INTRODUCTION

Drinking water is one of the most important pathways of exposure to arsenic in humanpopulation and groundwater as a drinking source is thought to be responsible for themajority of the world’s chronic arsenic related health problems. Arsenic contamination inground water has emerged as a major quality problem and health hazard in parts of thecountry. In India first reported case of arsenic contaminated ground water was detectedin the year 1978 in West Bengal and the first case of arsenic poisoning there werediagnosed in 1983 by the School of Tropical Medicine (STM) and All India Institute ofHygiene and Public Health (AIIH & PH). It was observed that the people suffering fromarsenical dermatosis , were consuming ground water and when the samples of groundwater of the respective area were chemically analysed, those were found to containarsenic beyond the permissible limit of 0.05 mg/l.

As per the BIS Standard for drinking water (BIS 1991 and subsequent modifications), themaximum permissible limit of Arsenic concentration in ground water is 0.01 mg/l. TheArsenic concentration in ground water >0.05 mg/l, as permissible limit has been taken upbased on recommendation of the task force set up in West Bengal. 0.05 mg/l has beenaccepted as regulatory limit for the country. This was also recommended as permissiblelimit by earlier BIS standard.

OCCURRENCE AND DISTRIBUTION OF HIGH ARSENIC IN GROUND WATER

With emphasis on drinking water related public health hazards and enhancement ofanalytical facility, large-scale analyses of spot sources have been taken up underdepartmental programme by several Central and State Government agencies and byvarious academic institutions and research organizations. Investigations by CentralGround Water Board (CGWB) reveals that arsenic contamination (> 0.05 mg/L) isaffecting the states of West Bengal, Bihar, Uttar Pradesh, Assam, Chhatisgarh. TheBengal Delta Plain (BDP) covering Bangladesh and West Bengal in India is the mostsevere case of groundwater arsenic contamination. Besides this high Arsenic groundwater has also been reported from Jharkhand and Manipur state.

The details of State wise occurrences are as follows:

West Bengal

The occurrence of high Arsenic in ground water was first reported in 1978 in West Bengalin India.In West Bengal, 79 blocks in 8 districts have Arsenic beyond the permissiblelimit of 0.05 mg/l. The most affected areas are on the eastern side of Bhagirathi river inthe districts of Malda, Murshidabad, Nadia, North 24 Parganas and South 24 Parganasand western side of the districts of Howrah, Hugli and Bardhman. Arsenic in groundwater is confined mainly in the aquifers upto 100 m depth. The deeper aquifers are freefrom Arsenic contamination. At present about 162.6 lakh people(35.48% of the totalpopulation of the state) live in the risk zone of potential threat in terms of Arsenic relateddiseases. The details of affected blocks with maximum concentration of Arsenic areproduced in Table-I

Some of the findings of studies taken up by CGWB &
• The top aquifer within 100mbgl is mostly arseniferous. Arsenic free deeperaquifers(below 100 mbgl) are potential with capacity to yield 5-20 litres persecond(lps) and cater to the need of both rural and urban water supply.

• Hydrogeological tests on arseniferous aquifers (within 100 mbgl) have beenconducted in different arsenic infested areas to observe the Arsenic concentrationin ground water consequence to pumping of Arsenic water from the tube well. Theresults indicate that there is not much impact on Arsenic concentration of groundwater due to pumping when the drawdown created remains within 6m (A.Ray,2009)

• Artificial Recharge studies at Ashoknagar, Habra-II block, North 24 Parganasdistrict, West Bengal has revealed that Arsenic concentration of 0.12 mg/l inground water has been diluted to
• A collaborative study between CGWB & BARC, Trombay in parts ofMurshidabad, Nadia, North and South 24 Parganas district of West Bengal usingenvironmental stable isotopes ( 2H, 18O, 34S ) and radio isotopes (3H, 14C)indicated that ground water from the shallow aquifers are mostly from modernrecharge (
• The major finding of the collaborative project between CGWB and United NationsIndustrial Development Organization indicates that Arsenic removal equipmentsare effective in bringing down the concentration of input water to less than 0.01mg/L. In few instances treated water had noticeable Arsenic concentration whichit can mainly be attributed to poor maintenance and monitoring rather than anyinherent weakness in the technology itself.

