Ecological and health risk assessment of potentially toxic elements in Ewaso Nyiro River surface water, Kenya

Ewaso Nyiro basin covers an area of about 210,226 km 2 , 36.3%, of Kenya drainage area and bears 5.8% of Kenya water potential with an annual yield of 1469 million m 3 . The river is the principal source of domestic and irrigation water to the arid north of Kenya. To determine metal and nutrient concentration of Ewaso Nyiro River surface water, a total of 30 water samples, 15 samples each for dry (February) and wet (August) seasons of 2019, were collected. Chromium, lead, iron, manganese, cobalt, cadmium, mercury, selenium, molybdenum, boron, copper, zinc, arsenic, nickel, aluminum, total phosphorus and nitrate were analyzed in the two seasons. Ecological risk assessment was determined by calculating contamination factor, pollution load index and ecological risk index. Multivariate statistical analysis was used to infer pollutants association and identify their potential sources. Cadmium, arsenic, lead, molybdenum, mercury, selenium and nickel were not detected in both seasons, while manganese, iron and aluminum were the main pollutants identified. Ewaso Nyiro irrigation water had a manganese contamination factor of 9.17, implying it was very contaminated. Twenty-seven and 40% of sampled sites in dry and wet seasons, respectively, had more than 0.3 mg/L of iron that is recommended by USEPA in drinking water. Herbicides, leached fertilizer and fuel leaking into the river water were the primary sources of anthropogenic pollution.