River Bonsa is located in the Tarkwa Nsuaem municipality of the Western Region of Ghana on latitude 4˚5'' and longitude 5˚5'' (Figure 1). The Municipality is often labeled the Hub of mining activities in Ghana because it lies on the Birimian and Tarkwaian geological formations, which are the two most economically important formations as far as mineral deposits are concerned. As such the Municipality hosts three major mining companies and many other small-scale mining companies.

Before the collection of sediments samples, a survey was conducted to identify suitable sampling sites taking into account factors such as accessibility, human activities and establishments near the river.

Ten sediment samples were collected at approximately 90 m along the river using a grab sampler and labelled BS1 to BS10. A handheld GPS receiver was used to record the coordinates of all the sampling points as shown in Table 1. Figure 2 shows sampling points along the river Bonsa. All samples collected

were stored in well labeled Ziploc bags, sealed and transported to the laboratory for pre-treatment and analyses.

In the laboratory, the sediment samples were oven dried at a temperature of 110˚C for one hour to remove the moisture content. The dried samples were allowed to cool and subsequently acid digested.

Aqua Regia (68% w/w Nitric acid (HNO₃) and 35% w/w Hydrochloric acid (HCl)) in the ratio 1:3 (i.e. 10 mL of HNO₃ and 30 mL of HCl) was added to 5 g each of the dried sediment samples and heated on a hot plate at 105˚C for 15 minutes. 10 mL each of the digests were transferred into 50 mL standard flask and topped up to the mark with distilled water. These were then filtered through 0.45 µm Whatman filter paper and used for heavy metal analysis.

Cu, Cd, Pb, Fe, Mn, Ni, Cr, Zn and Co were analyzed using the Varian AA240FS Fast Sequential Atomic Absorption Spectrometer (AAS) at absorbance wavelength of 324.8, 228.8, 217.0, 243.8, 279.5, 232.0, 357.9, 213.9, and 240.7 nm respectively. The results were then compared with the Australian and New Zealand Environment and Conservation Council (ANZECC) and National Oceanic and Atmospheric Administration (NOAA) Guidelines for fresh water sediment quality.

The ANZECC and NOAA guidelines both have two limits, the ISQG-low concentration also known as the trigger concentration and the ISQG high concentration. The trigger concentration is the threshold below which the probability of adverse effect is very low or negligible whilst the ISQG high concentration is the limit beyond which the heavy metals become bioavailable. Exceeding this value however does not mean adverse effects will occur in the sediments, but will require further investigation into other factors that influence bioavailability to confirm whether or not an adverse effect will be produced.

The study employed the geo-accumulation index (I-geo) and Enrichment Factor (EF) to determine the status and sources of heavy metal contamination of the sediments respectively.

The concentrations of zinc, lead, cadmium, chromium and copper in all the 10 samples analyzed were below their respective ANZECC trigger concentrations or threshold limits (Figures 3-8). The concentrations of iron in all the samples analyzed were found to be below its corresponding NOAA ARC TEL. This implies a very low probability of bioavailability of these metals and hence no ecological or human health impact.

With the exception of BS1 and BS2, all samples analyzed had Ni concentrations below the ANZECC trigger value (Figure 9). The Ni concentrations at BS1 and BS2 although greater than the trigger limit were all found to be well below the ISQG high concentration. The high Ni concentration at BS1 and BS2 could

be attributed to the disposal of domestic waste into the river at sampling points BS1 and BS2. These wastes usually include batteries and other Ni containing substances (Harasim & Filipek, 2015). The Ni concentration at BS2 could have been further enhanced by run offs from an auto mechanic workshop closely associated with BS2. Concentrations in between the low and high ISQG value do not necessarily mean adverse impact will occur. Other factors in addition, will help determine the bioavailability of the metal. More so, the average Ni concentration was found to be 13.645 mg/kg, which is below the trigger value of 20 mg/kg, hence the frequency of occurrence of the adverse impact will be very low.

Mn recorded a maximum concentration of 1221.602 mg/kg at sample point BS2 and a minimum concentration of 273.216 mg/kg at BS1 (Figure 10) with an average concentration of 745.097 mg/kg. Apart from BS1, BS7 and BS9, all other samples recorded Mn concentration above NOAA ARC TEL (p < 0.05). According to (El-Madani & Hacht, 2017, Barceloux, 1999), about 0.1% of the earth’s crust is composed of Mn. Moreover, the high Mn concentrations are as expected, as the study area is high in Mn deposits and the metal is being mined in commercial quantities, about 15.4kilometers from the river (Google Map, 2018). The high Mn concentrations are attributable to geogenic sources as was later indicated by the calculated EF (Table 3). The use of Mn gangue for construction activities in the catchment areas may have contributed to the differences in the concentration levels recorded at the different sampling points. Mn concentrations are within natural limits and will not impact human and ecological health negatively.

Cobalt had maximum and minimum concentrations of 20.039 mg/kg and 6.401 mg/kg respectively (Figure 11) with an average of 10.805 mg/kg. Both ANZECC and NOAA guidelines for fresh water sediment quality do not state any value(s) for cobalt concentrations in sediments. As Co is naturally occurring, all aquatic environment contains trace concentration of this element, sometimes referred to as background concentration. Additionally, the mean concentration of 10.805 mg/kg recorded is much lower than 15.42 mg/kg mean Co concentrations recorded by (Li et al., 2018) although background concentrations differ from region to region.

The mean concentrations of the various heavy metals were compared with their corresponding ANZECC or NOAA ARCS TELL threshold guideline values using a one-sample t-test as shown in Table 2. It was observed that the mean concentrations of Cu, Pb, Cd, Zn, Ni, and Cr were significantly lower than their respective ANZECC threshold values (p < 0.05). The mean Fe concentration was also significantly lower than the NOAA threshold, however there was no significant difference between Mn mean value and the corresponding NOAA guideline value. However, it was close to the NOAA threshold value. These higher Mn levels corroborate the economic deposit of Mn in the study area.

Based on the individual elements’ measured concentrations and their background values, geo-accumulation indices and enrichment factors were calculated to determine the levels and sources of contamination, respectively, and the results are shown in Table 3.