Estimation of non-point source of pollution loads in the River Song, a tributary of River Ganga, India.

The Himalayan rivers are particularly vulnerable to regional climate changes and anthropogenic influences, which can significantly alter both water quality and quantity, jeopardizing the fragile river ecosystems. This study investigates the hydrochemical characteristics of the Song River, a tributary of River Ganga focusing on non-point source (NPS) pollution, during the period June 2022 to November 2023. Monitoring of river discharge was carried out water samples were collected weekly during the monsoon (June to September), bi-weekly in the post-monsoon (October & November), and monthly during lean periods (December-May) from three monitoring stations. The study revealed that Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) eventually exceeded the criteria limits (3 mg/L for BOD and 10 mg/L for COD) prescribed by the Central Pollution Control Board (CPCB). The chemical composition of the river water at the monitoring stations revealed Caand Mg as the dominant cations, while HCO⁻ and SO⁻ were identified as the major anions. Gibbs plot suggesting the dominance of rock weathering as the major natural controlling mechanism of chemical composition in the basin. Carbonation and sulphide oxidation are two proton-producing reactions controlling the chemical weathering processes. C-ratio plot suggested that dominance of carbonate dissolution in the tributary Suswa, while Song River showed dominance of sulphide oxidation, particularly in its upstream region. Spatial and temporal analysis identified nutrient pollution (NO₃⁻, NH₄⁺, PO₄⁻), organic loads (BOD, COD), and other parameters (TSS, Cl⁻) as key contributors to water quality deterioration at the monitoring stations. A chemical mass balance (CMB) approach based on mass conservation has been applied for estimating the NPS pollution loads on the Song River. CMB calculations carried out for major cations such as (Na, K, Ca, Mg, and NH) and major anions (Cl⁻, SO⁻, HCO⁻, NO⁻, and PO⁻) across a 21 km stretch of the river, encompassing approximately 305 sq. km. and observed that the contribution of uncharacterised load was maximum in dry season and minimum during monsoon season. Seasonal variations in ion concentration and flux were strongly correlated with hydrological processes, highlighting the need for comprehensive monitoring and management of NPS pollution in the Song River to safeguard its water quality and downstream impacts on the Ganga River system.