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Arsenic in Ground Water of North 24 Parganas District, West Bengal

Author: 
Tapan Talukdar, Asit Kr. Ghosh, K.K. Srivastava
Source: 
Bhujal News Quarterly Journal, April-Sept, 2009

INTRODUCTION


Arsenic Contamination in ground water used for drinking purposes is a severe health hazard in the state of West Bengal since late seventies. At present arsenic pollution in sporadic manner has been reported in ground water from eight districts of West Bengal in 79 administrative blocks and nearly 16.2 million people are in the risk zone. Epidemiological studies conducted so far by various organizations have established that intake of arsenic contaminated water over a long period results in arsenic poisoning in human body. Different Government / Non government Organizations / Institutions have come forward to identify the arsenic affected tube wells yielding arsenic contaminated water and also trying to monitor the magnitude and extent of the pollution to mitigate the problems. In this respect detailed studies have been Carried out by CGWB in North 24 Parganas district of West Bengal to understand the causes and mobilization of arsenic in ground 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 people out of the total population of 35.85 lakhs are in the risk zone of arsenic pollution. In total 253 number of Mouzas out of 1606 are arsenic affected.

HYDROGEOLOGY


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

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

From the periodical monitoring of arsenic in ground water from the shallow aquifers in time and space it is observed that arsenic content in ground water is maximum during pre monsoon period (April - May) and is minimum in monsoon period (Aug-Sept) which indicates the effect of dilution due to rainfall recharge during monsoon period. It is also interesting to note that arsenic content in ground water in same aquifer varies within a very short span of distance even within 100m and the extent of arsenic variation is very high to very low as (as 2.98 mg/l from a tube well at Adhata village of Amdanga block to 0.02mg /l in another tube well of same depth of 45m bgl at a distance of 100m ). This leads to the problem for arsenic conc. contouring in space in a regional scale and is also difficult to demarcate the source of the arsenic in the area. As the source of arsenic in ground water is geogenic, the actual nature of release of arsenic from geological horizon to ground water is still confusing which has been discussed later. The variation of arsenic concentration in space, from the same aquifer can be explained as (a) During the course of movement of arsenic water through the aquifer, local clay lenses may adsorb arsenic from the arsenic rich water (b) The nature of release of arsenic in ground water from the geological horizons may depend on local factors like presence of organic matter 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 delineation of arsenic free deeper aquifers. It has been observed that arsenic free aquifers are separated from the upper arseniferous aquifers by a thick clay bed. Construction of suitably designed tube well tapping the arsenic free deeper aquifer with cement sealing technique yields arsenic free water. The details of the exploratory drilling and arsenic free aquifer identified has been presented in Table 1.

Tabel-1 Based on the sub-surface lithological correlation diagram (Fig.1 & Fig 2) of the this district it has been observed that three aquifer systems exist in the area: The first aquifer system exists down to the depth of 80 m below ground level. The second aquifer system starts below 100m below ground level and extends up to the depth of 180m bellow ground level and the third aquifer system exists down the depth of 200- 335m bgl out of the explored depth of 350m bgl. Furthermore, each aquifer system is consisting of one or more aquifers which are interconnected within a short distance.

The first shallow aquifer system (within depth of 80 m bgl) consists of two to three aquifers in which thickness of individual aquifer ( 10m to 40m ) varies from place to place and individual aquifers are separated by thin clay layers which are not extended regionally. The aquifer material is fine to medium grained sands in the upper part and coarser in the lower part. There is a lot of facies changes within this aquifer system. This aquifer is potential enough to yield good quantity of ground water having sporadic occurrences of arsenic in ground water in this aquifer in most of the places.

Fig-1 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 system consists 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.

