Monsoon-controlled hydrogeochemical processes govern seasonal water quality in the downstream Bagmati River, India

Understanding seasonal hydrochemical variability is essential for sustainable water resource management in monsoon-dominated regions experiencing increasing anthropogenic pressure. Although the Bagmati River has been extensively studied within the Kathmandu Valley (Nepal), comprehensive hydrochemical investigations of its downstream Indian reach remain limited. This study investigated the seasonal hydrochemical characteristics, water quality, and controlling geochemical processes of the Bagmati River in the Sheohar district of Bihar, India, using physicochemical analysis integrated with the Water Quality Index (WQI), principal component analysis (PCA), Pearson correlation analysis, and Gibbs diagram interpretation. Surface water samples were collected from four representative sites during the pre-monsoon (April 2025) and post-monsoon (October 2025) seasons, and major physicochemical parameters, nutrients, microbial indicators, and selected heavy metals were analyzed following standard APHA procedures. Pronounced seasonal differences were observed between the two sampling periods. Post-monsoon samples showed higher turbidity, total hardness, calcium, magnesium, nitrate, fluoride, and dissolved oxygen, whereas pH, alkalinity, total dissolved solids, ammonium, chloride, and microbial indicators decreased. Water quality improved substantially after the monsoon, with WQI values decreasing from 163.30–220.94 (poor to very poor) during the pre-monsoon season to 52.60–73.16 (good to medium) during the post-monsoon season. Multivariate statistical and hydrogeochemical analyses consistently indicated that seasonal rainfall primarily modified river chemistry through hydrological dilution, whereas rock weathering remained the dominant geochemical control. Overall, the study demonstrates that monsoon-driven hydrological processes strongly influence seasonal water quality while preserving the underlying rock-weathering control on river hydrochemistry. These findings provide an important hydrochemical baseline for the downstream Indian reach of the Bagmati River and support future seasonal monitoring, pollution management, and sustainable river basin management.