Results and discussion

Literature survey shows that Bangalore had about 379 waterbodies in 1973, due to urbanisation  it is reduced to about 246 in 1996 [Deepa et al, 1997] with about 81 major waterbodies [Lakshamana Rau et al, 1986]. Recent studies also showed that about 40% of the waterbodies in Bangalore were sewage fed [Krishna et al, 1996]. In view of this the present study attempts to assess the pollution status and draft suitable management strategies for wetlands.

The results of present investigations showed that the most of the analysed parameters of five tanks (Ulsoor, Hebbal, Yediur, Kamakshipalya and Madivala) exceeded the limits set by Indian Standard for Industrial and sewage effluents discharge (IS: 2490 -1982).

The colour of the waterbody was mostly greenish largely due to algal blooms on account of Microcystis, due to sewage and domestic effluents. In Eutropic systems the plankton and other dead organic matter in high levels would contribute highly towards an autochthonic turbidity source.

Turbidity a significant parameter determining the primary productivity levels is inversely related to productivity.  Turbidity in the lakes were noticed mainly due to silt, organic matter, sewage from both industries and domestic and other effluents contributing to turbidity.

The pH values of most water samples showed to range from alkaline,   (7.6 - 9.3) to acidic (Kamakshipalya, 6.6). High alkaline values were noticed in Yediur, and Ulsoor. The varying values of pH may be indicative of the entry of acidic or basic industrial effluents.

The dissolved solids consist mainly of bicarbonates, carbonates, sulphates, chlorides, nitrates and possibly phosphates of calcium, magnesium, sodium and potassium. High values of Dissolved Solids are well correlated with the EC of lake waters and can have negative influences on the biological production efficiencies of lake ecosystems.  The high-suspended solids ranging from 52.2 to 278.3 mg/L are a result of silt, organic matter in suspension. Autochthonous sources mainly plankton also directly influence the light penetration into the aquatic system and hence affect primary production efficiencies in lakes.

The DO levels were seen to range from 1.2 in Kamakshipalya to 9.1 mg/L in Ulsoor Lake. The DO levels of 5 mg/L are ecologically recommended minimum for sustaining a waterbody. The recommended DO concentration for a healthy and ideally productive lake waterbody as 8 mg/L [Wetzel., 1973]. The high values of DO were noted due to higher photosynthetic activities.

The contents of phosphates were found to be very low in the lakes surveyed from 0.06 to 1.3 mg/L. The standard is 0.2 mg/L for surface inland water [Chakarapani et al, 1996]. This parameter is very crucial and ecologically elusive as it as the tendency to precipitate by many cations occurring in lakes and accumulates at the bottom of the lake inaccessible to the phytoplankton.

The total nitrogen estimated as nitrates ranges from 0.1 to 2.7 mg/L. The standard for inland surface water is 0.1-mg/L [NEERI, 1988]. The parameter is very significant from the algal productivity in lakes.

The analysis of heavy metals of the lake waters indicated that among the metals such as iron, zinc, lead and chromium. Iron and lead were present in greater concentrations than the other two (zinc and chromium).

In order to make a quantitative estimate of the extent of pollution two lakes were selected for detailed parametric statistical analyses.  Preliminary investigations of physico-chemical parameters indicate that Yediur lake is highly polluted compared to Sankey. In view of this, Yediur and Sankey lakes with different levels of pollution loads were examined in detail. 

The multiple regression analysis of Conductivity, Magnesium, Calcium, COD, Chlorides, Total Hardness verses pH and Sulphates, Magnesium, Potassium, Calcium, Total hardness verses COD for Yediur lake and Sankey lakes were carried out. The results are as shown in the table below. The step wise regression carried out to determine the relationship of various parameters (Conductivity, Magnesium, Calcium, COD, Chlorides, Total hardness) with  pH  and (Chlorides, Sulphates, Potassium and Total hardness) with COD . The probable relationship for Yediur and Sankey lakes are as given in the table. The contribution by parameters such as Chlorides, Sulphates and Total hardness are higher in Yediur lake as compared to Sankey owing to pollution from domestic and industrial sectors.

Yediur:

Sankey lake: