Path to sustainability

Bengaluru mega city, with an extent of 800 square kilometres and population of around 10 million, is located on a basin divide forming a part of the Ponnaiyar river catchment (435 sq km ) in the east and Arkavathi river (365 sq km) catchment of Cauvery Basin in the west. Its peri-urban activities spread beyond 30-35 km from its peripheral parts and the 300-350 sq km green belt around it has virtually disappeared.

The city has no water resource of its own, except the 66,400 hectare metre (23.45 thousand million cubic feet, TMC) rainwater it receives when there is normal annual monsoon rainfall of 830 millimetres.

Of this, nearly 17,500 hectare metre (6.18 TMC) or 25 per cent of annual rainfall is the precious surface run-off that flows through the stormwater drains called rajakaluves. These have now become the perennial flow path for untreated and / or partially treated urban wastewater / sewage / and other disposals.

The natural topographic features and the drainage characteristics were once congenial for uninterrupted flow of stormwater. The effects of urbanisation have played a devil’s role in the city! Disposal of urban waste pollutes the groundwater, surface water and atmosphere. Original stormwater drains have been tampered, encroached and grossly reduced both in width and depth. Many of the 1st and 2nd order streams have disappeared on ground.

The centuries old lakes with cascading effect constructed by the erstwhile rulers across the natural stormwater drains to conserve water, serve the irrigation and domestic needs during lean season and as groundwater recharge structures have now become the repositories of untreated and / or partially treated urban sewage / wastewater discharges.

Bengaluru is no more a city of those highly glorified lakes. Small water bodies have vanished. Many medium and large lakes have been irretrievably reduced in extent due to human encroachments and solid waste dumping. The potential lakes are heavily silted. Original designed reservoir capacity of each of the existing lakes have been highly reduced.

The natural landscapes are altered beyond anybody’s imagination! There are no old spatial gardened villas. The most desired avenues have disappeared. No adequate space is left for air to circulate. The apartment complexes and multi storeyed buildings have come up along the length and breadth of the city. In the Ponnaiyar catchment, the mushrooming of apartment complexes and high towered buildings are with no assured water source during the lean season. Dry weather persists for longer than before.

There is a strong contrast in the geomorphological, geological, geo-hydrological, pedological and even climatological features between the Vrishabhavathi stream system of Arkavathi catchment in the west and the dominant Hebbal and Koramangala-Challaghatta (KC) valley systems of Ponnaiyar river catchment in the east in Bengaluru.

While the structurally trained steep to moderate rock cut course of the Vrishabhavathi channel system with its prime tributary ‘Nagarbhavi-Thorai’ flows southerly and south-westerly, the flow systems in the predominant Hebbal and KC valleys of the Ponnaiyar catchment are easterly and gentle over a raised plateau like topography.

The drainage system of Hebbal and KC valleys are characteristically broad, shallow and unstable. The gneissic rocks in the Ponnaiyar catchment, which have undergone intense chemical weathering since the geological past are altered to clayey ‘saprolite’ to a thickness of about 25-30 metres from the land surface at 885 metres above the mean sea level.

The concrete urban cover followed by clayey laterite soil of 2-3 metre thickness over thick clayey saprolite have all together prevented water to infiltrate. Further, unaltered part of the parent rock between impervious clayey saprolite above and the massive rock body at around 300 metres below the surface, form a confined zone, largely with no rainwater infiltration.

The water, whatever may have been tapped through borewells in that part, is accumulated before and during the process of chemical weathering. This ‘long-stayed static water resource’ under ‘confined conditions’, which has been extracted continuously over the past two decades through countless borewells, has either become dry or is on the verge of becoming dry.

Many builders and land-owners, not looking into the ground condition of such places, have got borewells drilled even beyond 300-350 metres. They have not struck water or the little volume they managed to tap into was short lived.

In contrast to the Ponnaiyar part of the catchment, the hydrogeological characteristics of the gneissic aquifers between the right bank of the south-flowing Vrishabhavathi River and the western fringe of intrusive granite, has been a better water-yielding zone. But the continued indiscriminate extraction of groundwater resources beyond the annual recharge over a period has made the groundwater ‘not a sustainable resource’.

The precious stormwater flow of nearly 17,500 hectare metres of rainwater during the period of normal annual rainfall can serve about 2.5 million people at 140 litres / head / day, if action is taken to protect from pollution and degradation, and used judiciously.

The stormwater drains, which are now the perennial flow path for sewage and other urban discharges, cannot now sustain the stormwater load of even short durations. But the scenario is such that during rainy days, stormwater, along with the urban sewage and other discharges, flood the valley zones / low-lying areas, and the buildings or any other structure present there.

Bengaluru is susceptible to the vagaries of rainfall. Rainfall has been erratic in recent years. El Nino has been a matter of concern to the people and the administrators concerned.

The weather phenomenon may be recurrent and people may feel the pinch of water scarcity in each El Nino year. El Nino sometimes go on for 4-5 successive years and may result in a drought.

More than a decade ago, hydrogeologists KC Subhash Chandra and GV Hegde, after an intensive hydrogeological study, had made the following suggestions to the authorities in 2011, 2012, 2014 and 2015:

✼ Prevent and plug the enroute leakage and transmission loss of 30-40 per cent of Cauvery River water getting pumped to Bengaluru for meeting the city water requirement;

✼ Conserve, protect and free from pollution the precious storm water of 17,500 hectare metre (6.18TMC) of Bengaluru;

✼ Effectively implement rooftop rainwater harvesting in all the houses and buildings, which exceed 100 square metres of roof top; and

✼ Treat 70 per cent of the sewage / wastewater generated in the city up to potability level after tertiary-level treatment.

The cumulative impact of these measures would meet the water requirement of 14 million people in Bengaluru, without any dependence on groundwater. Had these suggestions been discussed at an appropriate level and measures taken for implementation, probably there would not have been any cry for water scarcity as now being largely heard!

Immediate solutions

People are feeling the brunt of summer from mid-February itself. Both the southwest and northeast monsoons failed during the year 2023. Each year, when summer months approach, there is naturally a public outcry for water.

But the authority concerned investigates to overcome the problem only through certain temporary measures like supplying water through tankers! The quality of water needs to be evidently tested when the source is not known! Ironically, while the groundwater source in the city aquifers has already hit the rock bottom, suggestions from some corners are to drill fresh public borewells!

The authorities are trying to overcome the water scarcity on a war-footing, more so now that the posh apartment complexes in the Ponnaiyar catchment part of the city are facing the brunt. These neighbourhoods rely mainly on groundwater from the borewells deeper than 300 metres, which have become defunct or are on the verge of becoming unusable.

The authorities shoulder treat the only available source of water whatever stored in the major lakes like of Ulsoor, Hebbal, Yele Mallappa Shetty, Agara, Bellandur, Vartur, up to tertiary level, bring it on to potable level in a fool-proof manner and supply. Even if people hesitate to use such water for domestic purposes, it can serve other demands.

It is the moral responsibility of every citizen to use the available precious water resources judiciously. When mining of groundwater resources is in full-swing, attempts for groundwater recharge measures remain meaningless.

The city should protect and conserve as much stormwater as possible and adopt rooftop rainwater harvesting. Limitless expansion of the city only aggravates the problem of water availability and resource management. DTE

Views expressed are personal