Results and Discussion

It maintains the acidic or basic property and is a vital characteristic of any aquatic ecosystem by controlling all biochemical activities and physio-chemical attributes (Jalal and Kumar 2012). The value of pH below 6.5 causes discontinuation in the making of vitamins in human body. When pH becomes more than 8.5, the taste of water becomes more salty and causes eye irritation and skin disorder (Gupta et al. 2017). The mean values were 7.7-8.2 during summer 7.3-7.9 (monsoon) and 7.1-8.3 during post monsoon (Fig. 3a). S1 showed least (summer) and S4 had higher (post monsoon) pH values than other sites during the study period. Similar trend was reported by Deepa et al. (2016), Ajayan and Kumar (2016), Luharia et al. (2016).

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

TDS is determined for measuring the amount of solid materials (inorganic and organic) dissolved in water. Any alteration in the balance of ionic concentrations by natural or anthropogenic activities causes detrimental effects (Tiwari 2015). The increase in TDS increases the apparent color of the water, water temperature and decreases the rate of photosynthesis (Chauhan and Sagar 2013). The mean values of TDS ranged from 595 to 612 mg/l during summer, 472 to 511 mg/l during monsoon and 603 to 705 mg/l during post monsoon season (Fig. 3b). During study period S1 had highest TDS in post monsoon and S3 the least in monsoon season respectively.

Electrical conductivity is an ability of aqueous solution to carry electric current. The depends on presence of ions their total concentration, mobility, valence, relative concentrations and temperature of measurement. Electrical conductivity is directly correlated with the total dissolved solids. Figure 3c summarizes the variations in electrical conductivity over the study area. The values of EC in summer season varied from 840 to 1208 µS. The values ranged fron 725-890 µS and 833-1201µS during monsoon and post monsoon respectively. S1 had the highest and S2 had the least. It was increasing from monsoon to summer with summer season having highest value.

Dissolved oxygen is an important on site parameter that gives an indication of pollution in water body. The combined activity of dissolution of oxygen and photosynthetic activity maintains the level of DO in water bodies. It depends upon the water temperature water agitation, types and number of aquatic plants, light penetration and amount of dissolved or suspended solids. DO levels between 5-8 mg/l are satisfactory for the survival and growth of aquatic organisms. In the present study the values range between 0 to 12 mg/l (Fig. 3d). In summer the DO varied from 2.5 (S1) to 12 (S2) mg/l. It varied from 0 (S1) to 7.9 mg/l (S4) in monsoon and 2.5 (S1) to 9.6 mg/l (S4) in post monsoon.

Hardness is the capacity of water to react with detergent. It is mainly because of calcium and magnesium salts. The permanent hardness is due to chloride and sulfate whereas the temporary hardness which can be removed by boiling is due to carbonates and bicarbonates. Total hardness is used to describe the effects of dissolved minerals (mostly calcium and magnesium) determining suitability of water for domestic, industrial and drinking purpose attributed to presence of strontium, ferrous iron, bicarbonates, sulfates, chloride and nitrate of calcium and magnesium (Deepa et al. 2016). High values of hardness are probably due to regular addition of large quantities of detergents used by nearby residential localities which drains into water bodies. The hard water can cause indigestion problem and possibilities of forming calcium oxalate crystals in urinary tracts. Average total hardness values varied as follows: 116 (S3)-196 mg/l (S1) during summer; 138 (S4)-164 mg/l (S1) during monsoon and 164 (S3)-296 mg/l (S1) during post monsoon (Fig. 3e). It was within prescribed limits by BIS at all sites during the study period. S1 had the highest concentration (post monsoon) and S4 the least (summer).

