25-Modelling health of a lake - a case study

Abstract

To improve the availability of both quality and quantity of water supply systems, it is necessary to improve the knowledge on tropical lakes and reservoirs. Modeling of lakes and reservoirs is extremely complex and is totally different from modeling of rivers. Water quality in lakes and reservoirs is subjected to the natural degradation process of eutrophication and its impacts of societal development. The major nutrients responsible for the process of eutrophication are phosphorus (P) and nitrogen (N). They are mainly reaching the water bodies through fertilizers, detergents, agricultural run off, domestic and industrial wastes. The effects of eutrophication are considered negative and often reflect human perceptions of good versus bad water quality.

A case study was carried out at Lake Porur, which is located in Chennai, Tamil Nadu, with special emphasis on eutrophication. This lake assumes great importance as most of the imported water for the city water supply has been stored in this lake. Representative samples were collected from the lake and analyzed for the horizontal and vertical variations of temperature, dissolved oxygen (DO), turbidity and phosphates. Temperatures showed a reduction of 3°C within a depth of 2.6 m. The corresponding increase in the concentration of DO was from 6.9 mg/L to 7.9 mg/L. Turbidity was found to reduce by 50% from surface to the bottom. The variation of the concentration of phosphates also showed an increasing trend towards the bottom. With an average value of 0.09 mg/L, the concentration of phosphate was found to exceed the limiting value of 0.03 mg/L, throughout the lake. A mass balance approach showed that the phosphate loading to the lake is about 847 kg per year. It is found that Lake Porur has been polluted beyond its assimilating capacity. The reasons for the pollution were analysed and some potential remedial measures are suggested in this paper.