Fig-2 The third deeper aquifer (within the depth range of 200 to 335 m bgl) is separated from the upper second deeper aquifer system by thick clay layer of 10 to 30 m. This aquifer system consists of one aquifer (5 m to 20 m thick) but the thickness varies from place to place. The aquifer material is medium to coarse grained sands in places enriched with gravels and of Pleistocene to Tertiary in age. The yield of this aquifer is relatively less than the upper aquifer systems. The regional extension of this aquifer could 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 of the district and properly designed tube well with cement sealing techniques can be capable of yielding sufficient quantity of safe potable and arsenic free ground water for drinking purposes. Transmissivity(T) of arsenic free deeper aquifers are determined as 2000-3000 m2/d.

CAUSES & MOBILIZATION OF ARSENIC AND ITS MITIGATION


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

Arsenic in Vadose water zone


Arsenic water in most parts of the district is mainly used for irrigation purposes and there 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 water samples have been collected through hand auger drilling and vadose zone sampler for arsenic, iron, phosphate and sulphate analysis. It is observed that in vadose zone water arsenic concentration is <0.001mg/l, iron concentration <0,1 mg/l, Phosphate concentration 0.025 to 1.25mg/l and sulphate concentration is low whereas in sediment samples iron concentration ranges between 8-112mg/kg and arsenic concentration 0.05 to 6.5 mg/kg. This indicates that irrigation through arsenic water does not increase the arsenic content in the vadose zone water. As soon as the arseniferous water is exposed to air and top soil, some of the arsenic may absorb by the plants & soils and rest may decomposes to arsine gas through bioorganic activity. Again, use of phosphatic fertilizer may not responsible for the release of arsenic in ground water as the vadose water contains very low phosphate. However, detailed works is essential to establish the fact.

Artificial Recharge Study


Experimental study has been conducted at Ashoknagar, Habra I block where recharge of arsenic free rain water from a recharge pit has been allowed through a small diameter shallow tube well ( depth 16m ) to recharge the shallow arseniferous aquifer ( within depth of 20mbgl ). The results indicates that initial arsenic concentration of 0.128mg/l could be diluted to 0.08mg/l in one month and < <0.001 mg/l in a span of 3 months during October to December. The rate of natural recharge in this case has been estimated as 7.11 liters/hr. The recharge rate has been increased to 115 liters/min by pumping of arseniferous water ( dischage 0.5 lps for 180 minutes). This indicates that recharging of arsenic free water in arseniferous aquifer may reduce the arsenic concentration in ground water.

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


The behaviour of arsenic concentration consequent to pumping has been studied from six locations in Barasat, Gaighata, Habra blocks .It has been observed that there is no definite relation of arsenic concentration consequent to pumping. However, arsenic concentration comes down in some of the pumped wells as 0.036 to 0.026 mg/l after 120 min in Gopalnagar, 0.128 to 0.069 mg/l after 180 min in Gajna (both the tubewells are of 22-24 m depth). Consequent with the pumping (pumping discharge 29 to 31 m3/hr),in some places arsenic concentration fluctuates consequent to pumping as seen in Ashoknagar and Mayna. Arsenic concentration increases (0.65 to 0.87 mg/l) in the observation well located close to pumping well consequent to pumping as observed in Mayna.

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

In some places, iron concentration, are also decreases consequent to pumping (7.0 to 5.7 mg/l at Gajna, 10.28 to 10.02 at Mayna). Therefore, it can be concluded that iron concentration 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 no definite trend consequence to pumping of arsenic rich water, pH increasing (pH 6.6 to 7.12 whereas EC & Dissolved Oxygen reducing ( EC 620 to 281 micromhos/cm & DO 1.9 to 0.5 mg/l). This also indicates that release of arsenic from sediments to water is instantaneous and not from a distance sources.

