INTRODUCTION

Arsenic Contamination in ground water used for drinking purposes is a severe healthhazard in the state of West Bengal since late seventies. At present arsenic pollution insporadic manner has been reported in ground water from eight districts of West Bengal in79 administrative blocks and nearly 16.2 million people are in the risk zone.Epidemiological studies conducted so far by various organizations have established thatintake of arsenic contaminated water over a long period results in arsenic poisoning inhuman body. Different Government / Non government Organizations / Institutions havecome forward to identify the arsenic affected tube wells yielding arsenic contaminatedwater and also trying to monitor the magnitude and extent of the pollution to mitigate theproblems. In this respect detailed studies have been Carried out by CGWB in North 24Parganas district of West Bengal to understand the causes and mobilization of arsenic inground water and suggestion on mitigation for suitable management of ground water.

STUDY AREA

The study includes in the arseniferous blocks (Bongaon, Bagdah, Gaighata, Habra I & II,Barasat I & II, Amdanga, Deganga, Rajarhat, Barrackpur I & II, Baduria, Haora,Swarupnagar, Basirhat I & II ) of North 24 Parganas district. About 35.63 lakhs peopleout of the total population of 35.85 lakhs are in the risk zone of arsenic pollution. In total253 number of Mouzas out of 1606 are arsenic affected.

HYDROGEOLOGY

Hydrogeologicaly the North 24 parganas district is mainly within the upper delta plain ofGanga-Bhagirathi river systems. Arsenic in ground water is mainly restricted in shallowaquifer (depth 15 to 70mbgl) which is mainly built up of sediments deposited bymeandering streams and levees. The migration of the meander belts, lag front bars, leveeback swamp with fining upward sequence in a vertical section depending upon thedensity of channel network the composition of sediments changes laterally across deltaplain even within a few meter to several hundreds of metres. This may responsible forlateral variation of arsenic in ground water.

In the present study area, the main water bearing formations are Quaternary formationsmainly Recent and Pleistocene alluvial deposits and the aquifer materials comprising ofsand of varying grades and gravels. Ground water occurs within water table and in semiconfined to confined conditions. It is also noticed that the nature of aquifer materials inhorizontal extent is not uniform and it changes from place to place. This is due to thefacies variation during the sedimentation.

From the periodical monitoring of arsenic in ground water from the shallow aquifers intime and space it is observed that arsenic content in ground water is maximum duringpre monsoon period (April - May) and is minimum in monsoon period (Aug-Sept) whichindicates the effect of dilution due to rainfall recharge during monsoon period. It is alsointeresting to note that arsenic content in ground water in same aquifer varies within avery short span of distance even within 100m and the extent of arsenic variation is veryhigh to very low as (as 2.98 mg/l from a tube well at Adhata village of Amdanga block to0.02mg /l in another tube well of same depth of 45m bgl at a distance of 100m ). Thisleads to the problem for arsenic conc. contouring in space in a regional scale and isalso difficult to demarcate the source of the arsenic in the area. As the source of arsenicin ground water is geogenic, the actual nature of release of arsenic from geologicalhorizon to ground water is still confusing which has been discussed later. The variationof arsenic concentration in space, from the same aquifer can be explained as (a) Duringthe course of movement of arsenic water through the aquifer, local clay lenses mayadsorb arsenic from the arsenic rich water (b) The nature of release of arsenic in groundwater from the geological horizons may depend on local factors like presence of organicmatter in the geological horizons, local geochemical conditions like radox potential ( Eh ),hydrothermal conditions and physico-chemical characters etc.

GROUND WATER EXPLORATION

Ground water exploration has been carried out (CGWB) for identification and delineationof arsenic free deeper aquifers. It has been observed that arsenic free aquifers areseparated from the upper arseniferous aquifers by a thick clay bed. Construction ofsuitably designed tube well tapping the arsenic free deeper aquifer with cement sealingtechnique yields arsenic free water. The details of the exploratory drilling and arsenic freeaquifer identified has been presented in Table 1.

