Depth profile of reservoir water quality in the Southwest of Ethiopia

Bizuneh Woldeab; Argaw Ambelu; Zewdu Efrem; Siyoum Deribe; Moa Megersa; Tibebu Alemu; Seid Tiku Mereta

doi:10.1016/j.heliyon.2023.e17474

Depth profile of reservoir water quality in the Southwest of Ethiopia

Heliyon. 2023 Jun 24;9(7):e17474. doi: 10.1016/j.heliyon.2023.e17474. eCollection 2023 Jul.

Authors

Bizuneh Woldeab¹, Argaw Ambelu², Zewdu Efrem³, Siyoum Deribe³, Moa Megersa⁴, Tibebu Alemu⁵, Seid Tiku Mereta³

Affiliations

¹ Department of Biodiversity Conservation and Ecotourism, College of Agriculture and Natural Resource Madda Walabu University, P.O. Box 247, Robe, Ethiopia.
² Division of Water and Health, Ethiopian Institute of Water Resources, Addis Ababa University, Ethiopia.
³ Department of Environmental Health Science and Technology, Jimma University, Jimma 378, Ethiopia.
⁴ Department of Biology, School of Natural and Computational Sciences, Madda Walabu University, P.O. Box 247, Robe, Ethiopia.
⁵ Department of Natural Resource Management, Jimma University, Jimma, 378, Ethiopia.

Abstract

The objective of this research was to investigate the variation of water quality parameters at different depths of the Gilgel Gibe I reservoir in Oromia Jimma zone during wet seasons. Five stations within the reservoir were selected, and water quality parameters were determined at four different depths (surface, 5 m, 10 m, and 15 m). Water quality parameters were analyzed on-site using a HACH, HQ40d portable multi-meter, and turbidity was measured using Wag tech turbidity meter and in the laboratory using the standard method. Comparison of water quality parameters among depths were conducted using one-way ANOVA and Tukey's pairwise comparisons with 5% significance level. The probable contributing source of the investigated physicochemical water quality parameters at different depth was identified using Principal components analysis. The results show that depth wise except for total suspended solids (TSS), total dissolved solids (TDS), total phosphorus (TP) and soluble reactive phosphorus (SRP) the rest showed statistically significant difference at p < 0.05 level. Negative correlations were found between depth and dissolved oxygen (DO), water temperature, pH, nitrate (N0₃^-) and chlorophyll a (Chl.a) while positive correlations were found between electrical conductivity (EC), biochemical oxygen demand (BOD₅), turbidity and N0₃^-. The study revealed that the release of nutrients associated with increased concentration of BOD5 at the bottom depth caused low concentration of dissolved oxygen due to oxygen consumption. This was further aggravated through the decomposition of organic matter, indicating organic pollution resulting from runoff from the catchment. The presence of dense masses of blue-green algae in the pelagic zone of Gilgel Gibe I reservoir suggested the presence of ample nutrients for its blooming and significant reduction of water quality, indicating possible eutrophic conditions. Therefore, catchment management is required to protect aquatic life and the reservoir function as a whole from reservoir water quality degradation.

Keywords: Chlorophyll a; Eutrophication; Gilgel gibe; Organic matter; Physicochemical.