Study of Arsenic rich litho sample :


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

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

The major element composition of the core lithological samples indicate differing proportions 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 to sub-rounded in shape with mineral assemblages of biotite, garnet, lignite, opaques indicating a provenance of dominantly metamorphic origin. Sand grains in the arsenic rich aquifer coated with iron and arsenic rich material. Presence of clay with enriched organic matter in the sediments having high iron and arsenic sulphides deposited in the reducing environment is the principal source of high arsenic concentration in ground water. Arsenic is released to solution by reductive dissolution of FeOOH and release of its sorbed arsenic to ground water.

Study of the Efficacy of Arsenic removal equipments :


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

Tabel-2 Back wash samples from three locations of different community plants have been collected and analysed for arsenic, iron an aluminium concentration. The results are presented in Table 3A.

Tabel 3a Thus it can be stated that-

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

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 be attributed 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 that were analyzed. It is recommended from ‘research and Development study for their safe disposal otherwise back washing may form a secondary source of contamination.

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

Heavy metal analysis :


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

Tabel-4 Concentration of heavy metal in the ground water of shallow aquifer is within the range and 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 (As concentration 0.06 and 0.40 mg/l, 13 numbers of food items of Joypur Village, Barasat I block 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 using sodium boro hydride and GBC atomic absorption spectrophotometer. And results are presented in Table 5

Tabel-5 On perusal of the results obtained it is observed that almost all the food items do contain arsenic in appreciable amount. The average arsenic level in the food items are likely to be variable depending upon the level of element in the soil. Krishnamurti (1987) has reported a range of 0.02-0.3 mg/kg of arsenic in vegetables. The values obtained varies from 0.4 to 2.0 mg/kg which is much higher than normal value.

Arsenic enters in the human body through the biogeochemical and biochemical pathways. If the soil contains significant levels of arsenic content then the food items may be directly affected by that (Buat Menard, 1987). Animals feeding on the arsenic contaminated 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 more Attention.

Arsenic is similar to phosphorus in chemical behaviour therefore; it is likely to compete with phosphorus in normal metabolic process. The toxicity of arsenic is due to the inhibition of critical sulfhydryl (SH) group of proteins, complextion with coenzymes and uncoupling of phosphorylation (Peer, 1973). Organo-arsenic compounds like Arsenobetain 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 which definitely needs extensive research.

MITIGATION OF ARSENIC PROBLEM IN THE STUDY AREA


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

Proper designing of tubewells (Fig 3) by putting cement sealing against appropriate thickness of clay bed can prevent vertical percolation of arsenic rich water from shallow aquifer into deeper aquifer. The leakage of arsenic water from the shallow aquifer should be prevented adopting proper cement sealing techniques. The cement sealing is an effective sealing for separation of upper aquifer from the deeper aquifer. The aim of this technique is to place thoroughly mixed cement slurry against the impervious layer between the casing and the wall of the borehole either by gravity or under pressure through pump. Practically it is the filling up of openings principally to retain the impervious character so that there is no percolation from the upper aquifer to the lower aquifer. The cement slurry consists of ordinary /quick settling cement and water. A little clay is added in cement water mix to improve the flowing properties. 40 kg of cement should be mixed with 20-25 liters of water with a specific gravity of 1.8. About 2 to 6 per cent of bentonite is added to this slurry to improve its workability. About 30 cm of fine sand layer should be placed at the top of the gravel packing before the cement grouting operation. Settling time of 72 hrs for ordinary cement and 30 hrs for quick settling cement 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 by Central Ground Water Board few years back has been monitored for change in concentration of arsenic in deeper aquifer. It is worth mentioning that all these wells are not contaminated by arsenic as on date. This also indicates that proper cement techniques can be prevent the percolation of arsenic from upper arseniferous aquifer and the deeper aquifer is still arsenic free after several years.

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

Fig-3 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 entire population for drinking purpose only has been assessed for some of the blocks of the study area which has been presented in Table 6 (Considering 10 liters per capita per day consumption for drinking purposes). It has been observed that in total 48 tube wells in North 24 Parganas district can meet the supply of safe arsenic free water in all affected inhabits of seven arsenic infested blocks.