Based on the sub-surface lithological correlation diagram (Fig.1 &
The first shallow aquifer system (within depth of 80 m bgl) consists of two to threeaquifers in which thickness of individual aquifer ( 10m to 40m ) varies from place to placeand individual aquifers are separated by thin clay layers which are not extendedregionally. The aquifer material is fine to medium grained sands in the upper part andcoarser in the lower part. There is a lot of facies changes within this aquifer system. Thisaquifer is potential enough to yield good quantity of ground water having sporadicoccurrences of arsenic in ground water in this aquifer in most of the places.

The second aquifer system (within the depth of 100 to 180 m bgl) is separated.from the upper shallow aquifer system by a thick clay layer of 10 to 30 m thick. This aquifer systemconsists mainly of one or two aquifers whose individual thickness varies from 5 m to 30 m in place to place and the aquifer are separated by thin clay layers which are also not extended regionally. The aquifer material is mainly medium to coarse grained sands and is of Pleistocene alluvial deposits. The aquifer is regionally extended and potential enough to contribute good quantity of arsenic free ground water.

The third deeper aquifer (within the depth range of 200 to 335 m bgl) is separated fromthe upper second deeper aquifer system by thick clay layer of 10 to 30 m. This aquifersystem consists of one aquifer (5 m to 20 m thick) but the thickness varies from placeto place. The aquifer material is medium to coarse grained sands in places enrichedwith gravels and of Pleistocene to Tertiary in age. The yield of this aquifer isrelatively less than the upper aquifer systems. The regional extension of this aquifercould not be assessed in details due to insufficient data.

From the above table it is observed that arsenic free deeper aquifer exists in most parts ofthe district and properly designed tube well with cement sealing techniques can becapable of yielding sufficient quantity of safe potable and arsenic free ground water fordrinking purposes. Transmissivity(T) of arsenic free deeper aquifers are determined as2000-3000 m²/d.

CAUSES & MOBILIZATION OF ARSENIC AND ITS MITIGATION

The studies have been carried out in different parts of the district to identify the causesand mobilization of arsenic:-

Arsenic in Vadose water zone

Arsenic water in most parts of the district is mainly used for irrigation purposes andthere is a chance of arsenic rich zone in the top soil and in vadose water zone.Considering this view vadose zone ( upto 3m b.g.l.) litho samples as well as vadose watersamples have been collected through hand auger drilling and vadose zone sampler forarsenic, iron, phosphate and sulphate analysis. It is observed that in vadose zone waterarsenic concentration is

Artificial Recharge Study

Experimental study has been conducted at Ashoknagar, Habra I block where recharge ofarsenic free rain water from a recharge pit has been allowed through a small diametershallow tube well ( depth 16m ) to recharge the shallow arseniferous aquifer ( withindepth of 20mbgl ). The results indicates that initial arsenic concentration of 0.128mg/lcould be diluted to 0.08mg/l in one month and

Arsenic and Iron concentration consequence to pumping of arsenic water fromarseniferous aquifer:

The behaviour of arsenic concentration consequent to pumping has been studied from sixlocations in Barasat, Gaighata, Habra blocks .It has been observed that there is nodefinite relation of arsenic concentration consequent to pumping. However, arsenicconcentration comes down in some of the pumped wells as 0.036 to 0.026 mg/l after 120min in Gopalnagar, 0.128 to 0.069 mg/l after 180 min in Gajna (both the tubewells are of22-24 m depth). Consequent with the pumping (pumping discharge 29 to 31 m3/hr),insome places arsenic concentration fluctuates consequent to pumping as seen inAshoknagar and Mayna. Arsenic concentration increases (0.65 to 0.87 mg/l) in theobservation well located close to pumping well consequent to pumping as observed inMayna.

The behaviour of iron concentration consequent to pumping has been studied and it hasbeen observed that ferrous iron increases consequent to pumping (5.8 to 7.4 mg/l inAshoknagar, 62.5 to 7.10 mg/l in Dhakuria, 15.4 to 23.4 mg/l in observation well ofMoyna, 3.25 mg/l in Gopalpur).

