**1. Introduction**

Industrialization and urban development are among the major causes of metal pollution in a natural aquatic ecosystem [1]. The persistent nature and bioaccumulation ability of heavy metals in trophic levels make these metals serious pollutants of the aquatic environment [2–4]. The presence of heavy metals in an aquatic ecosystem either through adsorption or co-precipitation can threaten its biodiversity, and the health of humans depending on the resources of that ecosystem [5,6]. However, biological and chemical factors influence the mobility of heavy metals in aquatic environments by desorption from sediments into the surface water [7]. This made the surface water a major intermediate source of metal pollutants in benthic sediments, which is the definitive receptor [8,9].

Heavy metal concentrations of sediments and benthic organisms such as freshwater mollusc, speciation and several analytical techniques are often used in the evaluation of probable ecological risks in benthic sediments and their effect on the biota [10,11]. Several environmental factors and pollution indices such as geoaccumulation index (*Igeo*), ecological risk factors (ErF) and sediment quality guidelines (SQGs) have been used by several authors. It was used to measure the level or degree of pollution caused by metal and the ecological risk posed by metals in benthic sediments [12]. Although chemical speciation is ignored, it can be considered as subjective when evaluation of the level of pollution is computed based on *Igeo* and ErF. This is done because of the efficiency of these

indices in utilizing both the concentration and toxic effect of metals to draw valuable conclusions on risk assessments [13,14].

The discharged industrial and domestic effluent containing toxic metals in aquatic ecosystems is a major concern to the survival of aquatic biota. Freshwater molluscs are not an exception. Although they have devised a means of bioaccumulating the metals even at high levels, due to their high tolerant ability to metal concentrations [15]. This ability makes them a good bio-monitor and bio-indicators of metal pollution in aquatic ecosystems [15,16]. The need to involve freshwater mollusc in ecological risk assessment studies does not stop at their ability to accumulate metal pollutants in high concentration. They also form an important link in the metal cycle for aquatic ecosystems and occupy a trophic level in the aquatic food chain [17,18].

Heavy metal concentrations in high levels can lead to the generation of reactive oxygen species such as H2O2, OH, RO2 among others [19,20]. The manifestation of oxidative stress as a result of oxidative damage to proteins, nucleic acids, and lipids in aquatic organisms exposed to metal-polluted environments is triggered by reactive oxygen species (ROS). This happens when the process of detoxifying metals by organisms has to do with redox cycling reactions [20]. However, another important biomarker of oxidative stress known as defensive antioxidant enzymes is found in organisms [20,21], to scavenge ROS produced in organisms because of metal pollution and oxidative stress. Therefore an increase in ROS formation as a result of an increase in metal pollution triggers an increase in the production of antioxidant enzymes [22,23]. This phenomenon leads to the utilization of antioxidants as biomarkers of environmental pollution, which is a source or primary cause of oxidative stress in organisms [23].

In this study, five sites were selected to evaluate the risk assessment of heavy metals in the River Kaduna. Hence, to achieve the aim of this study, we determined concentrations of heavy metals in sediments and the freshwater mollusc *Bellamya unicolor*, pollution indices and antioxidant enzyme activities in *Bellamya unicolor* across the five sites.
