LEVELS AND POSSIBLE RISKS OF HEAVY METALS IN THE LOWER OMO RIVER AND DELTA (L. TURKANA) IN SOUTHERN ETHIOPIA, INCLUDING AN ASSESSMENT OF SOME OTHER WATER QUALITY FACTORS

Abstract

Heavy metal concentrations in water and sediments can be biomagnifyied and impact human health through consuming of contaminated water or fish. Human activities in the Omo River and Delta have caused increased heavy metal levels in the freshwater ecosystem, adversely affecting the freshwater quality. Studies have shown higher levels of heavy metals in Lake Turkana freshwater ecosystem including in the fish tissues and water on the Kenyan side, but information gaps exist on the Ethiopian side. Commercially exploited fish species, such as Lates niloticus (Nile perch) and Oreochromis niloticus (Nile tilapia), may accumulate heavy metals from the freshwater, leading to health risks in humans. Apparently, no study has been reported on the water quality status of the freshwater ecosystem of the Lower Omo Basin especially with respect to heavy metals. Therefore, the current study aimed to investigate the levels and possible risks of the nine heavy metals (Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb, and Zn) including an assessment of some other water quality factors from from the Lower Omo River and Delta (L. Turkana) in southern Ethiopia. Surface water, surface sediment, irrigated soil, and two fish species (L. niloticus and O. niloticus) of the freshwater ecosystems were examined for heavy metals. The study was carried out at the Lower Omo River Basin near Omorate town and the Omo Delta where River Omo joins Lake Turkana. To assess the levels of heavy metals and the water quality from this freshwater ecosystem, thirty sampling points were taken of which 15 sampling points were designed to represent the River water and 15 the Delta. About 120 fish samples of each of the two fish species (L. niloticus and O. niloticus) were taken. Liver and muscle tissues of the two fish species were analyzed for heavy metals. Sediment samples were also taken from the same sites where the water samples were collected. Soil samples were collected from the vicinity of Lower Omo River where irrigation is intensively used. The levels of heavy metals in water; fish tissues, sediment, and soil samples were analyzed with Flame Atomic Absorption Spectrometer (FAAS, novAA400p). The other water quality parameters were analyzed using the standard methods for each parameter as described in the protocol or guidelines APHA (2017). The analysis was carried out at Arbaminch university in the laboratories of chemistry, water and environmental engineering. XVIII The eight heavy metals (Mn, Zn, Cu, Co, Cr, Pb, Ni, and Fe) were detected in the different environmental compartments of the freshwater ecosystem from Lower Omo River and Delta. Accordingly, the mean values of the measured heavy metals detected in the River waterin mg/L were 0.439, 0.1, 0.168 , 0.393, 0.318 , 0.007 , 8.926, and 0.06 whereas the respective values for Delta were 0.43, 0.118 , 0.166 , 0.382 , 0.338 , 0.008, 8.684 and 0.064 for Mn, Zn, Cu, Cr, Pb, Ni, Fe, and Co respectively. Both the River and Delta water had mean levels of lead (Pb), manganese (Mn), Cobalt (Co), and chromium (Cr) that exceeded WHO's permissible limits for water. The target hazard quotient (THQ) value greater than 1 was examined for Cr, Pb, and Mn both in children and adults through ingestion and dermal route from the River and Delta Water. The CRs for both children and adults via ingestion of the River and Delta water followed the order Cr > Pb. According to the CRI value, both the River and Delta water could be classified as high environmental risk in terms of the detected heavy metals heavy metal levels under consideration. In terms of heavy metal levels of freshwater fish, the Pb level in muscle tissues ranged from 0.597mg kg-1 to 0.890mg kg-1, with a greater value in the Omo Delta sample, which could be attributable to the water character of the Omo River and the Delta. The mean Pb levels in the muscle tissue of O. niloticus in the Omo River and the Delta were above the FAO/WHO recommended limits in the human diet. The high values of Pb could be due to intensive anthropogenic activities like use of agrochemicals (pesticides and fertilizers), petrol from fishing boats that contain lead, Car washing, gas/fuel station, solid wastes, and effluents from factories. It’s also possible that heavy rains carried the wastes down; contributing to the greater heavy metal levels of the study area. The target hazard quotient (THQ) and hazard index (HI) indices were used to assess the noncarcinogenic health hazards associated with the detected heavy metals in children and adults who consumed L. Niloticus and O. niloticus muscle from the Lower Omo River and Delta. In all XIX of the samples tested, the THQs for heavy metals in fish muscle consumed by adults and children were less than one. However, the Hazard Index (HI) for the detected heavy metals for children was larger than one. Pb had the highest THQ levels in both L. niloticus and O. niloticus, while Fe and Ni had the lowest values in O. niloticus muscle and L. niloticus muscle, respectively. In terms of sediment pollution, the mean concentrations of the detected heavy metals in mg/kg in the sediments of River and Delta Lake were respectively 2.947 and 2.904 for (Mn), 0.801 and 0.809 for (Zn), 0.278 and 0.278 for (Cu), 0.437 and 0.434 for (Cr), 0.054 and 0.058 for (Pb), 0.009 and 0.008 for (Ni), 19.553 and 19.515 for (Fe), and 0.236 and 0.223 for (Co). The order of the mean values of the detected heavy metals in the river sediment were Fe > Mn > Zn > Cr > Cu > Co > Pb > Ni, but in the Delta Lake, the order was Fe > Mn > Zn > Co > Cu >Pb > Cr > Ni. The mean pollution liad index (PLI) values of heavy metals in sediment revealed low metal contamination of the sediment (PLI < 1.0). Thus, the study highlighted the importance of monitoring heavy metal levels in sediments to ensure the safety of the freshwater ecosystem. Regarding the heavy metal level in soil, the mean values of the measured heavy metals in mg/kg in the soil irrigated by the lower Omo River were 4.4 for Mn, 1.142 for Zn, 0.2 for Cu, 0.43 for Cr, 0.424 for Pb, 0.004 for Ni, 23.5 for Fe, and 0.588 for Co. The order of the typical concentrations of heavy metals in soil were Fe > Mn > Zn > Co > Cr > Cu > Ni. The EI results for the detected heavy metals in the soil irrigated by the Lower Omo River in decreasing order were: Fe > Cu > Mn > Pb > Co > Zn > Cr > Ni. Except for Fe, the CFvalue showed a low level of contamination which was less than one. Concerning the other water quality factors, the mean BOD5 levels in the upstream and downstream of this study were 16.268±1.47 mg/L and 16.28 ±1.133 mg/L, respectively. The COD value of the XX river, which is upstream, was higher at 376.06 ± 130.45 mg/L than that of the Lake, which is downstream, at 136.00 ± 41.52 mg/L. Fluoride ion levels in the river were 0.89 ±0.0135, while those in the Lake were 2.026 ±0.064 mg/l on average. The phosphate (PO4-3 ) concentrations in the River and Lake (Delta) were 1.866 ± 0.625 mg/L and 5.108 ± 0.975 mg/L, respectively. The mean NH3 readings of the River and Lake were 0.54 ±0.361 and 1.354 ± 0.655, respectively. The higher values of the water quality factors could be due to anthropogenic activities such as agrochemical use, and domestic wastes. The finding of the water quality index (WQI) also revealed that the water quality status was poor and unsuitable for drinking particularly in the Omo Delta.The Delta Lake’s WQI value was found to be 142.47, which was beyond the threshold value for water quality index and Unsuitable fordrinking