Percolation pits push up Lalbagh groundwater by 20 feet

The method has helped overcome the water shortage problem faced in the summer

Percolation pits at Lalbagh to harvest rainwater and recharge groundwater. Credit: DH Photo/ S K Dinesh

Lalbagh Botanical Garden, the premier lung space in the city, has overcome a potentially serious water problem ahead of the summer. The Horticulture Department has used an ingenious method of rainwater harvesting to water the thousands of plants in the park.

Lalbagh is located on a rocky patch of what is called the Bangalore plateau in geologic parlance. In early 2020, its groundwater table fell to 30 feet, setting alarm bells ringing among authorities. The Horticulture Department came up with an ingenious solution: over the last eight months, it dug 209 rainwater percolation pits of different sizes across the park. The method not only ensured water security but also raised the water table by about 20 feet (about six metres).

A percolation pit is nothing but a hole dug into the ground. It helps draw water downward through the soil, recharging groundwater.

The success has prompted the officials to decide digging another 500 percolation pits at Lalbagh as well as a few in Cubbon Park, another premier lung space in the city.

B Fouzia Tarannum, Director, Department of Horticulture, said the pits would raise the underground water table which would, in turn, ensure adequate water supply to trees and shrubs with shorter roots. The pits have been dug scientifically in order to check waterlogging and soil erosion during heavy rainfall, she added.

Nonprofit United Way of Bengaluru and German multinational Bosch funded 129 pits under the CSR initiative.

While the NGO and the multinational dug 18-foot-deep pits, the ones bored by the Horticulture Department have a depth of 12 feet but are a little wider. The 18-foot-deep pits can hold about 3,600 litres of water from just one day of rain. The capacity of 12-foot-deep pits is about 4,268 litres due to the wider diameter. In all, the 209 pits can help percolate 2,42,77,000 litres of water downward during just 30 days of rainfall in a year, an official in the Horticulture Department explained.

Digging an 18-foot-deep pit costs about Rs 38,000 and a 12-foot-deep pit Rs 35,000. The expenses include soil excavation, concrete rings, iron meshes and covering slabs.

Geologists have found that the groundwater table at Lalbagh has now improved from 25-30 feet to less than 10 feet. Back in May, the groundwater was available at a depth of 7.4 metres (about 25 feet). In November, it was available at a depth of just 1.8 metres (about six feet).

From an average of six metres eight months ago, the groundwater is now available at an average depth of about one metre due to effective percolation through pits, officials said.