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Overlay of vector layer of Varthur lake shoreline (Figure 5) with the remote sensing data (of 2006), it is seen that the general shape and surface area of the lake is virtually the same in 2006, 1998 as it was between 1971 and 1974, in the SOI map. The SOI map was used as a reasonable approximation of the current shoreline of the lake and the shoreline of the lake has not changed significantly since 1998. The results of the morphometric analysis reveal that Varthur is a shallow lake, with a very large surface area in relation to its depth. The total area of the lake is estimated to be 1,478,000 m2.The shoreline of Varthur lake does not appear to have changed considerably between the early 1970’s and present day, unlike many other tanks in the district that have decreased drastically in size due to sedimentation and encroachment. Figure 6 is a bathymetric map of Varthur lake and shows the lake has an estimated maximum depth of approximately 2.0 meters. The mean depth is estimated to be 1.05 m and the lake bottom exhibits a very gradual downward slope from west to east, with maximum observed depth occurring near the dam wall. These results are consistent with sedimentation patterns common to dammed reservoirs. Table 2 summarised the volume computation based on Method A. Figure 7 presents the depth profile produced using Method B, which involved software-assisted volumetric analysis incorporating the same contour map as well as the actual data points collected in the field to extrapolate a depth profile of the lake in grid form. This profile produced a maximum estimated depth of 2.55 m, which is 0.55 m greater than that of the previous contour map. Volume computed based on calculus-based formulas - Trapezoidal, Simpson's, and Simpson's 3/8 Rule were applied to the above profile, resulting in three separate but highly similar estimates of the volume. The results of these calculations are summarised below in Table 3. Table 4 lists other morphometric parameters that emphasise the fact that the whole of Varthur lake is shallow in relation to its surface area. The lake exhibits low shoreline development consistent with the lack of topographical diversity in the region; this factor contributes to the regularity of the sedimentation patterns within the lake as there are few formations to interfere with the water currents.
The velocity and turbidity of the water decreases considerably due to the increase in cross-sectional area and the presence of large mats of water hyacinth as silt and sediment-laden water enters Varthur lake from the Bellandur Canal,. At this point, the water no longer contains sufficient energy to displace or carry larger suspended particles. These particles are deposited on the lake bottom near the inlet, forming a delta. Smaller suspended particles are deposited further away from the inlet where the velocity and turbulence decrease further. This forms a gradual downward-slope along the length of the reservoir, with the deepest section occurring near the dam. The velocity of the water increases as its approaches the northeast and southeast outlets, and these areas appear to accumulate less sediment than the main body of the lake. Sediment deposits in Varthur lake is lesser compared to other tanks in the area due to desilting by local residents around the edges of the lake. This activity was observed in several areas along the northern shoreline while conducting field sampling. Varthur lake has a catchment area of 1.8 km2, the second largest in the Bangalore South taluk (Govt. of Karnataka, 1990). This catchment area contains a substantial human population engaged in agriculture and various industries and, therefore, the potential for accelerated sedimentation due to anthropogenic causes is substantial. Without previous depth profiles of the lake, it is difficult to estimate the rate of sedimentation. However, even if the historical depth of Varthur is very shallow, its lack of depth makes it highly susceptible to increases in sediment loading caused by human development within the catchment area. Loss of depth and volume would reduce the water supply available to local farmers who continue to use Varthur as a primary water source. It would also have a detrimental effect on the quality of water in the reservoir and degrade habitat for fisheries and wildlife. The ability of the lake to moderate the local climate would be reduced, as the amount of energy absorbed and released by the lake would decline along with its depth and volume. Accumulation and impaction of silt on the lake bottom also has the potential to impede the infiltration of rainwater into the aquifers below. This infiltration is the main water source of groundwater recharging in the Bangalore area. Varthur lake represents a major local reservoir of rainwater and a reduction in the permeability of its benthic layers would decrease the water resources available from local open and bore wells. These wells are the primary source of domestic, potable, and agricultural water, and their decline would be detrimental to the people living in the area.
