Powai Lake : When green is grievous

Powai Lake is an artificial body of water that was constructed in 1891 to meet Mumbai’s drinking water requirements. The scheme was implemented on a tributary of the Mithi River as a measure to address anticipated water shortages. However, in 1893, the lake was declared unfit for drinking due to pollution from sewage and silt. Since then, its use has been restricted to industrial purposes, cattle sheds, and recreational activities like fish farming and AnglingIt is synonymous with fishing..

Every day, nearly 7 Olympic-sized swimming pools’ (18 million litres) worth of sewage is discharged into Powai Lake ³. This continuous inflow of wastewater into the lake has led to significant nutrient enrichment. This results in eutrophication, which manifests as a green surface cover, attributed to species such as algae, that alter the lake’s ecosystem.

The algae choke the gills of the aquatic organisms. According to a field investigation of Powai Lake conducted by the Maharashtra State Angling Association ⁴, certain species of blue-green algae were identified as key contributors to fish deaths in the lake. In addition, some of the toxins produced by algae can be transmitted through the air, posing a potential hazard to terrestrial fauna and people residing near the lake ⁵.

While algae seems to be a more widely associated species with eutrophication, a significant portion of the apparent green cover is attributed to a floating grass-like species, called ‘hyacinth’. In fact, algae and hyacinth are competing species as they feed on the same nutrient: phosphorus. Hyacinth, being larger, consumes more water and nutrients, outperforming algal growth on the surface.

The problem with hyacinth does not end with mere invasion of the water surface. Studies show that EvapotranspirationIt refers to the combined processes by which water moves from the land surface to the atmosphere through evaporation and transpiration. from water hyacinth mats is nearly four times that from open water surfaces, resulting in lower water levels ¹¹. The mats and the complex network of roots trap sediments and silt ¹². As they accumulate over time, they can potentially reduce the lake’s depth. According to a report published by the Maharashtra Engineering Research Institute, the water-holding capacity of Powai Lake has decreased by 30%*. In the future, this reduction may continue to reduce the rate of groundwater replenishment, worsening the already existing water scarcity ¹³.

Apart from the disappearance of water, the survival of aquatic life is further threatened by the lake’s suffocated state. The decomposition of the aforementioned eutrophic species utilises a lot of oxygen. According to a 2021 study ¹⁴, the DO Dissolved Oxygen (DO) refers to the amount of free oxygen available in the water. It is measured in mg/L. in the lake was found to be as low as 50% of the minimum levels required for the survival of aquatic life. This is exacerbated further when the floating green sheet blocks access to both air and sunlight for underwater species. Consequently, it is evident that eutrophic species lead to a reduction in both water quality and quantity, having far-reaching implications.

Gates 17 and 18, located in front of Powai Plaza, are closed drains that open within the IIT premises and outflow directly into the Lake. Two-lined drains from the Perubaug area and slum pockets serve as inlets to untreated sewage, contributing roughly 2 MLDMLD stands for ‘Million Litres per Day’. It is used to quantify the capacity of sewage influx in the given context. of waste. Ongoing metro construction has also damaged the existing sewer line, diverting more flow into the Lake.

The lake, like any other ecosystem, should strive to attain an equilibrium state, where populations of species are relatively constant in number and do not undergo abrupt changes in short time periods. However, under current conditions, this equilibrium will correspond to a polluted state rather than a healthy one, characterised by the presence of species that thrive in nutrient-rich waters. Bacterial contamination in Powai Lake, measured by CFUCFU (Colony Forming Unit) is a measurement used to estimate the number oof counts, is nearly 25 times higher than the safe limit for recreational use.

To quantify a lake’s water quality, certain parameters are determined through standard laboratory experiments, and their values are then compared with those set for a healthy water body. The following data points ¹⁵ reflect the poor water quality of the Lake, marked by high eutrophication.

