EVALUATION OF SPATIO -TEMPORAL WATER QUALITY STATUS AND POTENTIAL RISK OF HEAVY METALS ON AQUATIC ECOSYSTEM: THE CASE STUDY OF WABE RIVER, ETHIOPIA M. Sc. THESIS BY TILAHUN KASA

Abstract

Wabe River is one of the most important tributaries of the Gibe River basin. It is the main source of drinking and irrigation for the nearby communities throughout the stretch. However, the river is polluted due to soil erosion, sedimentation, waste disposal, agricultural activities and urban runoff. The study was designated with an objective of evaluating the spatio-temporal water quality status for multipurpose uses and potential risk of heavy metals on aquatic ecosystem. A total of 60 water samples were collected during both wet and dry seasons from selected 10 sample points. The physicochemical and biological parameters, such as, pH, DO, TDS, EC, BOD5, COD, TH, Turbidity, Cl-, SO42-, HCO3-, CO32-, Ca2+, Mg2+, Na+, K+, PO43-, NO3-, Cd2+, Cu2+, Zn2+, Pb2+, and Ni2+were analyzed for all samples by using APHA standard methods. All measured water quality parameters were compared with WHO and ESA, FAO, and CCME guidelines for drinking, irrigation, and aquatic life, respectively. Both Weighted Arithmetic Water Quality Index and Metal index were used to evaluate the suitability of the river water for drinking purpose. Whereas, the aptness of the river water for the irrigation was assessed by computing indices such as SAR, PS, PI, MR, KI, RSC, and %Na. Moreover, in order to ascertain the suitability of the river water for aquatic life, and the risk level of heavy metals on the aquatic ecosystem Metal Index (MI), and PERI were used. The WAWQI value of Wabe River for drinking in the wet season was under the unsuitable category (>100) in all sample points, whereas during the dry season, it varies from poor to unsuitable (51-134.9). On the other hand, the computed irrigation water quality indices revealed that the water quality of the river was appropriate for agricultural application except at a sample point T1 during the wet season. Similarly, the output from MI for both drinking and aquatic life showed that the river water was not safe (MI>1) in all sample points during both seasons. Also, the risk level posed by those metals according to PERI was ranged from 436.9 (Strong) to 1714.1(Very high) and 288.5 (Moderate) to 704.3 (Very high) during both wet and dry seasons, respectively. BOD5, COD, PO4, Turbidity, and Cadmium were the influential variables for deterioration of the river water quality for drinking purpose. Likewise, Cadmium, Copper, and Zinc have exceeded CCME guideline values in all sample points. This was possibly due to irrigation activity in the watershed, soil erosion, surface runoff, municipal waste, and weathering of rocks. Therefore, it is essential to prevent pollutants inflow to the river.