Tabel-6 Similarly arsenic problem in rest of the district can be meet up with construction optimum number of deep tube tube wells using proper cement technique. The deep exploratory wells constructed by Central Ground Water Board has ultimately handed over to Public Health Engineering Department/municipalities for safe drinking water supply. Apart from deep tubewells Public Health Engineering Department, Govt. of West Bengal also 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 Parganas district is geogenic and the it is observed that with the presence of an effective clay barrier, the deeper arsenic free aquifer can meet up the demand of arsenic free water for the present drinking water requirement. The tube well construction should be properly designed by cement sealing techniques against the impervious clay separating the upper arsenic rich and deeper arsenic free aquifers. In this regard, age of ground water has been detected which indicates that arsenic rich younger water from shallow aquifer has no hydraulic connection with the arsenic free old water from the deeper aquifer.

• Proper cement sealing will prevent vertical percolation of arsenic rich water from the upper arsenic contaminated aquifer into the deeper arsenic free aquifer. The water quality should be monitored periodically to assure arsenic free water supply. Sometimes it is also reported that some of the deep tube wells are contaminated with arsenic, due to not properly designed with cement sealing Techniques or both the upper contaminated & deeper arsenic free aquifers to get higher discharge.

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

• Recharging of arsenic free water in arseniferous aquifer can reduce the arsenic concentration in ground water. Large scale artificial recharge projects can be undertaken specially in areas of high arsenic ground water to bring down the level of 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 any adverse impact of these arsenic containing food items on human health or not, is yet to be established

ACKNOWLEDGEMENT


The authors are thankful to Chairman, Member(SML), Member(SAM), Regional Director(ER),Central Ground Water Board for kindly permitting the authors to publish this 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 contribution specially during ground water analysis and ground water exploration studies. The authors 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
• CGWB,1999, “High Incidence of Arsenic in Ground Water in West Bengal” Report by Dr. D.K.Chadha & Dr. S.P.Sinha Ray,
• J.M.McArthur,et al. 2002“ Pollution of ground water by arsenic in Bengal basin” ,Workshop on “Arsenic Hazards in Ground Water of West Bengal- Steps for ultimate solution”, Science City, Kolkata, 7th February 2002, organised by CGWB, Kolkata.
• Mukherjee, S., Kumar B.A., and Kortvellessey, L. (2005). Assessment of groundwater Quality South 24 Parganas, West Bengal, Coast, India. Journal of Environmental Hydrology (USA), Paper 15 Volume 13 pp 1-8,IEAH, San Antonio, USA.
• Tapan Talukdar et al:2002, “Arsenic removal equipments installed in arsenic infested area of West Bengal” , Proceeding of Workshop on “Arsenic Hazards in Ground Water of West Bengal- Steps for ultimate solution” , Science City, Kolkata on 7th February 2002, organised by CGWB, Kolkata.
• T.Talukdar et al , 2000:“ Application of artificial recharge technique for in-situ dilution of Arsenic concentration in shallow ground water – A case study in North 24 Parganas district, West Bengal ”,Proceedings of International workshop on “ control of Arsenic contamination in Ground water ” Kolkata on 5th & 6th January 2000, organised by PHED, Govt. of West Bengal
• M. McArthur, D.M. Banerjee, K.A. Hudson-Edwards, R. Mishra, R. Purohit, P.Ravenscroft, A.Cronin, R.J. Howarth, A.Chatterjee, T.Talukder, D.Lowry, S. Houghton, and D.K. Chadha,2004: "Natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water: the example of West Bengal and its worldwide implications". Applied Geochemistry 19, 1255-1293 published in 2004
• T.Talukdar et al,1999: “ A new approach of Ground Water Management in supplying safe drinking water in Arsenic infested area- case studies ” , Proceedings of National seminar on Ground water, held at Science City, Kolkata on 22nd January 1999 organised by IWWA.

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

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