Results of the water analysis for samples taken near the northeast outlet during October, November and January are presented in Table 5. The pH of the water was found to be slightly alkaline (approximately 7.5 to 8.0) for all water samples. November water samples exhibited a strong ability to neutralise acids in solution due to the presence of bicarbonate. The acidity of the samples was much less than their alkalinity. Total hardness showed little variation during the sampling period, indicating that the overall concentration of calcium and magnesium salts is fairly constant; hardness due to calcium carbonate ranged from 59 to 68% of total hardness for November and January samples. In November, light was able to penetrate the upper 19 to 24 cm of the water column. Transparency was substantially reduced during January. Further examination of physical properties revealed high concentrations of suspended and dissolved solids. The concentration of total dissolved solids (TDS) showed substantial seasonal variability, increasing three-fold between November and winter sampling periods. This increase in TDS corresponds to a similar increase in electrical conductivity. Moderate to high concentrations of total suspended solids (TSS) were also present in January samples. Water from the middle of the lake exhibited the highest concentration of TSS by far. The wide variation between TSS concentrations for various sampling sites could be due to the presence of organic floatables observed during collection of the samples. The presence of these clumps of matter could significantly increase the TSS value for a sample in comparison to a similar sample without clumps. Turbidity from organic and inorganic suspended matter in Varthur has the potential to impact the ecology of the lake in several ways. Many toxic contaminants, such as heavy metals and some pesticides, could potentially find their way into Varthur by adhering to solids in solution. Eventually, much of the suspended matter will settle in the bottom of the lake where they smother benthic organisms and contribute to siltation. Turbidity is also the most important factor in prolonging the survival of faecal coliform in water bodies because the particulate matter shelters bacteria from harmful solar radiation (DWI, 1995).
Dissolved oxygen (DO) levels in Varthur lake were extremely low. Water temperature ranged from 22 to 26C prior to 9:00 AM on all sampling dates. The high BOD of the water samples indicates that decomposition of organic matter is one of the main factors leading to the low DO concentrations observed in the lake. Much of the remaining oxygen is likely consumed through nighttime respiration by aquatic plants. Eutrophic lakes similar to Varthur often experience a daily cycle of hyper- and hypooxygenation due to the high concentration of photosynthetic algae that produce oxygen during daylight hours and consume oxygen at night. However, this requires further investigations to confirm diurnal-nocturnal fluctuations in DO. The low DO content limits diversity of animal life, which can survive in the lake. Anoxic conditions also affect many other chemical processes within the lake that can be detrimental to organisms, such as the conversion of organic nitrate to toxic ammonia. The high BOD values imply that virtually all the organic matter contained in the samples were biologically degradable, and that the combined concentrations of sulphates, nitrates, ferrous iron, and other organic components that cannot be oxidised by bacteria are comparatively low. Based on these findings, only a small proportion of the organic pollution in Varthur could have its origin in industrial effluents. The majority of organic pollution likely comes from animal and plant sources, such as sewage and plant death within the lake. In addition to sewage, several aquaculture ponds are seasonally drained into the lake also have the potential to contribute substantial amounts of nutrientrich organic debris.
The concentration of chloride ions in November samples averaged 102 mg/l. In January samples, these values increased 60 to 70 percent. October lake water samples contained less than 0.2 mg/l of residual chlorine. Sulphate concentrations in the lake were consistently low, however, a substantial decrease in sulfate occurred between November and January sampling dates. Sodium concentrations for November were only moderately high. Elevated levels of potassium were observed in November samples. January samples were well within standard range for unpolluted surface waters. Potassium is also an essential element for plant growth. Elevated levels of potassium were observed in November samples, indicating potential contamination from industrial effluents or fertiliser. Potassium concentrations dropped substantially in January, possibly due to uptake by the increasing macrophyte population. A similar trend was observed for sulfate and could be caused by winter plant uptake as well. Elevated chloride values could be due to many factors, including sewage, industrial effluents, and agricultural runoff. The seasonal variation may due to the fact that January concentrations were not diluted by monsoon rainwater.
