This study represented the first comprehensive assessment of the physicochemical water quality status of the entire Yamuna River stretch in India. The upper zone had "excellent-to-good" water quality index (WQI) with mean 5-day biochemical oxygen demand (BOD5) values of 2.1 and 2.4 mg/L during monsoon and non-monsoon, respectively. The middle region was described by "poor-to-marginal" WQI with average BOD5 values of 13.1 mg/L (monsoon) and 32.3 mg/L (non-monsoon). The low WQI observations at the midstream region were due to the negative impact of two major drains, namely Najafgarh and Shahdara, that carry partially treated effluents from industrial units. Further, BOD5 decreased to 1.9 mg/L (monsoon) and 1.8 mg/L (non-monsoon) in the lower zone, and the WQI values improved to "good" and "excellent". The dilution and depuration effects of the Chambal, Sindh, Betwa, and Ken Rivers recovered the environmental conditions in downstream stations. The oxygen sag curve complied with the water quality status along the river stretch. Based on the principal component analysis, the Yamuna River was strongly influenced by dissolved mineral salts originating from atmospheric deposition, weathering of soils and rocks, and application of deicing chemicals and landfills. Moreover, organic and nutrient substances and biological activities resulting from the discharge of sewage, and the utilization of fertilizers in agriculture, were the second contributors to pollution. The statistical techniques employed in this work could be beneficial for decision-makers (government and stakeholders) to identify the pollution sources/factors and to determine the viability of water bodies for domestic applications.
Keywords: Indian water standards; Monsoon and non-monsoon; Multivariate analysis; Water quality index; Yamuna River.