Calcium is an element which exists in divalent form in water. It is the main component of different aquatic shells and bones of vertebrates. Usually Ca is found in all natural waters, but the discharge of sewage and wastewater enhances its concentrationSeasonal variation of calcium is shown in Fig. 3f. The range of Ca in the study varied from 14.4-39.6 mg/l during summer, 29.2-38.4 mg/l during monsoon and 30.5-73.7 mg/l during post monsoon with S1 having the highest value (post monsoon) and S3 the least (summer). It was within permissible limits during study period.

It is also one of the important element of usually it is found along with calcium but in very low concentration and contributes to hardness of water along calcium. It is main component of chlorophyll. Magnesium values were as follows-17-24 mg/l during summer; 15.85-17.55 mg/l during monsoon; 18-27.29 mg/l during post monsoon. Highest Fig. 3g) Mg was observed at SI during post monsoon and least at S4 during summer.

In natural waters alkalinity is due to dissolution of CO₂ in water (Patil et al. 2018). High alkalinity indicate the presence of strongly alkaline industrial waste water and sewage. The degradation of plants, living organisms and organic waste also be the reason for increase in carbonate and bicarbonate levels thereby increasing alkalinity. The observed alkalinity values ranged between 108-250 mg/l summer; 170-450 mg/l monsoon and 300-460 mg/l post monsoon (Fig. 3h). It was above the standard limits given by BIS in all seasons at S1. Highest alkalinity was observed in S1 during post monsoon and lowest was at S4 during summer season.

The high chloride concentration in water is an index of pollution. It is mainly present in sewage, effluents farm drainage and remains unaltered during purification of sewage (Patil et al. 2018). Excess chloride would reduce the Do content of water, which turns harmful to aquatic organisms (Deepa et al.2016). Figure 3i gives the seasonal variation of chlorides during the study period. Chloride values ranged between 188-295 mg/l; 158-186 mg/l and 178-218 mg/l during summer, monsoon and post monsoon seasons respectively. It was above desirable limit during summer at S2 and S4. Highest chloride concentration was observed at S2 (summer) and lowest was at S1 (monsoon).

It is important part of assessing the organic pollution of aquatic ecosystem. It is the measure of quantity of oxygen required by bacteria and other microorganisms under aerobic condition in order to biochemically degrade and transform organic matter present in the water bodies. The consequences of high BOD include aquatic organisms becoming stressed, suffocating and eventually death (Dhinamala et al. 2015). Figure. 3j illustrates the variation in BOD at sampling sites. The lowest seasonal average BOD was 4.5 mg/l at S4 while the highest value was 56.9 mg/l at S1 during monsoon season respectively. It was obove reference values set by BIS in all sites during study period except S4 during summer.

Nitrate is an important nutrient which plays key role in deciding the productivity of aquatic ecosystem and accelerates growth of algae and macrophytes. Nitrate occurs in water from various natural sources and due to human activities like food production, agriculture and manure, disposal of domestic and industrial sewage (Lodh et al. 2014).Excess amounts of nitrates in drinking water can create serious health problems in humans. It can change normal hemoglobin to methaemoglobin which reduces the ability of the blood to transport oxygen to cells. In severe cases it can lead to respiratory and heart problems and death (Deepa et al. 2016). The average values of nitrate were 0.10.12 mg/l during summer; 0.02-0.11 during monsoon and 0.05-0.1 mg/l during post monsoon respectively (Fig. 3k). The highest value was 0.12 mg/l at S2 and lowest average value was 0.02 mg/l at S4. Water quality index (WQI) First step in calculation of WQI using weighted arithmetic index given unit weight for each parameter. The unit weight of each parameter and standard values for each parameter as per BIS is given in Table 2. The summary of WQI values of the water samples from all the 4 sampling sites for all seasons are given in Table 3. The results showed that water samples from S1 fall under unsuitable water category (WQI>100) during all seaons, S2 had very poor

Fig. 3 Seasonal variations in Mg among sites, Seasonal variations in alkalinity among sites, Seasonal variations in chlorides among sites, Seasonal variations in BOD among sites, Seasonal variations in nitrate among sites.