Results and Discussion

The variations in physicochemical characteristics of lakes during the study period are presented in figure 3. Water temperature ranged from 23.6 to 34.2°C, which showed diurnal and seasonal variations. Water temperature affects the physicochemical characteristics of lake water and accelerates the metabolic/biological activities of aquatic organisms. In the present study, total dissolved solids and electrical conductivity ranged from 150.5 - 1230 mg/L and 311 - 2814 µS/cm respectively. Less TDS in water indicates of lower pollutants and can be used for domestic purposes (Bhatia et al., 2018; Ravikumar et al., 2013). pH is an important parameter which influences the availability and release of nutrients like ammonia, phosphate, iron and trace metals into lake water. pH ranged from 7.08 - 9.43 in the monitored lakes. Total alkalinity ranged from 102.6 - 595 mg/L and gives the measure of bicarbonates, carbonates, phosphates and hydroxides etc. in water samples. Dissolved oxygen (DO) which represents the amount of oxygen present in water that supports aquatic life, varied from 0 - 16.53 mg/L. Oxygen enters water from the air and through photosynthesis by algae and aquatic plants. The increase or decrease in DO depends on photosynthesis and decomposition activities in lakes.

Organic pollution in lakes represented by chemical oxygen demand (COD) and biochemical oxygen demand (BOD), which ranged from 8 - 164 mg/L and 4.07 - 99.59 mg/L respectively. Total hardness, calcium and magnesium in monitored lakes ranged from 79 - 738 mg/L, 21.11 - 224.45 mg/L and 5.03 - 49.07 mg/L respectively. Chloride ions exist in natural waters as the salts of sodium, potassium and calcium. In the monitored lakes, chloride ranged from 46.86 - 1586.14 mg/L. Polluted lakes receiving sewage water, industrial effluents and agricultural run-off have higher amount of chloride. Basapura lake - 2 had higher values of water temperature, total dissolved solids, electrical conductivity and BOD, which indicated ionic and organic pollution. Ambalipura Kelagina kere had higher ionic contents with chloride, total hardness and calcium. Lesser ionic pollution was evident in Yediyur lake with lower values of chloride, total hardness, magnesium, electrical conductivity and total dissolved solids. Handrahalli lake had lowest organic pollution compared to others evident from minimum values of BOD and COD. Dorekere lake had high nutrient loadings of ortho-phosphate and nitrate. Domestic wastewater mainly containing detergents, organic wastes, industrial effluents and agricultural run-off contribute to higher levels of phosphates in surface waters (Iscen et al., 2008).

Pearson’s correlation coefficient (r) helped to identify the correlation among water quality parameters. In the current study, strong positive correlation was found between EC - TDS (r = 0.95); COD - BOD (r = 0.98); total hardness - chloride (r = 0.81); calcium - chloride (r = 0.80); magnesium - total hardness (r = 0.90) and nitrate - orthophosphate (r = 0.73).

Figure 2: Restored lakes of Bengaluru, Karnataka, India

Figure 3: Spatial variation in water quality parameters

Environmental variables affecting water quality of lakes

Principal Component Analysis (PCA) aided in identifying the most important environmental variables that significantly influence the water quality of lakes. PCA performed on normalized water quality data of forty lake samples described by fourteen physical and chemical parameters (14 variables), yielded 4 PCs with eigenvalues >1. The eigenvalues were 5.75, 3.08, 1.60 and 1.49 and the % variance were 41.07, 22.02, 11.41 and 10.67% for PC1, PC2, PC3 and PC4 respectively and these components explained the variability in water quality variables (table 2). PC1 has strong positive loading on pH, dissolved oxygen, orthophosphate and nitrate whereas negative loading on total hardness. PC1 indicates non-point sources of pollution due to the presence of nutrient and organic pollution. PC2 influenced by chloride which indicates pollution in lakes through domestic sources. PC3 influenced by BOD and COD, which indicate domestic as well as industrial pollution of lakes. PC4 influenced by water temperature and TDS, showed the influence of climatic factors. Thus, PCA revealed that the physicochemical parameters like pH, dissolved oxygen, orthophosphate, nitrate, chloride, calcium, BOD, COD, water temperature and TDS played an important role in the present study.

Table 2: Loadings and eigenvalues of water quality parameters on significant principal components

WQI status of restored lakes

Monitored lakes were grouped under three different WQI status like good water quality (10%); poor water quality (37%) and very poor water quality (53%). WQI results revealed that only 4 lakes such as Jakkur, Devasandra 1, Ullal and Handrahalli had good water quality. About 15 lakes fell under poor water quality i.e., Dasarahalli (Chokkasandra), Kattigenahalli, Narsipura - 20, Kodigehalli, Kattigenahalli kere - 136 (Palanahalli), Narsipura - 26, Sowl kere, Dorekere, Sankey, Yediyur, Rachenahalli, Ulsoor, Vijnanapura, Mangammanapalya kere and Yelahanka. Rest of the lakes such as Ambalipura Melina kere, Kasavanahalli, Haraluru, Chinnappanahalli, Herohalli, Munnekolalu, Parappana Agrahara, Garudacharpalya, J.P. Park, Sheelavanthakere, Kaikondrahalli, Seegehalli, Basapura - 2, Uttarahalli, Allalasandra, Malagala (Ballehannu), Devarabisanahalli, Kowdenhalli (Gangadharkere), Ambalipura Kelagina kere, Deepanjali Nagara kere and Puttenahalli lake had very poor water quality (figure 4).

Most of the restored lakes were polluted as they continue to receive pollutants in the form of untreated or partially treated sewage from the catchment and dumping of industrial effluents. Inflow of sewage and industrial effluents resulted in chemical pollution and microbial contamination of groundwater in Bengaluru city, causing health risks (Sheeba et al., 2017).

Rejuvenation is expected to improve the overall quality of lakes. Comparison of the status of Devarabisanahalli lake before and after restoration activities, it was found that the water quality of Devarabisanahalli lake has not improved, evident from the WQI of very poor water quality during the two scenarios. This shows a lacuna in the current restoration measures. This also underlines an urgent need to maintain these restored lakes before further deterioration. Awareness programs and implementation of environmental norms is required to arrest further deterioration (Ladu et al., 2018).