Delhi is facing the risk of land subsidence. And uncontrolled and illegal groundwater extraction is to blame!

What is land subsidence

Land subsidence is a gradual settling or sudden sinking of the earth's surface due to removal or displacement of subsurface earth materials. The principal causes include:

• Compaction of aquifer-systems due to extensive groundwater withdrawals
• Drainage of organic soils
• Underground mining of minerals, oil and gas
• Natural compaction or collapse, such as with sinkholes or thawing permafrost, earthquakes

Land subsidence is causing more and more damage every year informs this paper titled 'Tracking hidden crisis in India’s capital from space: implications of unsustainable groundwater use' published in Nature Scientific Reports. More than 80 percent of land subsidence across the world is due to excessive groundwater extraction which increases inter granular stress and causes rearrangement of soil particles resulting in aquifer compaction and eventual land subsidence.

Groundwater extraction rates continue to increase in India to meet the drinking water, agricultural and industrial needs of the growing population. Delhi National Capital Region (NCR), is one of the fastest-growing metropolitan cities and as high as 6,25,000 households do not have access to piped water supply. People have to rely on groundwater or private tankers for their daily water needs.

At some areas in southwest Delhi, the groundwater tables have plummeted to 80 m below ground and continue to fall at the rate of 3–4 m/year. The Central Ground Water Authority (CGWA) has introduced ‘Water Conservation Fees (WCF)’ for groundwater extraction for domestic and industrial purposes. The penalty varies depending on the amount of extraction and the exploited zone, but exempts individual households and agricultural users, which does not help in preventing excessive extraction of groundwater.  The current policies only focus on mitigating the water scarcity problem in Delhi, but do not take into account the hazardous effects of land subsidence.

The paper discusses the findings of a study that explored the extent of land subsidence between 2014 and 2020 in Kapashera, Faridabad and Dwarka areas in Delhi.  

The study found that:

High land subsidence rates found in Kapashera and Faridabad

The largest subsidence was found in Kapashera area along southwest Delhi that shares its borders with Gurgaon. The rate of subsidence was found to be increasing with the subsidence velocity found to be 11 cm/year during the years 2014-2016, which increased by almost 50 percent in the next two years to around 17 cm/year by 2016-2018. The trend remained almost same during 2018–2019.

In addition, a few slow subsidence zones were found in the areas such as Mahipalpur village, Bijwasan Harijan Basti; Sanjay gram colony; chack Sadhu and Nathupur, Delhi. Most of these areas were subsiding between 15–40 mm/year and expanding gradually towards the IGI Airport Delhi, causing a threat to the structure.

In Faridabad, the industrial capital of Haryana and one of the fastest growing cities in the  world, data showed an accelerating rate of subsidence throughout the study period. From 2014 to 2016, the maximum subsidence rate was relatively low, around 2.15 cm/year and the spatial extent was small. However, during subsequent years, the deformation rates surged dramatically, to an approximate rate of 5.3 cm/year by the end of 2018 and 7.83 cm/year for the year 2018–2019.

The extent of subsidence increased continuously with time, which could be attributed to deepening groundwater levels, and the high rate of groundwater extraction. The primary reason for the plummeting groundwater levels was illegal pumping through more than 100,000 illegal connections in the city. The area that was the most affected was New Industrial Town (NIT) Faridabad.

Land subsidence shows a decreasing trend in Dwarka

Dwarka presented a different picture with the region showing subsidence at a rate of 3.5 cm/year in 2014-2016, which changed leading to the gradual uplift in 2016–2018 and 2018–2020. The main reason for this change was the swelling of the soil due to the rise in groundwater table and the consequent reduction of effective stress in the soil.

This positive change could have to do with Delhi government introducing strict policies to improve the groundwater condition in the area. For example, Delhi Jal Board (DJB) in July 2016, made it compulsory to install a rainwater harvesting system for people who owned 500 square meters to as an alternative to groundwater extraction and penalize illegal pumping by imposing heavy fines. In addition to this, residents of Dwarka revived a 200-year-old water body named ‘Naya Jhod’ of 10 million storage capacity in 2015, which not only reduced water demand but also acted as a very good source of groundwater recharge.

Subsidence can harm

The paper informs that subsidence can lead to subtle changes in the land gradient affecting storm drainage or sewer lines that flow under gravity. In many parts of Delhi NCR, sewer lines, water mains, etc are laid beneath the roads due to the shortage of space. Whenever there is damage to these utilities, the water seeps in making the lower soil layer soft and after some time, the roads cave in.

The best example is that of the Old Delhi-Gurgaon Road, a 7.5 km-long stretch linking Delhi and Gurgaon - a very busy road full of cracks and potholes, which has subsided by more than 70 cm in the past five years. The major effect of  subsidence is the increased risk of flooding and more frequent rainfall-induced floods waterlogging. Waterlogging has been observed to be increasing in Delhi and Faridabad following rains in recent years.

The north-west and south-eastern parts of Delhi having a high population density show a high subsidence gradient as compared to other areas and high groundwater extraction has been found to be the primary cause of land subsidence. A total area of approximately 100 km² is found to be at high risk of ground displacement. It includes Bijwasan, Samlkha, Kapashera, Sadh Nagar, Bindapur and Mahavir enclave from Delhi; Dundahera, Sector 22A, and Block C from Gurgaon; and Pocket A, B, C of Sanjay Gandhi Memorial Nagar from Faridabad.

Most of these locations have a high population density and do not access to piped water leading to a gap of 750 million litres a day between demand and supply. Illegal extraction of groundwater is rampant. Delhi receives an average annual rainfall of 611 mm, mostly in July, August, and September. Harvesting rainwater will not only bridge the gap between demand and supply but will also replenish the falling groundwater levels that can help in curbing the increasing rate of land subsidence and the risks associated with it, argues the paper.