Physicochemical and microbial evaluation of water quality in the Tinau River Basin, Lumbini Province, Nepal

Abstract

Monsoon runoff can dilute and degrade river water quality in Himalayan catchments by mobilizing sediments, organic matter, and fecal contaminants. The Tinau River in western Nepal supports domestic abstraction, irrigation and riparian livelihoods, yet its monsoon-season suitability for these uses remains insufficiently resolved. This study assessed the twenty-four sites (Tinau River Basin) spanning upstream (TR01-TR10), midstream (TR11-TR16) and downstream (TR17-TR24) reaches during the third week of September 2025 using physicochemical parameters, fecal indicator bacteria, a weighted arithmetic water quality index (WQI), and major-ion-based irrigation indices. Water was neutral to slightly alkaline (pH = 7.10-8.60) and moderately mineralized (EC = 212-503 μS/cm; TDS = 106-252 mg/L), but turbidity exceeded 5 NTU at 18 sites. BOD5 and COD ranged from 8.11-40.54 and 10.0-52.4 mg/L, respectively, indicating notable organic loading. Total coliform and Escherichia coli ranged from 20-247 and 7-102 CFU/100 mL, respectively and E. coli occurred at every site. WQI classification identified 1 good site, 11 poor sites, 8 very poor sites and 4 unsuitable sites. Hydrochemical facies were dominated by Ca-HCO₃ water, consistent with carbonate weathering. Irrigation indices suggested low salinity and sodicity hazards, with 23 samples in C2-S1 and 1 in C1-S1, although RSC values of 1.76-2.96 meq/L indicated carbonate hazard at several locations. The Tinau River is therefore generally suitable for irrigation with drainage and soil management, but direct drinking use during the monsoon is not advisable without clarification and effective disinfection.