Application of automated QUAL2Kw for water quality modeling and management in the Bagmati River, Nepal

Abstract

The Bagmati River in Kathmandu Valley (Nepal) receives seven major polluted tributaries. Discharges of wastewaters containing degradable organics and nutrients have resulted in decrease in DO concentrations along its course. A one-dimensional stream water quality model Q`UAL2Kw was calibrated and confirmed using the data in 2000. The model represented the field data quite well with some exceptions. The sensitivity analysis showed the model was highly sensitive for water depth and moderate to point sources flow, TN, CBOD and nitrification rate. The model was applied to simulate various water quality management strategies during critical period to maintain the targeted water quality criteria (minimum DO at or above 4 mg/L; maximum CBOD, TN, TP and temperature at or below 3, 2.5 and 0.1 mg/L and 20 degrees C, respectively, and pH range 6.5-8.5) considering: (i) pollution loads modification (ii) flow augmentation and (iii) local oxygenation. Except for CBOD, all the stated quality limits were achieved with 30 mg/L CBOD, 5 mg/L TN, 0.25 mg/L TP limits at point sources and with flow augmentation of 1 m(3)/s and local oxygenations at three critical locations. The simulated maximum CBOD was 8.5 mg/L. It was considered reasonable for the developing country, Nepal, as the European water quality with maximum CBDO of 3 mg/L is difficult to achieve. The results showed the local oxygenation is effective to maintain minimum DO concentrations in the river. The combination of wastewater modification, flow augmentation and local oxygenation is suitable to maintain the acceptable limits of water quality criteria. (c) 2007 Elsevier B.V. All rights reserved.