Nitrate concentrations present in October samples were low, averaging only 0.24 mg/l. The average concentration of nitrate increased to 1.00 mg/l and 1.27 mg/l in November and January, respectively. Ammonium was estimated to be in excess of 3.0 mg/l for three of the four October samples. Phosphorus concentrations from January samples were very high, averaging 15.1 mg/l. Varthur contains significant amounts of the macronutrients required by aquatic plants in large quantities in order to survive and grow, especially phosphate. Excess amounts of phosphorus could be the result of contamination from sewage and/or fertilisers. Eutrophication has resulted in large populations of algae to develop in Varthur, which imparts a green colour. This process has also assisted in the intrusion of Eichhornia crassipes (water hyacinth). Although the amount of lake surface occupied by this plant fluctuated dramatically between sampling dates, the western portion of the lake was consistently covered with mats of hyacinth, as were the two main outlets. Overall, coverage by water hyacinth increased during the winter months. The concentration of nitrate was slightly higher than standard values for unpolluted waters in October samples, but increased substantially in November and January. The relatively low nitrate concentrations observed in Varthur could be a result of several biological processes. Loss of nitrate in Varthur could be the result of ammonification, the conversion of organic nitrogen to ammonium during the decomposition of organic matter. High concentrations of ammonia observed in October samples support this explanation. Under anoxic conditions, nitrate may also be converted to nitrite; it is likely that such conditions exist near the bottom sediments of Varthur lake, given the extremely low oxygen levels of the surface layers, and that this process may be partly responsible for the lower concentrations of nitrate in the water. Loss of nitrate also occurs through uptake by macrophytes and algae; during periods of high plant growth, this process may significantly reduce nitrate concentrations in the lake. Ammonia concentrations during November were high enough to be toxic to many forms of aquatic life. When water samples from January were viewed under a microscope, the most dominant zooplankton by far was Daphnia, a species that is highly tolerant of ammonia.
Bacterial culturing confirmed the presence of the bacteria E. coli in the lake. The bacterium Escherica coli is indigenous to the intestines of animals, including humans. Its presence in Varthur indicates that faecal matter contaminates the lake. Faecal contamination is often associated with other types of pathogenic bacteria and viruses found in untreated sewage. The turbidity of the lake water, along with its warm temperature, mildly alkaline pH, and low oxygen levels, could lead to prolonged survival of pathogenic bacteria for up to several days. The water sample taken from the Bellandur Canal in November was very similar in composition to those taken from Varthur lake, and it is likely that many of the contaminants that enter Bellandur lake from its own substantial catchment area eventually make their way to Varthur.
Characterisation of Groundwater :
Results from the groundwater survey are presented in Table 6.
These wells were located at opposite ends of the lake, approximately 250m and 750 m from the
southeastern and southwestern shorelines, respectively. The groundwater parameters were found to
be within the limits set by the 1983 Indian Standards Specification for Drinking Water: ammonia,
chloride, electrical conductivity, fluoride, nitrate, and pH. Two of the samples tested positive for minor
concentrations of coliform bacteria. There is a possibility that coliform bacteria present in two of the
samples could have originated from sewage effluent in the lake.