Bihar

Arsenic groundwater contamination in Bihar was initially detected in the year 2002 fromSemariya-Ojhapatti villages of Bhojpur district. Detailed investigations in the GangeticPlain of Bihar revealed its wide occurrence on both the banks of the river Ganga. Arsenicdistribution is marked with wide spatial variability resulting in patchyness indistribution. Fifty seven blocks, in 15 districts, located on both the banks of Ganga areaffected (CGWB &PHED, 2005). The list of affected blocks of high Arsenic in ground waterin Bihar revealed through survey by CGWB & State agency are given in Table-II

• The patches of high groundwater Arsenic (>0.05 mg/L) zones are confined inNewer Alluvial belt along the river Ganga affecting both the Active and the OlderFlood plains. The contamination, however, is confined within the top ~ 50 m ofthe thick multi-cyclic sand, clay, sandy clay and silty clay sequence.<br /><br />• The Arsenic affected aquifers represent young groundwater. In a Joint Study withBARC, Trombay in Younger alluvial belt of Sone Ganga interfluves area coveringBhojpur district the Tritium (3H) concentration of the shallow groundwater(generally 3.42-10.13 TU) reveals a substantial component of modern rechargeand the age has been estimated as less than 40 years. Based on 14Cconcentrations, the age of the groundwater from deeper low-Arsenic aquifers havebeen worked out as ~3000 years. The older age supports less permeable nature ofthe middle clay, holding the deeper aquifers under semi-confined to confinedcondition”. (CGWB &amp; BARC, 2009).<br /><br />• The studies in Bhojpur, Buxar and Patna district (GSI, 2008) reveal that therecould be broadly three separate depth zones, (i) down to 50ft (15.24m) from thesurface (Arsenic content in ground water mostly 500ppb approx. and mostly with high incidences of Fe in localized pockets),(iii) 200ft (60.96m) to 300ft(91.44m) depth (Arsenic content in ground water<br /><h3>Chhattisgarh</h3><br />Arsenic contamination in ground water is reported along the N-S trending 80 km stretchof Kotri lineament from Chhattisgarh State. The severity is found in eastern part ofAmbagarh Chowki block of Rajnandgaon district. The high Arsenic in ground water isrestricted to small isolated area and are in cluster of few villages extended over 330sqkm area and is confined to the Proterozoic rocks. The worst Arsenic affected villagesare Kaurikasa, Joratari, Sonsaytola, Jadutola, Muletitola.<br /><br /><b>Some of the findings of studies conducted by CGWB &amp; Other agencies are as follows:-</b><br />• The severely affected villages are situated on rhyolite and granite rocks close toshear zone. Elevated Arsenic in ground water is being controlled by variousgeological, structural, hydrogeological and geochemical factors. Ground waterfrom basic rock aquifer is found invariably low in arsenic (0.009 to 0.049 mg/l).(CGWB, 2006).<br /><br />• Bore wells and hand pumps are more affected than dug wells in general. This ismore particularly applicable to the area where arsenic contamination is less.However in severly contamination area large numbers of dug wells are also foundarsenic contaminated.• Ground water with higher mineralization (EC&gt; 800 μs/cm) are found invariablycontaining lower arsenic value, below 0.05mg/l (CGWB, 2006). Such controlmay also contribute toward providing safe and alternative drinking water.<br /><br />• The high Arsenic in ground water in the area varies in the range of 0.050 to 1.89mg/l (NEERI,2000)<br /><br />• Large temporal variation in concentration of Arsenic is observed in the area. TheArsenic level in individual contaminated wells shows variation up to 35 times indifferent seasons. Arsenic concentration in ground water is influenced by dynamicground water levels. (CGWB, 2006)<br /><br /><h3>Uttar Pradesh</h3><br />Arsenic in ground water (0.05 mg/l) in Balia district was reported in 2003 and furthersurvey indicated the presence of contamination in 7 blocks of the district of Gazipur,Balia and Varanasi (School of Environmental studies(SOES), Jadavpur university). TheArsenic affected districts are mainly aligned along a linear tract along the river Ganga asobserved in Bihar and West Bengal. However the reasons for same are not established. The presence of Arsenicin ground water has been observed by CGWB and state agenciesas given in Table-III<br /><br /><a data-flickr-embed="true" href="https://www.flickr.com/photos/hindiwater/26067786780/in/dateposted-public/" title="Tabel"><img src="https://farm2.staticflickr.com/1708/26067786780_58c9d94faa_o.jpg" width="424" height="600" alt="Tabel" /></a><script async="" src="//embedr.flickr.com/assets/client-code.js" charset="utf-8">

Jharkhand

High Arsenic contamination in ground water (>.05mg) are reported from 3 blocks ofSahebganj district of Jharkhand(SOES). CGWB has reported in its studies during 2006-07 that one block i.e Sahebganj block of Sahebganj district was effected with HighArsenic. Later on studies by PHED & CGWB confirmed that three blocks of the districtsi.e Rajmahal, Udohwa & Sahebganj located on the alluvial deposits.

