Stream water quality modelling for pollution load capacity in Roshi river- rural Nepal

Abstract

A rural catchment stream, Roshi (Nepal), is experiencing pollution loads from non-point pollution sources: agricultural runoffs and stone quarry sediment loads requiring immediate concern. This study is focused on developing a well-calibrated and validated water quality model to determine the pollution load capacity of Roshi River at peak rainfall periods using software QUAL2kW concerning pH, Temperature, DO, FSS, NO3-, NH4+, TP, BOD, and Turbidity. A pollution-dominant stretch of 1.95km was selected and segmented into three segments. RMSE values showed the model satisfactory in predicting actual in-field scenarios. Scenario analysis showed that in the existing pollution source flow and streamflow, the pollution load capacity is 275 mg/L for BOD, 33 mg/L for NH4+, 1250 mg/L for NO3-, 10 mg/L for TP, and 0 mg/L for DO. The FSS in the Headwater must be limited to 55 mg/L to maintain the stream water quality within standards signifying the immediate need to reduce the upstream stone quarry load. Policymakers are urged to implement stringent regulations on upstream quarries concerning sediment load discharge into the stream. The developed model can be a valuable tool for predicting the stream's response to potential emissions from future development activities during the monsoon period.