In some places, iron concentration, are also decreases consequent to pumping (7.0 to 5.7mg/l at Gajna, 10.28 to 10.02 at Mayna). Therefore, it can be concluded that ironconcentration consequent to pumping does not reflect any linear trend and it fluctuates.It has been observed that Oxidation Reduction Potential (ORP –75 to –60 mv) has nodefinite trend consequence to pumping of arsenic rich water, pH increasing (pH 6.6 to7.12 whereas EC & Dissolved Oxygen reducing ( EC 620 to 281 micromhos/cm & DO 1.9to 0.5 mg/l). This also indicates that release of arsenic from sediments to water isinstantaneous and not from a distance sources.

Study of Arsenic rich litho sample :

Determination of arsenic content and other chemical constituents in lithological sampleshas done from the bore hole samples down to depth of 70.02 m drilled by CGWB atJoypur village, Barasat block and has been analysed in the laboratory of Geologicalsciences, London, U.K. (Ref .J.M. Mc Arthur of Geological sciences, UCL, London, UK)The lithogy of the bore hole is as follows:-

The 0.66 metre-thick unit between 29.21 and 29.87 metres depth contains 18% organicmatter (7.2% TOC). The concentration of arsenic is high in clay rather than sand samplesThis value is very high compare to the values found in the aquifer sands. Concentrationsof extractable-arsenic in the sediment profile are lowest in Aquifer B and show minimarelated to organic matter concentrations that confirm the operation of solubilization ofarsenic via reduction of iron oxides. The concentration of arsenic is high in clay ratherthan sand samples. Concentrations of Zn, Cu, Ni, correlate well with each other andshow an association with traces of sedimentary iron sulfide, which is authigenic in originin stable in a reducing environment that is maintained as anoxic by the highconcentrations of organic matter in the aquitards and organic-rich layer. As expected,concentrations of Arsenic show associations with both FeOOH and iron sulfides. Theseinfluence can be separated using measurements of TOS (total sulfur), which residesprimarily in iron sulfides. Similar organic rich horizon has been identified in arsenicaffected parts of North 24 Parganas district within the depth of 30m and it seems toplay an important role for the release of arsenic in ground water.

The major element composition of the core lithological samples indicate differingproportions of mica, feldspar and quartz in the different aquiclude and aquifer units.Distinct differences are seen between the aquifer above organic-rich horizon (Aquifer A)and that below it (Aquifer B).

Arsenic rich aquifer constituted of grey coloured medium to fine sand, sub-angular tosub-rounded in shape with mineral assemblages of biotite, garnet, lignite, opaquesindicating a provenance of dominantly metamorphic origin. Sand grains in the arsenicrich aquifer coated with iron and arsenic rich material. Presence of clay with enrichedorganic matter in the sediments having high iron and arsenic sulphides deposited in thereducing environment is the principal source of high arsenic concentration in groundwater. Arsenic is released to solution by reductive dissolution of FeOOH and release ofits sorbed arsenic to ground water.

Study of the Efficacy of Arsenic removal equipments :

The efficacy of different arsenic removal equipment installed by different agencies showresults of different arsenic removal equipments. The results are presented in Table 2.

Back wash samples from three locations of different community plants have beencollected and analysed for arsenic, iron an aluminium concentration. The results arepresented in Table 3A.

Thus it can be stated that-

1. All the arsenic removal equipments installed in Joypur Moyna and Sibalaya village, arecapable of removing arsenic as presented in Table 3B.

The values presented above, have been arrived from the maximum arsenic concentration,each technology has been able to remove at the available input water concentration.Sometimes treated water contains arsenic as in Joypur (0.16 mg/l), this may beattributed to poor maintenances and any inherent weakness in the technology itself.

2. The concentration of both arsenic and iron is high in the backwash samples thatwere analyzed. It is recommended from ‘research and Development study for their safedisposal otherwise back washing may form a secondary source of contamination.

3. Parameters of general chemistry of both raw and treated water (table 31) fallwithin the normal prescribed limit

Heavy metal analysis :

To study the correlation of arsenic concentrations with the heavy metals in the studyarea, samples have been collected and analysed. The data on heavy metal analysed ispresented in Table 4.

Concentration of heavy metal in the ground water of shallow aquifer is within the rangeand desirable limit of drinking water standards.

Arsenic in Food grains and cooked food:

To understand the mobilization of arsenic in food items irrigated by arsenic rich water (Asconcentration 0.06 and 0.40 mg/l, 13 numbers of food items of Joypur Village, Barasat Iblock have been collected and analyzed. Food items include spinach, Bean, Tomato,Cabbage, Potato, Chilli Brinjal, Pumpkin, Papaya, Banana, wheat and mustard).

