Applicability of MIKE 21 to assess temporal and spatial variation in water quality of an estuary under the impact of effluent from an industrial estate

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Abstract
This study aims at analysing the impact of wastewater load of industrial units in Haldia on the water quality of the Hoogly estuary using numerical modelling techniques. Modelling was never attempted in this region because it was generally felt that simulating such a complex system would not be easy with limited data availability but MIKE 21, a hydrodynamic and water quality model, was used to simulate BOD and DO profiles in the study area and simulation provided reasonably good predictions. A scenario assessment was also carried by increasing the flows from various sources to understand the site-specific relationships between pollution sources and water quality conditions and also to calculate the assimilative capacity of Hoogly with respect to waste discharged from the industrial units at Haldia. The results suggested that water quality of the canal towards closed Oil Jetty was highly deteriorated due to stagnant pond condition and discharges from a refinery. But large part of the canal was found to be well drained under tidal influence as a consequence of which both DO and BOD conform to the prescribed water quality standards most of the times. The impact of industrial waste load on the Hoogly was found to be negligible and the radius of influence was limited to about 200–300m across the estuary and about 1.6km along the shore during ebb conditions. Assimilative capacity of the region was estimated to be 837 kg/d as against the present load of 121 kg/d. The study suggested that the estuary had good dilution capacity and intense tidal mixing helped in rapidly diluting the pollutants.