Comprehensive Evaluation of Water Quality and Nutrient Dynamics Using Coagulation Experiment and Trophic Status Index in Northern Lake Tana, Ethiopia

Abstract

Water quality assessment is crucial for safeguarding aquatic ecosystems and ensuring the sustainable use of freshwater resources, particularly in regions experiencing increasing anthropogenic pressures. This study comprehensively evaluated the water quality, trophic status, and bacteriological safety of Lake Tana using Carlson’s model approaches, including physicochemical and heavy metal analyses, nutrient profiling, and Jar test experiments. Eight strategically selected sites revealed significant spatial variability in key parameters: temperature (21.4–26.3°C), pH (6.7–9.1), dissolved oxygen (5.2–8.4 mg/L), turbidity (14–88 NTU), and electrical conductivity (180–395 µS/cm). Nutrient concentrations also varied considerably nitrate (0.4–1.3 mg/L), phosphate (0.3–2.8 mg/L), and ammonium (0.05–1.12 mg/L) with ANOVA (p < 0.05) confirming statistically significant differences among sites. Correlation analysis revealed strong positive associations between nutrients and algal indicators, and Trophic State Index (TSI) values ranging from 56.1 to 78.4 indicated mesotrophic to hypereutrophic conditions, especially near urban and agricultural runoff zones. Heavy metal analysis showed copper (0.56–0.77 mg/L), iron (0.35–1.70 mg/L), manganese (0.17–0.86 mg/L), and zinc (0.45–0.75 mg/L), with iron and manganese exceeding WHO and USEPA guidelines at several sites. In bacteriological analysis the highest counts were observed at sites S7 and S8, while S5 showed no coliform presence. The mean values (188.75 ± 115.35 CFU/100 mL for total coliforms and 120.25 ± 92.54 CFU/100 mL for fecalcoliforms) exceeded permissible limits for surface water, with statistically significant differences among sites (p < 0.05). These results indicate fecal contamination likely from sewage discharge and livestock activity. Jar test experiments demonstrated effective turbidity and removal >85%) using alum (Al₂(SO4)3) at an optimal dose of 3 ml at low-to-moderate turbidity sites; however, sites with high turbidity (50–70 NTU) showed limited treatment response. While zinc levels remained within acceptable limits, elevated copper, iron, manganese, and coliform levels pose potential health and ecological risks. Overall, Lake Tana’s water is conditionally potable but requires site-specific interventions, including disinfection, oxidation-filtration, and public health monitoring. The findings underscore the urgent need for integrated watershed management, pollution control, and continuous monitoring to preserve the lake’s ecological integrity and protect public health. Keywords: Lake Tana, Water Quality, Trophic Status, Nutrients, TSI, Jar Test, Eutrophication