The water crisis in Chennai has been compounded by the rock bottom ground water level. With the water level in the reservoirs almost ground zero, Chennai depends heavily on groundwater.
Water for the state is managed through 19 lakh deep borewells, 30 per cent of which has already gone dry. Drinking water from 27 per cent of these wells have become brackish. In the rest of the wells, water can be drawn only for four to six hours.
The advent of deep borewells has brought down the cultivable land under tank irrigation to 10 lakh acres in 2018, from 22.5 lakh acres in 1960 and 15.75 lakh acres in 2000.
Groundwater levels are alarmingly less not just in Chennai but in other districts of Tamil Nadu as well.
In an effort to conserve rain water, Tamil Nadu enacted a law in October 2002 and followed up with an ordinance in June 2003, making rain water harvesting compulsory in all existing buildings on or before October 11, 2003, the first state in the country to enact such a law.
As Chennai is dependent on groundwater for 60 per cent of its water needs, RWH was made mandatory in both old and new buildings.
According to Metrowater officials, “In Chennai, when RWH structures were first implemented, groundwater level was at 8 metres. The RWH structures prevented groundwater levels to dip below 8 metres until now.”
There are nearly 8,20,000 RWH units in Chennai.
The most widely used RWH structure is the recharge wells because it has high water retention. It is also easy to maintain as it needs to be cleaned only once in three years. The RWH units perform at 100% efficiency in the sandy coastal areas of Chennai such as Adyar, Coouam and Neelangarai. In clay soil areas such as Anna Nagar, they have 80 per cent efficiency, he added.
During the South-West monsoon season, groundwater retention is at 100 per cent but during the North-East monsoon season the retention level is only 70 per cent because the water table is already high in this season. “Since rainfall in Chennai has been minimal in the last two years, groundwater levels are low,” the official said.
The average ground water level in metres below ground level has seen a steady increase from 2010 to 2019. This is an indicator of rapid ground water depletion in Tamil Nadu.
According to A Veerappan, ex- Special Chief Engineer, Tamil Nadu Public Works Department, “Around 200 MLD water is supplied from desalination plants and around 200 MLD of water is supplied from agricultural wells and quarries. In most parts of Chennai, water at bore wells has gone below 50 feet. Due to minimal rainfall in Chennai, rain water harvesting systems could not retain enough water and the groundwater levels are low.”
Recharge wells are mostly 15 feet deep. Cost of recharge well varies depending on the diameter. A three feet recharge well costs Rs 21,000, four feet costs Rs 32,000 and the five feet costs Rs 47,000.
Director of the NGO, Rain Center, Dr Sekar Raghavan, who was part of the high level advisory committee which was instrumental in legislating RWH says, “people are not aware of RWH systems and only 40 to 50 per cent of RWH systems are properly installed. Most of them are not properly monitored and maintained. Even then, it helped in increasing the ground water levels. According to an impact study performed in 2005 after a heavy rainfall in Chennai, the groundwater levels saw a six-metre rise.
Water levels increased in 39 temple tanks in Chennai. It happens only when groundwater levels increased. The NE monsoon failed only in 2018. We could easily overcome a year of monsoon failure, if RWH systems are properly installed and monitored in the city.
According to a data provided by the Rain Center, as of March 2019 groundwater monitoring wells in Anna Nagar, Choolamadu, Parthasarathy Temple, Vadapalani, Mylapore, Ashok Nagar, Nesappakkam, Shastri Nagar and Adyar Cancer Institute have dried up. In areas near Presidency College, the water level stood at 11.75m and in areas such as Valasaravakkam and Virugambakkam it is at nine metres.
As the shallow water table is drying up fast, many open wells in Chennai have dried out. They are closed and replaced by bore wells. Bore wells are designed to get water which is locked beneath the rock, deep inside the ground. Bore wells are dug very deep into the ground but the deeper we go, the quality of water will be low, with iron and increasing levels of total dissolved salts in the water. They also will go dry if not replenished. This can be done with slotted casing pipes in the bore wells, which allows water to seep into the ground. But most of the bore wells have only plain casings.
In order to quench Chennai's thirst, the government is planning to build two additional desalination plants, one with a capacity of 450 million litres per day (MLD) and another with a capacity of 100 MLD. Even though officials claim that the desalination plants would be built by adhering to State and Central government norm, these plants come with a price.
According to the latest UN backed study, “The state of desalination and brine production: A global outlook”, desalination technologies produce hyper salty (termed ‘brine’) water which is associated with negative environmental impact. The authors cite major risks to ocean life and marine ecosystems posed by brine greatly raising the salinity of the oceans, and by polluting them with toxic chemicals used as anti-scalants and anti-foulants in the desalination process (copper and chlorine are of major concern). Also, brine water discharge decreases the dissolved oxygen levels causing dead zones in the oceans, where aquatic life cannot thrive.
Says Dr Sekar Raghavan, “Our existing natural fresh water sources are like a bank and if we keep withdrawing water without making deposits, they will run out of water one day. We cannot just depend on desalination plants for water. A Cape Town like situation might not be imminent in Chennai right now but it can be a possibility in five years. People should be conscious about saving water not because of any law but for themselves. Then they can overcome water problems.” As the demand for water in Chennai keeps increasing, it is only a matter of time before Chennai faces a huge water crisis.
- Water needs for the state is managed through 19 lakh deep bore wells, 30 per cent of which has already gone dry
- There are nearly 8,20,000 RWH units in Chennai. These structures prevent groundwater levels to dip below eight metres until now
- Recharge wells are mostly 15 feet deep. Cost of recharge well varies depending on the diameter. A three feet recharge well costs Rs 21,000, four feet costs Rs 32,000 and the five feet costs Rs 47,000
- Rain water harvesting needs to be better managed and monitored