The determination of Arsenic in food chain was done by hydride generation method usingsodium boro hydride and GBC atomic absorption spectrophotometer. And results arepresented in Table 5

On perusal of the results obtained it is observed that almost all the food items do containarsenic in appreciable amount. The average arsenic level in the food items are likely to bevariable depending upon the level of element in the soil. Krishnamurti (1987) hasreported a range of 0.02-0.3 mg/kg of arsenic in vegetables. The values obtained variesfrom 0.4 to 2.0 mg/kg which is much higher than normal value.

Arsenic enters in the human body through the biogeochemical and biochemicalpathways. If the soil contains significant levels of arsenic content then the food items maybe directly affected by that (Buat Menard, 1987). Animals feeding on the arseniccontaminated vegetables and other food items might accumulate this element in them,which could then be transferred to human being. This is the area which requires moreAttention.

Arsenic is similar to phosphorus in chemical behaviour therefore; it is likely to competewith phosphorus in normal metabolic process. The toxicity of arsenic is due to theinhibition of critical sulfhydryl (SH) group of proteins, complextion with coenzymes anduncoupling of phosphorylation (Peer, 1973). Organo-arsenic compounds likeArsenobetain or Arsenocholine have been found in some cases and it has been demonstrated that arsenic replaces the phosphorus in the phosphate group of DNA(Lepp, 1981). These might have links with the problems of arsenic toxicity whichdefinitely needs extensive research.

MITIGATION OF ARSENIC PROBLEM IN THE STUDY AREA

Considering the seriousness of the problem several mitigation practices have beenintroduced by different organization/institutions/private companies. There are two typesof mitigation practices in the study area as (i) Short-term measures includesidentification of arsenic free tube wells in arsenic affected areas and installation ofarsenic removal equipments to remove arsenic from arsenic rich ground water and (ii)The long term measures includes (a) identification of deep arsenic free aquifer andconstruction of suitable designed tube well and (b) supply of purified surface water.From the Ground Water Exploration, it is clearly observed that arsenic free deeperaquifers are present in the arsenic infested study area. Isotope studies carried by BhabaAtomic Research Centre, Mumbai in a collaborative project with Central Ground WaterBoard in Ground water of North 24 Parganas district reveals that the age of the groundwater of shallow aquifer (within 80m) is of modern age i.e. within 50 years. On the otherhand the age of the ground water of deeper aquifer (l00-350m) is about 500 years. Thusthe upper shallow aquifer is completely different from the deeper aquifer. The deeperarsenic free aquifers is separated from upper arseniferrous aquifer by a thick clay bed.Proper designing of tubewells by putting cement sealing against appropriate thickness ofclay bed.

Proper designing of tubewells (Fig 3) by putting cement sealing against appropriatethickness of clay bed can prevent vertical percolation of arsenic rich water from shallowaquifer into deeper aquifer. The leakage of arsenic water from the shallow aquifer shouldbe prevented adopting proper cement sealing techniques. The cement sealing is aneffective sealing for separation of upper aquifer from the deeper aquifer. The aim of thistechnique is to place thoroughly mixed cement slurry against the impervious layerbetween the casing and the wall of the borehole either by gravity or under pressurethrough pump. Practically it is the filling up of openings principally to retain theimpervious character so that there is no percolation from the upper aquifer to the loweraquifer. The cement slurry consists of ordinary /quick settling cement and water. A littleclay is added in cement water mix to improve the flowing properties. 40 kg of cementshould be mixed with 20-25 liters of water with a specific gravity of 1.8. About 2 to 6 percent of bentonite is added to this slurry to improve its workability. About 30 cm of finesand layer should be placed at the top of the gravel packing before the cement groutingoperation. Settling time of 72 hrs for ordinary cement and 30 hrs for quick settlingcement may be allowed and no work is to be done till the cement is fully set.