The values exhibit a significant deviation from those of a healthy lake, providing a clear picture of the imbalance in the ecosystem. The high pH pH is a measure of the acidity or the basicity of a solution. Its value ranges from 0–14, with 7 indicating neutral nature of the solution. A lower value indicates a more acidic solution. value of the water indicates its basic nature, which favours the formation of ammonia from the ammonium ions present in the sewage discharge ¹⁶. This increases the ammonia-nitrogen toxicity on fish by a factor of 10, as ammonia can easily permeate through biological membranes and lead to fatal alterations in the organism’s biochemical systems. The ecosystem’s severe condition is further aggravated by low oxygen levels in the lake, as most of the oxygen is consumed during the decomposition of organic and inorganic matter. The large BOD Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen that bacteria consume while decomposing organic matter under aerobic conditions in water. A higher BOD indicates a higher organic load in the water, meaning dissolved oxygen is depleted as it is consumed during decomposition. and COD Chemical Oxygen Demand (COD) is a measure of the total quantity of oxygen required to oxidise all the organic matter into carbon dioxide and water. Unlike BOD, it does not distinguish between biologically available and inert organic matter. values indicate that this oxygen demand, or load, in the water body is extremely high. Overall, the CFU value confirms the presence of sewage contamination in the lake ¹⁷.

To understand the changes that have occurred in the lake over the past 10 years, Insight used Geographic Information System (GIS)GIS (Geographic Information System) is a framework for capturing, storing, processing, analysing, and visualising geographical data using computer-based tools.. This helped us analyse and correlate potential causes of the lake’s degradation. The satellite imagery (from Sentinel-2) was analyzed using remote sensingRemote sensing is the process of detecting and analysing the physical characteristics of an area by obtaining relevant information from a distance, typically via satellites. techniques and later validated through ground truthingIt is used to assess the accuracy of remote sensing data by comparing it with ground-level measurements. It thus involves collecting on-site data through sensors or in-situ measurements..

A clear seasonal pattern also emerges in growth trends. Winter, from December to February, is characterised by stable, clear water that supports robust algal blooms and intense plant growth. As temperatures rise during the pre-monsoon months of March to May, nutrient availability declines and growth slows, reaching its lowest levels in peak summer. The monsoon season, from June to August, brings heavy rainfall that increases dilution and turbidityTurbidity is a measure of the extent to which the water loses its transparency due to the presence of suspended particles. It is measured in NTU (Nephelometric Turbidity Units). A higher turbidity value implies a greater reduction in water clarity., suppressing algal activity. In the post-monsoon period from September to November, nutrient inflow continues while water levels decline, thereby increasing nutrient concentration. These conditions, hence, favour the development of fresh blooms. Overall, the growth of eutrophic species peaks in the winter and post-monsoon periods, and declines during the summer and monsoon months. This seasonal change helps to assess the nutrient enrichment pattern and model the hyacinth growth throughout the year. Furthermore, a seasonal pattern is also visible to a daily visitor at the lakeside.

The ecological damage, though it may start with one species, has a cascading effect on the entire ecosystem. Due to critically low levels of dissolved oxygen in the lake water and the presence of toxic algae, fish mortality is abnormally high. Presently, the actual fish yield in the lake is about a quarter of the potential yield ¹⁸. This has an adverse effect on the populations of animals that depend on them for their nutrition, either directly or indirectly. One such visible effect is on the bird population ¹⁹.

Additionally, the situation is particularly critical for the lake’s Marsh crocodile population, numbering around atleast 18⁴¹ as of 2022. The species has been classified as “vulnerable” and is protected under Schedule I of the Wildlife Protection Act. Furthermore, according to a study, pollutants from the lake also seep into the surrounding groundwater, affecting soil conditions and inhibiting the growth of nearby trees. Taken together, eutrophication in Powai Lake acts as a clear indicator of ecological stress, contributing significantly to the ongoing loss of local biodiversity.

The residue of glyphosate may remain in water and is toxic to aquatic organisms. Furthermore, merely killing the hyacinths does not significantly reduce the removal costs, since the quantity to be removed remains the same ²².

After the removal process, the hyacinths are dumped into low-lying areas in villages for disposal through BMC’s authorised contractors. However, attributed to its properties, there is significant potential for the utilisation of the biomass post-removal. Water hyacinth has been studied for its nutritional value and potential use as feed for livestock, poultry, and fish ²³. Its high cellulose, hemicellulose, and crude protein content make it suitable as a substitute or additive for animal feeds. Another use of hyacinth is conversion into biochar (through drying) ²⁴, which serves as a supplement to improve soil quality. Hyacinth, with its porous structure, enhances soil water retention and can be utilised in drought-prone areas. Dried hyacinth is also used as a raw material to make artefacts such as bags, flower pots, and mats.* In fact, there are Indian manufacturers in this sector ²⁵, like North Eastern Development Finance Corporation Limited, and some local enterprises like Ravi Exports in Andheri; it is a supplier of notebooks made from water hyacinth.

