Delineating Contaminant Hotspots Through Hydrochemical Assessment of a Severely Degraded Watershed in Nepal

Abstract

This project deals with the environmental assessment of the Bagmati River in Kathmandu Valley, Nepal. Rapid population growth and urban development in recent decades have turned this river into a highly polluted water body. To delineate the contaminant hotspots, water and sediment samples were collected from eleven (11) sites along the river. Water samples were analysed for temperature, pH, total dissolved solids (TDS), conductivity, dissolved oxygen (DO), total suspended solids (TSS), turbidity, E. coli, biochemical oxygen demand (BOD), nitrate, total phosphorus (TP), chloride, and heavy metals in sediments. The data showed considerable degradation of the aquatic system. The TDS increased from 52 mg/L in Sundarijal (near source) to 595 mg/L near Teku (city center). Simultaneously, TSS increased from 43 mg/L to 1233 mg/L, with a contamination hotspot near the Thapathali area. DO quickly dropped below 2 mg/L at all sites near downtown. E. coli increased from 4000 MPN/100 mL in Sundarijal to 46,000 MPN/100 mL in Teku. The high levels of E. coli and the low DO were attributed to the direct disposal of sewage, house-hold trash, industrial effluents, and wastes from hospitals and slaughter houses. In recent years, sediment accumulation of heavy metals has also gone up, namely Cr (35%), Cu (59%), Fe (7%), Pb (7%), and Zn (25%). Phosphorus ranges from 0.2 mg/L at the source to 6.2 mg/L near the city. Phosphorus comes from the area’s wastewater treatment plant, industrial discharges, and sewage. From people’s survey results, 23% said they dispose off part or all of their trash into the river or directly on the street. The urban impact is also evident in the dramatic rise of dissolved Cl in water from the suburbs (26.2 mg/L) to the central city area (73 mg/L). Based on the results, areas near Teku, Thapathali, and Kalimati should be prioritised for immediate remedial measures. Urgent recommendations include dredging of stream sediments, contaminant source cutoff, stringent industrial regulations, and buffer strips and filter beds along the stream banks.