*2.2. Collection of Samples*

Surface sediments (0–10 cm) were collected along a stretch of River Kaduna from five sites base on the type of anthropogenic activities, stages for the stretch of the river either downstream, upper or in between, and history of sediment pollution. Coordinates of the sites selected were determined and recorded using a T10 handheld Global Positioning System (GPS) receiver. At each site, five grab hauls were sampled using an Ekman's grab. The samples were sorted in the field to separate *Bellamya unicolor* from the sediment samples. *Bellamya unicolor* were identified as described by Brown and Kristensen [26]. After sorting, the samples were placed inside zip lock bags and stored in the icebox to be transported to the laboratory for further analysis.

**Figure 1.** Map of River Kaduna showing sampling sites.

#### *2.3. Treatment of Samples and Determination of Heavy Metals*

Sediment samples were freeze-dried and grounded while samples of *Bellamya unicolor* were treated and prepared for heavy metal determination and antioxidant enzyme activities assays; 0.4 g of grounded freeze-dried samples was digested in 8 mL of 1:1 HNO3:HCl and kept for 1 h in Anton-Paar PE Multiwave 3000 (microwave oven) set at 200 ◦C [1,27].

For the freshwater mollusc *Bellamya unicolor* samples, 0.3 g of the grounded samples were digested chemically in 3:1 HNO3:H2O2 and maintained for 45 min at 180 ◦C. The volume was then made up to 50 mL using ultra water. The solutions were left to stand after the mixture until the following day. The solute from the digested solution of sediment and *Bellamya unicolor* tissue was then transferred into polypropylene vials after filtration, stored between 4–5 ◦C and later analyzed for heavy metals using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo, Waltham, MA, USA).