The socio-economic survey revealed that the total land area irrigated using Varthur lake water is 622.27 hectares and the total number of farmers dependent on the lake water for irrigating their lands is 1159. In Varthur, Sorahumase and Valepura village, the land irrigated by the lake water amounts to 322.27, 223.12 and 76.57 hectares respectively. The type of crops grown in Varthur village and the area under each crop is as follows: Paddy – 312.27 hectares, coconuts –3.33 hectares, bananas –3.75 hectares, Beetle leaf –0.11 hectares, arecanut – 0.04 hectares and Floriculture – 2.15 hectares. In Siddapur village the main crops grown are vegetables and floriculture whereas in the nearby Ramagondanahalli it is vegetables, greens and flowers. All respondents used bore wells to meet their domestic water needs. 9 of the 22 households interviewed purify their drinking water with a filtration system, and one household boiled the water prior to drinking. 20 of the households, representing 83% of the survey population, relied on agriculture as their primary source of income. 12 of these households relied exclusively on lake water to irrigate their crops, and 2 more used both the lake and bore wells for this purpose. 10 of the houses that use the lake for irrigation reported a decline in both the quality and quantity of crops due to pollution of the lake water. 14 households raise cattle, primarily for milk. At least 11 of these farms rely exclusively on plants growing on and around Varthur lake to feed their cattle. 9 of these 11 farms rely on the sale of dairy products for part of their income; the percentage of total income derived from dairy products for these farms ranged from 1 to 74%, with mean and median averages of 32% and 40%, respectively. None of the households were involved in fishing the lake, however, one was actively engaged in aquaculture of carps in lake-water-filled dugouts near the shore. Another respondent indicated a desire to start a similar operation.
All of the residents surveyed indicated that their families had lived in the area for one generation or more. Duration of residency ranged from 30 years to more than 200 years and at least 60% of the families had lived in the area for over 100 years. 19 of the 22 households surveyed would actively support reclamation efforts for Varthur lake. 16 of the 22 households visit the lake on an annual basis to submerge idols during Ganesh festival. 86% of the respondents indicated that they had noticed deterioration in the quality of the lake. Although estimates of when this deterioration began varied widely, (from 6 to 40 years), over half of the estimates ranged from 15 to 20 years ago. 10 of the farms reported a reduction in the quality and quantity of their crops as a consequence of polluted lake water. 18 of the respondents indicated that the mosquito population around Varthur has increased in recent years. One respondent indicated that family members had suffered from malaria and dermatitis. Another household that did not filter or boil their drinking water reported problems with viral fever. The smell given off by the lake in winter months was considered to be a nuisance by 16 of the households involved in the survey. The effect of polluted lake water on crop production could very well be detrimental due to factors such as pathogens contained in the water. It is unclear whether aquaculture has become popular because of a decline in the population of fish in the lake or because of its comparative convenience and increased yield. Several residents lamented the fact that fish stocks have declined and they are no longer able to enjoy this resource.
Water hyacinth is often classified as a nuisance species in Bangalore tanks. However, it provides a significant and inexpensive source of cattle fodder for farmers around Varthur as well as a source of income for residents who gather and sell the water hyacinth. The majority of households in the villages surveyed maintain dairy cattle to feed their families and, in most cases, to supplement their income. While estimates of income derived from dairy varied widely, this income would be reduced if farmers had to purchase fodder outside the lake area. Loss of lake fodder may prevent some households from maintaining cattle at all.
Many residents relied on bore wells or open wells for all their water needs, a trend that increases rapidly as distance from the lake increases. Reliance on bore wells does not necessarily negate their reliance on Varthur lake, however, because Varthur could play an important role in recharging local aquifers in the area. 50% of the population represented in the survey does not filter well water prior to drinking. This makes them more susceptible to potential contamination of groundwater supplies by pollutants in the lake water.
Varthur exhibits several features that could have led to the increase in mosquito populations reported by local residents, including consistently warm water temperatures and large populations of water hyacinth that provide breeding habitat for these insects. Mosquitoes constitute both a nuisance and a public health risk in the vicinity of the lake, as they are carriers of diseases such as malaria, encephalitis, and dengue fever.
Few of the people living near Varthur do not have direct contact with the lake beyond the annual submersion of Ganesha idols. Many respondents were generally unaware of changes in the ecology of the lake unless they pertained to sight, smell, or mosquito populations. Most of the respondents were unable to provide information on wildlife populations, especially fish. Despite these observations, most respondents indicated a willingness to support efforts aimed at restoring Varthur lake to a less polluted state and hoped that their children would remain in the area around the lake to raise their families.
