` Land sub­si­dence is im­mi­nent'

Ge­ol­o­gist Pradip Sik­dar on dan­gers of overex­trac­tion of ground­wa­ter in In­dia

Down to Earth - - CONTENTS -

What is the relation be­tween land sub­si­dence and ground­wa­ter ex­trac­tion? Land sub­si­dence gen­er­ally oc­curs when ground­wa­ter is mined in an un­planned way. The im­pact is more ev­i­dent in rocks made of fine-grained sed­i­ments. De­cline of ground­wa­ter ta­ble causes a ver­ti­cal com­pres­sion of sed­i­ments bear­ing the wa­ter. Some­times, lat­eral com­pres­sion may also take place along with this ver­ti­cal com­pres­sion. Low­er­ing of the pore wa­ter pres­sure in a layer re­sults in an in­crease of the ef­fec­tive stress in the soil, re­sult­ing in con­sol­i­da­tion of the soil which man­i­fests as land sub­si­dence. What are the ef­fects of land sub­si­dence? The ef­fects can be set­tle­ment of up­per clay layer lead­ing to dam­age of in­fra­struc­ture (roads, bridges) and flood­ing due to in­ef­fec­tive drainage sys­tems of the city. Lines of weak­ness in the lay­ers can also be ac­ti­vated and this may cause earth­quake in the area. There can be in­di­rect ef­fects such as a change in gra­di­ent of streams or drains. Is land sub­si­dence a se­ri­ous is­sue? Land sub­si­dence is an ir­re­versible process.It can be very se­ri­ous.In China the av­er­age to­tal eco­nomic loss due to sub­si­dence is es­ti­mated at around US $1.5 bil­lion per year, of which 80-90 per cent are in­di­rect losses.In Bangkok many pri­vate and pub­lic build­ings, and un­der­ground in­fra­struc­ture are se­verely dam­aged by sub­si­dence. In 2006, the to­tal cost of sub­si­dence-re­lated dam­age, es­pe­cially in sub­si­dence-prone ar­eas, in the Nether­lands was es­ti­mated at over €3.5 bil­lion per year (`276 bil­lion at cur­rent rate). The US Ge­o­log­i­cal Survey also re­cently re­leased a re­port show­ing that ex­ten­sive ground­wa­ter pump­ing is caus­ing land sub­si­dence in Cal­i­for­nia. You were part of a study done in Kolkata on sub­si­dence. What did the study show? The area be­tween Kasba, Gari­a­hat and Dhakuria in south Kolkata has shown land sub­si­dence.The es­ti­mated mean sub­si­dence rate in the city, ac­cord­ing to a re­cent study, is 13.53 mm/year. There were few in­ci­dents of tilt­ing of build­ings. This sub­si­dence oc­curred due to a de­cline of ground­wa­ter. For ev­ery me­tre drop in ground­wa­ter level, the mean sub­si­dence is 3.28 cm. Another study, pub­lished in 2013,an­a­lysed land sub­si­dence be­tween 1956 and 2000 at Ul­tadanga, a lo­cal­ity in east Kolkata, and found that the area showed a sub­si­dence rate of 18.23 mm/ year due to a de­cline of 9 m of ground­wa­ter level.The stud­ies sug­gested that ex­trac­tion of ground­wa­ter should be car­ried out based on the ground­wa­ter po­ten­tial and the per­ceived threat of sub­si­dence in the area. The In­dian In­sti­tute of Re­mote Sens­ing re­cently signed an MoU with the Cen­tral Ground­wa­ter Board to study the ef­fect of land sub­si­dence in north­ern In­dia. Why is this study needed? North­ern In­dia com­prises plains and hilly ar­eas. Ma­jor rivers like the Ganga and its trib­u­taries criss-cross the area and de­posit fresh al­lu­vium on the north­ern plains.

ac­ti­vate fault lines and cause earth­quake in the area. There can also be in­di­rect ef­fects, such as a change in gra­di­ent of

Ur­ban­i­sa­tion and in­dus­tri­al­i­sa­tion in th­ese ar­eas are caus­ing a rapid de­cline in ground­wa­ter. Un­planned ex­trac­tion of ground­wa­ter from soft, fresh al­lu­vium un­der­ly­ing the north­ern plains can lead to land sub­si­dence. This means ci­ties like Lucknow, Agra, Mathura, Kan­pur, Al­la­habad, Varanasi, which are on the Gangetic plain, are prone to land sub­si­dence. A study done in 2009 in Lucknow had sug­gested that if the city con­tin­ued ground­wa­ter ex­trac­tion at present rate, there will be land sub­si­dence in 2026. The study was con­ducted by a re­tired sci­en­tist from the Cen­tral Ground Wa­ter Board (cgwb).

Till date there has been no ma­jor in­ci­dent of land sub­si­dence in north In­dia. Land sub­si­dence can­not al­ways be no­ticed as mostly the im­pact is spread over a large area. But ground­wa­ter ex­trac­tion from a sim­i­lar soil in Kolkata has shown land sub­si­dence and there is a high prob­a­bil­ity that land sub­si­dence will take place in north In­dia due to un­mind­ful min­ing of ground­wa­ter.

Ex­trac­tion of ground­wa­ter from ar­eas prone to land sub­si­dence may also cause ad­verse en­vi­ron­men­tal im­pact on the re­gion’s ecosys­tem. There may be a loss of wet­lands.It is im­per­a­tive to es­ti­mate the pos­si­ble rate of land sub­si­dence and chalk out a sus­tain­able ground­wa­ter man­age­ment plan based on ground­wa­ter po­ten­tial and es­ti­mated rate of land sub­si­dence of dif­fer­ent ar­eas, es­pe­cially in a frag­ile wet­land to min­imise the ad­verse en­vi­ron­men­tal im­pacts of ground­wa­ter de­vel­op­ment. The ci­ties of north In­dia may have lost their wet­lands due to ur­ban­i­sa­tion, but the Gangetic plains are dot­ted with ponds, marshes and swamps (oc­cur­ring in the peri-ur­ban and ru­ral ar­eas) which may be af­fected by land sub­si­dence. How is land sub­si­dence stud­ied? For the Kolkata study, we col­lected data on ground­wa­ter from pub­lished jour­nals, cgwb and wa­ter level data mea­sured by my­self and my stu­dents in wells in Kolkata and ad­join­ing East Kolkata Wet­land. The sub­si­dence of land was then cal­cu­lated us­ing dif­fer­ent math­e­mat­i­cal mod­els. But land sub­si­dence can be stud­ied more ac­cu­rately us­ing re­mote sens­ing. How will the study in north­ern In­dia help the peo­ple? The study will help cgwb to pre­pare a ground­wa­ter man­age­ment pol­icy. Strin­gent laws can also be im­posed by the state ground­wa­ter boards on the in­dus­tries which are the wa­ter guz­zlers.

In Bangkok, ex­treme land sub­si­dence by ground­wa­ter ex­trac­tion was suc­cess­fully re­duced by reg­u­la­tions and re­stric­tions. A spe­cific law (Ground­wa­ter Act) was en­acted in 1977. Most se­verely af­fected ar­eas were des­ig­nated as crit­i­cal zones, with more con­trol over pri­vate and pub­lic ground­wa­ter ac­tiv­i­ties. Ground­wa­ter use charges were first im­ple­mented in 1985 and grad­u­ally in­creased. Cur­rently, Bangkok uses a very small amount of ground­wa­ter. Only 10 per cent of wa­ter is sourced from the ground. The rate of sub­si­dence has gone dras­ti­cally down.


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