North Eastern States
Studies(by SOES) reveal that 27 villages of Dhemaji & Karimganj district of Assam andparts of Thumbil & Imphal district of Manipur are effected by Arsenic contamination(>0.05 mg/l).

GENESIS OF ARSENIC IN GROUND WATER

Genesis of arsenic in groundwater in India is broadly geogenic. Geogenicallycontaminated aquifers are embedded in deltaic, alluvial and lacustrine sediments of post-Pleistocene age. Geologically many differences exist between diferent regions ofdistribution of Arsenic in ground water, but most striking similarity is that, majority ofsuch high-arsenic groundwater provinces lie in unconsolidated sediments, generally ofthe Quaternary but often confined to Holocene age. In Chattisgarh, however, thecontamination is related to some sulphide belt of Dongargarh-Kotri zone in aPrecambrian terrain.

In Bengal Delta Plain(BDP), the sources of Arsenic may be i) Transported by the RiverGanges and its tributaries from the Gondwana coal, seams in the Rajmahal trap arealocated at the west of the basin can be of the order of 200 ppm. (Saha, 1991). (ii)Transported by the north Bengal tributaries of Bhagirathi and Padma from theGorubathan base-metal deposits in the eastern Himalayas (Ray, 1999). (iii) Transportedwith the fluvial sediments from the Himalayas (e.g., McArthur et al., 2004), which isconsidered today as the most accepted hypothesis.

In Bihar, the contamination is considered to be geogenic where arsenic in solid phase isreleased to groundwater under a redox controlled environment. Organic carbon rich claybeds facilitate dissolution of Arsenic. “The abundance of organic carbon in the shallowalluvial stratigraphy allows a part of it to be carried downward with the percolating waterfrom rainfall infiltration. The organic carbon stimulates microbial respiration and triggersreductive dissolution of As and Fe in solid phase. This process also generates HCO3- andso produce the relationship between As and HCO3- in shallow groundwater” (Saha et al.,2009). The Older alluvial deposits in the vast stretches of the South Ganga Plain and alsoin the North Ganga Plain are contamination free. The source of arsenic contamination inthe ground water appears to be associated with the sediments of the Himalayanprovenance brought down by the river Ganga and its tributaries of Extra Peninsular(Himalayan) origin (GSI, 2008)

The sources of Arsenic for the high Arsenic ground water in Chowki area, Rajnandgaondistrict, Chhattisgarh is being established in-situ and geogenic (Acharrya et.al 2001 and2005, Pandey et.al 2002).

SUM UP

• Arsenic contamination of groundwater and related health hazards are becoming ahigh-profile problems through out the world. Number of Aquifers have been identifiedwith problems of high Arsenic concentration in ground water.

• The Bengal Delta plain(BDP) covering Bangladesh and West Bengal in India is themost severe cases of ground water Arsenic contamination. The other parts of thecountry having Arsenic infested ground water include parts of Bihar,U.P,Chhattisgarh, Jharkhand, Assam & Manipur.

• Arsenic contamination in India is mainly geogenic and mostly occur inunconsolidated sediments except in Chhattisagrh where it occurs in aquifers inPrecambrian rocks.

• Arsenic infested area of West Bengal forms a part of Ganga-Bagirathi delta,comprising succession of thick Quaternary sediments and mainly restricted in theUpper Delta Plain within a shallow depth. In West Bengal Arsenic free aquifers hasbeen identified (by CGWB) in the depth zone of 120-160 m bgl and 200-250 mbglwhere tubewells have been constructed with suitable design to get Arsenic free water.

• In Bihar high concentration of Arsenic in ground water are confined in Newer Alluvialbelt along the river Ganga affecting the both the Active and Older Flood plains. Thecontaminated water is young in age(
• Studies in Chhattisgarh revealed that elevated Arsenic in ground water is beingcontrolled by various geological, structural, hydrogeochemical factors. Ground waterwith more than 800 μs/cm EC are found invariably containing lower arsenic value,below 0.05 mg/l. Such controls may also contribute toward providing safe andalternative drinking water in the area.

• Various Arsenic mitigation options include using surface sources, exploring andharnessing alternate arsenic free aquifer, removal of Arsenic from ground water usingtreatment filters and rainwater harvesting.

ACKNOWLEDEMENT

The authors are very thankful to Shri B.M.Jha, Chairman, CGWB for giving permissionto publish the paper in the journal

REFERENCES

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S.kunar,S.K.Jain,S.Shekhar & V.Sharma.
Central Ground Water Board, HQ, Faridabad