The impoundment under study is called Mandakalli lake, which is situated, adjacent to the Ooty-Mysore highway at a distance of about, 7.5-km from Mysore on the way to Nanjangud. It is a small impoundment situated next to Dalvoi lake, which is the main feeder to the lake. In order to assess the quality of the impoundment in the beginning, a reconnaissance survey was conducted to gather data on the physical features and dimensions of the lake. Temperature, dissolved oxygen and conductivity were measured at regular intervals in the lake.  A depth contour map was drawn to supplement this. Then preliminary assessments were made for collecting the samples. Four sampling stations were selected for the study and samples were taken at regular intervals for 8 weeks. The samples were analysed for nitrates and phosphates leading to mass-balance analysis followed by prediction of the trophic status of the lake. Nitrates, phosphates and heavy metals were analysed in the roots of aquatic plants and sediments. Further aquatic plants around the lake were identified which were found to be dominated by Cyperus and cropping was conducted to determine biomass density. Around 21% of the lake were found to be covered by biomass. Thermal heat budget was determined, which indicated the heat income of the entire lake expressed in heat units per unit area. From the results obtained average concentrations of nitrates and phosphates in the lake were found to be 11.92mg/L and 2.35 mg/L respectively. An annual seepage loss of 9474 mg of nitrates and 1114 mg of phosphates was determined. The heavy metal concentration was high in the sediments. The thermal heat budget of the lakes was 0.2966-cal sq. m. Lastly the trophic status of lake was determined from measured phosphate concentration thereby determining chlorophyll a concentration. All the results show that the lake has attained eutrophic status. 

Untreated domestic sewage from various localities of Mysore city are let into Dalvoi lake directly, which in turn finds its way to Mandakalli lake. Thus controlling inputs into Dalvoi lake is one of the remedial measures to save this lake. Minimising the use of fertilisers and pesticides in agricultural fields can control considerable entry of pollutants through runoff. A clear indication of transition of lake from mesotrophic during 1990 to eutrophic state as of now clearly indicates that it would be surprising if the lake meets its end in the near future, if concrete steps are not taken immediately to save it.

Address: Department of Environmental Engineering, 
Sri Jayachamarajendra College of Engineering, 
Mysore-570 006, Karnataka, India
Phone: 0821-431133 E-mail: vinubond@yahoo.com

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