The effect of artificial de-stratification on the water quality of a freshwater high altitude lake

The effect of artificial de-stratification on physico-chemical conditions were studied in Lake Nainital, India during January, 2006 to March, 2008. The lake is eutrophic and is situated at high altitude of Himalaya. In order to reduce the level of eutrophication, hypolimnetic aeration is being done with two compressors attached with thirty aeration discs, placed on the bottom sediment and operated regularly throughout the year. The aeration de-stratified the water column of the lake and produced almost homogenous physical and chemical parameters. Within ten days of aeration the thermocline was destroyed. Due to mixing of anoxic hypolimnion with upper layers the whole water column became anoxic during initial phase of aeration but after two days a sharp increase in dissolved oxygen concentration was noticed at all depths. There was significant change in the water quality of the lake due to aeration. The mud-water interface which was anoxic before aeration became oxic and Secchi disc transparency increased considerably. The BOD was reduced while the concentrations of ammonia-nitrogen, phosphate-phosphorus and nitrite-nitrogen increased during seven months of aeration. The increase in concentration of nutrients during aeration was due to upwelling of nutrients rich hypolimnetic water to the surface water. There was no significant difference in concentrations of nitrate-nitrogen of pre-aeration and aeration periods. The strong smell of H₂S and NH₃ during aeration indicated the release of these gases into the atmosphere. The results of the study suggested that withdrawal of hypolimnetic water in combination of hypolimnetic aeration will prove to be the best method of restoration of Lake Nainital. Since the hypolimnion has disappeared due to de-stratification and the whole water mass has mixed, removal of surface water may serve the same purpose.