In North 24 Parganas district, ground water from exploratory tube wells constructed byCentral Ground Water Board few years back has been monitored for change inconcentration of arsenic in deeper aquifer. It is worth mentioning that all these wells arenot contaminated by arsenic as on date. This also indicates that proper cementtechniques can be prevent the percolation of arsenic from upper arseniferous aquifer andthe deeper aquifer is still arsenic free after several years.

It is noticed from few places of the study area that deeper aquifer has been contaminatedwith arsenic after few years of construction. This may be due to the faulty construction ofthe tube well with out using proper cement sealing techniques. Apart from this, some ofthe deep tube wells were constructed in this area tapping both the shallow aquifer anddeep aquifer cumulatively and this situation leads to the leaking of arsenic from uppershallow aquifer to the deeper aquifer. Therefore, no tube well should be constructed infuture tapping both shallow and deep aquifers without cement sealing.

Considering the average yield of the arsenic free water from each constructed tube well,the number of tube wells required for the supply of arsenic tree water to the entirepopulation for drinking purpose only has been assessed for some of the blocks of thestudy area which has been presented in Table 6 (Considering 10 liters per capita per dayconsumption for drinking purposes). It has been observed that in total 48 tube wells inNorth 24 Parganas district can meet the supply of safe arsenic free water in all affectedinhabits of seven arsenic infested blocks.

Similarly arsenic problem in rest of the district can be meet up with constructionoptimum number of deep tube tube wells using proper cement technique. The deepexploratory wells constructed by Central Ground Water Board has ultimately handed overto Public Health Engineering Department/municipalities for safe drinking water supply.Apart from deep tubewells Public Health Engineering Department, Govt. of West Bengalalso implemented some surface water schemes for arsenic free drinking water supply.

CONCLUSIONS

• The causes of arsenic in ground water of the study area of North 24 Parganasdistrict is geogenic and the it is observed that with the presence of an effectiveclay barrier, the deeper arsenic free aquifer can meet up the demand of arsenicfree water for the present drinking water requirement. The tube well constructionshould be properly designed by cement sealing techniques against the imperviousclay separating the upper arsenic rich and deeper arsenic free aquifers. In thisregard, age of ground water has been detected which indicates that arsenic richyounger water from shallow aquifer has no hydraulic connection with the arsenicfree old water from the deeper aquifer.

• Proper cement sealing will prevent vertical percolation of arsenic rich water fromthe upper arsenic contaminated aquifer into the deeper arsenic free aquifer. Thewater quality should be monitored periodically to assure arsenic free watersupply. Sometimes it is also reported that some of the deep tube wells arecontaminated with arsenic, due to not properly designed with cement sealingTechniques or both the upper contaminated & deeper arsenic free aquifers to gethigher discharge.

• Arsenic treatment units (mainly as short term mitigation measures) which arecommunity based and can cater to the needs of 200 persons, have been installedin different arsenic infested areas to provide arsenic free water to the people. Mostof the Arsenic Treatment Units work effectively, provided they are monitored andmaintained effectively.

• Recharging of arsenic free water in arseniferous aquifer can reduce the arsenicconcentration in ground water. Large scale artificial recharge projects can beundertaken specially in areas of high arsenic ground water to bring down the levelof concentration of arsenic and its impact assessment in time.

• Arsenic content in some of the food items have been determined. However,whether the arsenic is in organic or inorganic form and whether there is anyadverse impact of these arsenic containing food items on human health or not, isyet to be established

ACKNOWLEDGEMENT

The authors are thankful to Chairman, Member(SML), Member(SAM), RegionalDirector(ER),Central Ground Water Board for kindly permitting the authors to publishthis paper. The authors are thankful to Shri A. Ray, Supt. Hg & Dr. B.C. Meheta, Sc ‘D’for their valuable guidance, Dr. P.K.Roy, Sc ‘B’ , Shri A.K.Chatterjee, Dr. P.K.Das, Shri S.Chakraborty, Shri S.M.Hossain, Shri T.Misra, Asstt Hg for their valuable contributionspecially during ground water analysis and ground water exploration studies. Theauthors are also thankful to the Dr.S.K.Jain, Editor, Bhujal News and Dr S.Shekhar, Sc‘B’ for the final shape and publication of the paper.

REFERENCES
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Tapan Talukdar, Asit Kr. Ghosh, K.K. Srivastava - Central Ground Water Board, Kolkata