Assessment of drinking water quality and irrigation suitability in the Marsyangdi River Basin, Nepal: Implications for SDG 6 and ecosystem sustainability

The Marsyangdi River Basin (MRB) in central Nepal is a crucial source of drinking water, irrigation, and hydropower; however, it is increasingly threatened by agricultural intensification, urban expansion, and climate change. This study evaluated the suitability of river water for drinking and irrigation in relation to Sustainable Development Goal 6 (Clean Water and Sanitation). A total of 84 water samples (42 per season) were collected during pre-monsoon and monsoon periods from the main river and its tributaries. Physicochemical and microbial parameters were analyzed following standard protocols, and water quality was evaluated using the Weighted Arithmetic Water Quality Index (WQI) and irrigation indices including sodium percentage (Na%), sodium adsorption ratio (SAR), magnesium adsorption rate (MAR), Kelly’s ratio (KR), permeability index (PI), and the cation ratio of soil structural stability (CROSS). Results showed that overall water quality for drinking was excellent (mean WQI < 50 across both seasons). However, Escherichia coli contamination was detected in more than 60% of samples, underscoring health risks and the need for treatment before consumption. Irrigation water was generally suitable (C1–S1 and C2–S1 classes on the Wilcox diagram), with spatial variation reflecting geology and land-use patterns: lower sodium but higher magnesium in leeward Manang, and higher sodium hazards in the intensively farmed districts of Lamjung and Tanahun. These findings highlight the dual challenge of ensuring safe drinking water and sustaining irrigation under changing hydro-climatic conditions. Continuous monitoring and localized water management strategies are essential to safeguard human health, agricultural productivity, and ecosystem resilience in Himalayan River basins.