Water Quality Predictions in the Mixing Zone of River Kabini at Nanjangud area, Karnataka, India

Abstract

The scenario of unauthorized encroachments and anthropogenic activities in the riverbed area brings our attention towards fast deterioration of water quality in rivers and subsequent damage to the ecosystems thereof. More precisely, number of drains discharging wastewater into rivers and solid waste dumping sites along the river banks pose threat to the aquatic community, recreational areas, quality of surface and ground water, and also results into problems, such as sedimentation and erosion. In view of the need for conservation of natural resources, developmental plans in the riverbed area are being proposed, due to this type of pollution, Dissolved oxygen (DO) concentrations in streams and rivers are below specified limits and adversely affect aquatic biota. The continuous discharge of wastewater effluents into the natural rivers demands careful control of the amount and quality of pollutants present in such effluents. In many situations, the discharge of industrial and municipal effluents into the rivers will results in the information of mixing zone. It consist of two zones: (a) Limited use zone (LUZ) and (b) Zone of Passage (ZOP), The present study deals with the water quality predictions in the mixing zone, with a drain from the city entering the river from the pipe through a bank outfall. The field data were collected during lean flow at selected points in river Kabini. The city's wastewater is being used to investigate the river's dispersion properties. In this survey, the data were obtained on the cross sectional distribution of dissolved solids (measured by specific conductance). The peak values of conductance occurred near the discharge shoreline during the study. Based on the results of conductivity observations, the width and lateral dispersion coefficients of the outflow plumes of various transects are determined. It was found that the width of the effluent plume flowing into Transect depends on the depth of the channel. The lateral variance coefficient is a function of the depth of the channel, not the breadth of the river under inquiry, according to the data analysis. The field data were also utilized to estimate decay rates of CBOD and NBOD as well as reaeration rate for each reach in the study segment of river.

The calibration of the model for lateral mixing is carried out using conductivity distribution data, which is followed by the calibration of DO model. Then the calibrated DO model was utilized for the prediction of DO in the river due to several viable technological options. The results indicate that, for given values of effluent CBOD and NBOD, the in stream DO would increase with increase in lateral limit use zone boundary.