Assessment of heavy metal contamination in water, sediment and fish from the Jimi Reservoir, Obuasi

This study was carried out to assess the levels of four heavy metals, Arsenic (As), Copper (Cu), Lead (Pb) and Cadmium (Cd) in the water, sediment and fish of the Jimi Dam in Obuasi over a four-month period from April to July, 2012. Heavy metal concentrations in the water were found to be very low, and in the cases of Arsenic and Cadmium, concentrations were found to be in trace amount and below detection. Mean Lead concentrations were however found to be well above the WHO, Drinking Water Quality Guideline Value of 0.01 mgL-1. The recorded Lead concentrations at some sampling stations were as much as approximately 10 times higher than the regulatory standard. Copper levels at all the sampling stations on the other hand were found to be low throughout the study period. The study also revealed very highly measurable concentrations of the studied heavy metals, especially in the bottom sediments and in the flesh of the two sampled fish species Tilapia zillii and Oreochromis niloticus harvested from the Dam. With the exception of Lead, all the heavy metal concentrations in the bottom sediments of the Dam were found to be above the USEPA Safety Reference Standards for all or some of the sampling month. Arsenic concentrations in the bottom sediments at all the sampling stations were all well-above the threshold value of 30 mgkg-1. The mean Arsenic concentrations at some of the sampling station were almost 3 times higher than the safety reference standard. Cadmium levels recorded in the bottom sediments were above the Safety Reference Value of 3 mgkg-1 at all the sampling points and fell within the levels for moderately polluted sites. The fish samples of the two species were categorized into two size classes, small (<10cm) and large (>10cm) for the metal analysis. The observed metal concentrations in the bottom sediments and fish were similar to those observed in areas under moderate to heavy pollution. The concentrations of As, Cu and Pb in the flesh of the two fish species were found to be above safety reference standards for human consumption stipulated by the WHO, and hence can pose a serious health threat to people who consume the fish from the Dam. Mean As concentrations in the flesh of the Tilapia zillii ranged from 15.83±0.19 to 16.35±0.07 mgkg-1 for the small size group and 16.05±0.21 to 26.65±0.35 for the large size class. As concentrations in the flesh of the Oreochromis were similar to the concentrations in the Tilapia zillii and ranged from 10.65±0.49 to 12.60±0.14 mgkg-1 for the small size class and 15.45±0.21 to 27.30±0.57 mgkg-1 for the large size class. Mean Copper concentrations in the flesh of the Tilapia zillii was found to range from 9.55±0.77 to 11.35±0.07 mgkg-1 and 8.30±0.00 to 11.70±0.56 mgkg-1 for the small and large size classes respectively. Copper in the flesh of the Oreochromis ranged from 10.65±0.49 to 12.60±0.14 mgkg-1 and from 13.10±0.14 to 13.30±0.14 mgkg-1 for the small and large size classes respectively. For the small size Tilapia zillii, mean Lead concentration ranged from 7.55±0.07 to 8.45±0.07 mgkg-1 while the large size class recorded mean concentrations ranging from 8.55±0.21 to 11.50±0.21 mgkg-1. Cadmium concentrations in the flesh of the two fish species were below the WHO Safety Reference Standard of 1 mgkg-1. Mean cadmium concentrations in the small and large size Tilapia zillii ranged from 0.55±0.07 to 0.65±0.07 mgkg-1 and from 0.50±0.00 to 0.80±0.00 mgkg-1 respectively. Mean Cadmium concentrations in the flesh of the Oreochromis niloticus ranged from 0.60±0.00 to 0.90±0.14 mgkg-1 for the small size class and from 0.70±0.00 to 0.85±0.07 mgkg-1 for the large size class. It is therefore imperative that fishing from the Jimi Dam is prohibited and consumption of fish from it discouraged because of the high levels of the heavy metals in the flesh of the two fish species. The study revealed no significant spatio-temporal variations in the concentrations of the four studied heavy metals as far as the water and biota were concerned indicating an even distribution of the metals in the dam.

Source: KNUST