, shipping activities and leaching oil from abandoned power plants. The integrated pollution and water quality indices revealed that approximately 90% of the sampling stations of both lagoons presented moderate to extreme contamination levels, rendering the water unsuitable for drinking, with potential uptake, accumulation, and ecological risks for important ecosystem functions. Multivariate analysis revealed that the elevated heavy metal concentrations in both lagoons originated primarily from diverse anthropogenic activities. The health risk assessment revealed that ingestion of Cd, Cr, and As poses the highest threat to human health, with most of the sampling stations, especially in Lagos Lagoon, presenting potential non-cancer and carcinogenic health risks, with children being the most vulnerable group. Our findings showed that both lagoons are highly contaminated irrespective of the varying anthropogenic influences. Thus, we recommend stringent monitoring of land use and contaminant sources, implementation of targeted remediation at high-risk sites, and development of integrated coastal-zone management strategies to protect the lagoon ecosystems and human health.