Seasonal comparative assessment of physio-chemical water quality of tap, bottled, river, and borehole water in Nairobi County, Kenya across wet and dry seasons

Abstract Water is fundamental to every life component on earth, including humans, animals, and plants. It is a component of food, an essential source of mineral nutrients, and plays a key role in various metabolic processes, hence underscoring the need for safe drinking water. However, research on the composition of water in cities like Nairobi, with its rapidly growing population, remains very limited. Therefore, there is a need to assess the water quality determining components in different years and seasons. The study assesses seasonal variations in water quality parameters, including pH, turbidity, conductivity, iron, manganese, total dissolved solids (TDS), and determines their safety for consumption. Nairobi River was sampled purposively since it is the main river, and the borehole, tap, and bottled water were sampled randomly in the selected study area. A total of 192 samples were collected from multiple locations representing each water source. The study employed standard laboratory methods for water quality analysis. Data were analyzed using SPSS, with a one-way ANOVA and post hoc Tukey tests to identify statistically significant differences between sources and seasons (α = 0.05). The study revealed significant differences in water quality parameters in the different water sources: tap, borehole, river, and bottled water (p < 0.05). River water showed the highest level in color turbidity, iron, and nitrate. During the wet season, river water exhibited high turbidity (14.37 ± 1.79 NTU), iron (0.46 ± 0.04 mg/L), and manganese (0.28 ± 0.04 mg/L). The turbidity and pollutant levels in river water significantly exceeded those in bottled and tap water, with bottled water showing the lowest turbidity (0.05 ± 0.03 NTU). Key findings revealed significant seasonal variations in river and borehole water quality. Borehole water demonstrated the highest conductivity (556.20 ± 43.79 µS/cm) and TDS (297.50 ± 21.94 mg/L), particularly in the dry season, due to the concentration of dissolved minerals as groundwater levels decreased. Sodium levels in borehole water were also notably high, reaching 149.2 ± 15.06 mg/L. Tap water, sourced from municipal systems, showed consistent quality across seasons, with minor increases in turbidity (2.39 ± 0.56 NTU) and color in the wet season. However, its overall conductivity (69.04 ± 2.33 µS/cm) and TDS (41.77 ± 1.33 mg/L) were lower compared to river and borehole water, indicating effective treatment. Bottled water was the most stable across all parameters and seasons, with conductivity at 94.23 ± 8.89 µS/cm and TDS at 56.56 ± 5.70 mg/L. In conclusion, while bottled and tap water remain the safest options for year-round consumption, river and borehole water present health risks, especially during the wet season when turbidity and pollutant levels increase. This shows the need for enhanced treatment systems and water management strategies, particularly for sources prone to contamination, such as rivers and boreholes. The study’s results provide crucial insights for public health policy and water safety, underscoring the necessity for interventions that ensure access to safe drinking water year-round.