Anaerobic digestion and briquette production from biomass can be employed to produce biogas, which can then be used for energy production. Biofertilisers can also be produced from hyacinth through mulching, composting, vermicomposting, and anaerobic digestion.* Microbial populations within the hyacinth facilitate the production of phytohormone, a hormone that supports plant growth ²⁶.

For areas larger than 5 acres, sediment basins are constructed to collect silts, metals, and separate water, then discharge it into a water body. The outlet is elevated to allow runoff to settle, and then the boundary is covered with various filters, depending on the purpose, such as metal extraction or chemical adsorption. After sediment basins are constructed, regular maintenance and inspection, along with adherence to specific construction specifications, are required to keep the basins operational ²⁸.

In constructed wetlands, plants and their associated rhizospheric microorganisms work together to remove pollutants from wastewater. Beyond improving water quality, these wetlands can also enhance the lakefront by providing additional green space along the landward side, contributing both to ecological restoration and urban amenity ³⁰.

However, studies ³¹ show that the lakeside construction of wetlands needs to be critically evaluated. A major drawback of constructed wetlands is that they require a large area. Constructing them at the lake’s boundary might affect the original area of Powai Lake, potentially turning the lake into a wetland. When discussed with Prof. Sanjeev Chaudhari from the ESED Department, he also shared a similar opinion regarding engineered wetlands.

Plantation of Indigenous Trees

According to a report published by Woodland Trust ³², the UK’s largest woodland conservation charity, plantation of trees can help reduce the influx of pollutants into the water body. The woodland buffers slow down water runoff and intercept sediments and nutrients, such as phosphates and nitrates, which are primarily responsible for eutrophication. Hence, the lake’s rehabilitation can be supported by planting certain tree species, along with indigenous grasses, which are already common in the forests surrounding the lake ³³.

In addition to some of the aforementioned measures, the report released by the Joint Committee ³⁴ proposed a sewerage diversion network to redirect sewage away from the lake’s 19 CulvertsA culvert is a tunnel that serves as an open drain under a road, railway track, etc. to existing or new treatment facilities. The plan involves laying 3,056 metres of sewer lines using both open-cut and Horizontal Directional Drilling (HDD) methods to minimise surface disruption.

In the Kuttanad region of Kerala, hyacinth invasion has extended to nearly 90% of the surface area of water bodies, rendering people’s lives extremely difficult. There has been a significant decline in native fish species, with more than 70% of the surveyed fishermen reporting reductions in fish catch ³⁵. Evidently, damage to the ecosystem incurs a high cost, both hidden and direct. If the eutrophication in Powai Lake is not addressed, these losses may intensify, just as happened in Kuttanad.

Alumni from the 1980 batch have launched an initiative to revitalise the lake, as a part of which they transformed the area behind the Convocation Hall lawns into a garden with natural trails. This garden is now known as Kshitij Udyan. As part of the subsequent phase of the project, the Alumni Association is coordinating with BMC to carry out desilting and cleaning of the water body ³⁶. Currently, Mr Kunnath has been leading the Powai Lake Rejuvenation Forum (PLRF) as the head of the Mumbai Chapters, with the ultimate goal of building a constructed wetland at the lake’s shore ³⁷. However, the implementation of these projects has been limited due to the scale required, as the efforts and funds were insufficient.

In response to a complaint filed by the NGO NatConnect Foundation regarding the compromised condition of the lake, the Ministry of Environment, Forests, and Climate Change has issued an order to the Maharashtra State Wetland Authority (SWA) to take the necessary actions to declare Powai Lake as a Ramsar Wetland ³⁸. This would enable the Lake to be recognised under the Ramsar Convention ³⁹, which is an international treaty for the adoption of sustainable practices regarding wetlands.

The need of the hour is for policy-makers and government stakeholders to adopt a more conscious and sensitive mindset toward the choked state of the lake. If there is collective effort in the right direction, a healthy equilibrium state can be restored in the body, as seen in the case of the Great Lakes of Canada ⁴⁰.