**2.3 Sampling, preservation and analysis**

Water sampling, conservation, transportation, and analysis have all been carried out in accordance with the standard methods specified in APHA [16]; Morufu and Clinton [13]; Raimi and Sabinus [12]; Olalekan et al. [14]; Morufu et al. [2].

### **2.4 Ground water collection**

Ground water samples were obtained in pre-rinsed 1litre plastic containers for analyses of physico-chemical characteristics. Prior to storage, pre-rinsed ground water samples for trace metal analyses remained obtained in 1litre containers with nitric acid and treated with 2 ml nitric acid (assaying 100%, Fisher Scientific, Trace Metal Grade). These steps were taken to keep the metals oxidation settings stable. For Dissolved Oxygen (DO) and Biological Oxygen Demand (BOD) assays, groundwater samples remained obtained in two groups of 250 ml glass-stoppered-reagent bottles per sampling site. The BOD samples were carefully filled without air trapping and the bottles were wrapped in black polythene bags. This was done to exclude the presence of light in the samples, which was capable of creating DO by autotrophes (algae). The BOD samples were cultured for 5 days before being added to 2 ml of each sample. Winkler solutions I and II apply various dropping pipettes to each sample to slow

#### **Figure 2.**

*A schematic illustration of quantification methodology adopted for the current study.*

down additional biological activity. To precipitate the floc that was at the bottom of the bottles, the bottles were thoroughly shaken. Furthermore, Winkler solution I is a manganese sulphate solution, whereas solution II is a mixture of sodium or potassium iodide, sodium azide (sodium nitride), sodium or potassium hydroxide as well as sodium hydroxide. The samples of DO were collected in transparent bottles with tight-fitting stoppers. With dissolved oxygen samples kept on the spot using Winkler I and II solutions identical to those used for BOD samples [16]. For simple identification, all samples had remained carefully identified and kept at 4°C. On-site measurements were taken to determine the unstable concentrations and sensitive water quality indicators such as pH, electrical conductivity (EC), total dissolved solids (TDS), alkalinity (Alka.) as well as temperature (Temp). As a result, **Figure 2** depicts the key methodologies for researching groundwater composition.

### **2.5 Quality assurance and quality control (QA/QC)**

Furthermore, using high purity analytical reagents and solvents, all analytical methods remained closely monitored using quality assurance and control methodologies. The instruments were calibrated using calibration standards. The analytical technique validation included the use of triplicate analysis, procedure blanks and the examination of certified reference materials (CRM). The limit of detection (LoD), precision, reproducibility, repeatability and accuracy of each organic pollutant from the groundwater samples were determined.
