Materials and Methods

Hebbal Lake is situated in Bangalore urban district (13002´47.57´´ N latitude and 77035´13.66´´ E longitude) of Karnataka state India (Fig. 1). The lake is situated at 839 amsl. It is surrounded by 3 villages and located along Bangalore Bellary Road in the Northern part. The area of the lake is 192.48 acres. The lake is mainly used for fishing and washing purposes.Water sample collection and physicochemical analysis

Fig.1. Study area

Fig.2. Sampling locations

Bangalore and its urban conglomerates comprise of many man-made wetlands that were built for various hydrological purposes to serve the needs and water demand of the city. Bangalore wetlands have been experiencing anthropogenic stress especially due to the sustained inflow of sewage altering the chemical integrity of wetlands. The present study was carried out to evaluate seasonal variation in the physico-chemical parameters of water quality and to compute water quality index (WQI) of Hebbal Lake, Bangalore.

Sampling was conducted monthly from June 2016 to July 2017. Surface water samples were collected from 4 sites (Fig. 2; inlet, middle and outlet) using acid washed, dried polythene 1 liter bottles. Parameters like dissolved oxygen (DO), Total dissolved solids (TDS), pH, Electrical conductivity (EC) were measured on site using probe. Samples were taken into lab and preserved at 40C. Further analysis of various physico-chemical parameters(Nitrate, BOD, Total hardness, Ca, Mg, chlorides and alkalinity) were carried out in laboratory as per standard methods (Singh and Jayakumar 2016, APHA 2005, Trivedi and Goel 1986). Seasonal variations in water quality parameters were recorded compared with standards and pollution status was studied.

Table 1. Water quality index scale.

Water quality index provides a simpler model for expressing the water quality and it has played important role in water resource management (Lumb et al. 2011, Mohebbi et al. 2013, Sutadian et al. 2016). It defines the whole status of water body by a single number and informs the public about its state (Simoes et al. 2008). This index gives aggregate data on various water quality parameters at different time and places and converts the whole information into a single value giving information about that particular space at a particular time (Alobaidy et al. 2010). In this study Weighted Arithmetic Mean WQI (Horton 1965) is used to calculate WQI. Eleven parameters are chosen for the calculation. They were pH, EC, TDS, DO, BOD, nitrate, total hardness, Ca, Mg, chlorides, alkalinity. The season wise mean values were considered. The standard for drinking water was recommended by BIS (Indian standard specification for drinking water 2012). The weighted arithmetic mean approach calculates the WQI using 3 steps. In the first step unit weight (W_i) for various parameters were calculated by using the following formula (Tiwari and Mishra 1985)

W=K∈ (1/S); Where, K= 1/∑(1/S₁ +1/S₂+ S₃+ ...... +1/S_n)

In the second step, the quality rating scale (Q_i) for each parameter was calculated using the equation :

Q_i = {(Q_act -Q_ideal)/ (S_std-Q_ideal)}*100

For all the parameters except DO and p^H, the value of Q_ideal are zero, as it is the value of that parameter in pure water. For p^H, value of Q_ideal is 7.0 and for Do, it has a value of 14.0. After calculating the weight of the parameter and quality rating scale, WQI has been calculated using the formula:

W_i = Unit weight of each water quality parameter, K = Proportionality constant, Q_i = Quality rating scale for each parameter, Q_act= Estimated concentration of ith parameter in the analyzed water, Q_ideal = Value of the parameter in pure water, Sstandard = Standard value of ith parameter, n = No of water quality parameters. The final water quality index obtained can be summarized in a table to obtain the overall water quality status of water body as in Table 1. Table 2 lists BIS standards and unit weights. p^HIt maintains the acidic or basic property and is a vital

Table 3. Summary of water quality index of Hebbal Lake.