POSSIBLE ARSENIC CONTAMINATION FREE GROUNDWATER SOURCE IN BANGLADESH

D. Chakraborti*, B. K. Biswas, G.K. Basu, U.K. Chowdhury, T. Roy Chowdhury, D. Lodh, C.R. Chanda, B. K. Mandal, G. Samanta, A. K. Chakraborti, M. M. Rahman, K. Paul

S. Roy, S. Kabir, B. Ahmed, R. Das, M. Salim, Q. Quamruzzaman

* Corresponding Author, Tel: 91 33 4735233, Fax: 91 33 4734266, Email: dcsoesju@vsnl.com

Abstract

Our present study with respect to arsenic in water from different environment of aquatic systems considering both surface and sub-surface geological condition with an emphasis to surface erosion and deposition of soil sediment in the Bangladesh Flood Plain and Deltaic Region reflects some clue for possibility of huge arsenic free reservoir water.

The present communication is the out come of our last 5 years field survey in 64 districts of Bangladesh and on the basis of 25897 hand tube-wells analysis for arsenic from four existing geomorphological regions of Bangladesh. In this report we will highlight (1) the area in Bangladesh where groundwater is not arsenic contaminated and possibility to use shallow hand tube-wells as potential groundwater resource for drinking purpose (2) arsenic contamination scenario of deep tubewells (above 100 meters) all over Bangladesh and suitable depth in arsenic contaminated area to get safe water.

Geomorphologically Bangladesh is mainly divided in four regions (a) Deltaic (including coastal) (b) Flood Plain (c) Tableland (d) Hill Tract. Total sixty-four districts of Bangladesh are in these four regions. In first phase during our last 5 years field survey up to May 2000 we had analysed 24145 water samples for arsenic from hand tubewells by FI-HG-AAS (95% confidence of arsenic determination limit in our FI-HG-AAS system is 3 m g/l) from all these four regions (Table 1). It appears from the results of Table 1 that groundwater of Hill Tract, Tableland are almost free from arsenic contamination and Flood Plain and Deltaic Region including Coastal Belt are highly arsenic contaminated.

Although we could not identify any sample above 50 m g/l from Tableland area but we have found 28 samples having arsenic between 10 and 40 m g/l from 3 police stations (Table 1) out of 23 police stations we had surveyed. So in 20 police station in Table 1 all samples (937) were below 10 m g/l. The reason why these 28 (2.9%) samples have some amount of arsenic in tubewell water can be that we had collected the samples without filtering through membrane filter and thus small invisible particle containing some arsenic compound had dissolved when preservative added (1 ml of concentrated HNO₃ per liter of sample) or the area of these police station are slightly contaminated. However, over all data of 937 tube-wells of 20 Police stations from Tableland and 489 samples of Hill Tract indicate the area is free from arsenic contamination (below WHO guideline value of arsenic in drinking water¹).

To be sure whether groundwater of Tableland is free from arsenic contamination or not we collected further 1611 hand tube well water samples from 21 new Police stations during July 2000 from 11 districts in Tableland area. Here we had also found 15 samples (0.93%) having arsenic between 21& 40 m g/l. It is important to mention that these 15 samples were again from 4 police stations out of 21 police stations we had covered. Thus 2533 water samples from 38 police stations we surveyed appears safe to drink according to WHO guideline value¹.

During our study we noticed that fringe area of Tableland with Flood Plain might get contamination so also if some river passes or history of river eroded the Tableland. The probable reason of contamination may be heavy deposition of Holocene sediments due to severe surface erosion of Tableland and creating aquifer. Again confusion arise when we try to find out the actual extension of Tableland with respect to geographical boundary of the area. The available Tableland map of BGS/DPHE² and Bangladesh Physical Map³ differs to some extends mainly in bordering area.

When we compare our total 2576 water sample analysis from 44 Police stations from 11 districts of Tableland area with the available report published by BGS/DPHE² about arsenic contamination, we find that in 40 police stations out of 44 we reported BGS/DPHE also did not report contamination but in rest 4 police stations [Sripur of Gazipur district (2 samples); Mirzapur of Tangail district (1 sample), Porsha of Thakurgaon district (3 samples), Panchagar Sadar of Panchagarh district (1 sample)] they reported arsenic contamination above 50 m g/l). But all these police stations are in fringe area of Flood Plain with Tableland. Moreover, BGS/DPHE arsenic analysis report² also mentions 20 more police stations in Tableland area free from arsenic contamination. The recent report published by Disaster Forum⁴ also reported Nachole police station of Nawabganj district and Godagari police station of Rajshahi district are free (no. of samples 188) from arsenic contamination. This also supports our findings (Table-2).

When further we look to the available hand tube-well arsenic analysis report in Bangladesh we find that BGS/DPHE had also reported analytical results for 2887 samples from 65 police station² which according to our assessment and BGS/DPHE map is in Tableland and results show 51 police stations are arsenic contamination free. But 30 samples out of 790 analysed from 14 police stations showed contamination above 50 m g/l. 12 police stations out of these 14 according to our assessment are mainly in fringe area of Flood Plain and Tableland and the rest 2 police station which are according to us in Tableland. Out of 136 samples analysed from these 2 police stations only 2 samples show arsenic above 50 m g/l. The report of BGS/DPHE mentions above 50 m g/l, so we do not know exact arsenic concentration of these two samples.

So far up to July 2000 from all 4 geomorphological regions of Bangladesh we had analyzed 1217 hand tube-wells from 100 to 415 meters depth. Table-3 shows the arsenic concentration with different depth from each geomorphological region. It appears that deep tube-wells of Tableland and Hill Tract are arsenic contamination free. Deep tube-wells of both Deltaic region (including coastal belt) and Flood Plain are arsenic contaminated but over all result shows higher the depth lower is the arsenic concentration⁵.

Our 1217 hand tube-wells analysis above 100 meters indicate that out of 931 hand tube-wells above 200 meters 185 have arsenic between 10 and 49 m g/l and 58 tubewells (6.2%) have arsenic above 50 m g/l. The available deep tubewell report above 200 meters from BGS/DPHE² shows out of 909 deep tubewells from all over Bangladesh 34 (3.7%) are above 50 m g/l. Our result of Table 3 indicating that at depth above 300 meters (except one in Flood Plain and 3 Deltaic Region) the rest are free from arsenic contamination (< 10 m g/l). To get a better understanding of arsenic concentration in deep tubewells we were looking for a police station in a district where maximum numbers of deep tubewells above 200 meters are available. We finally found police station of Kalkini of Madaripur district where more than 1500 deep tube-wells between depth 200 & 310 meters were installed during last 2-3 years. We made a field collection in Kalkini police station on and from 4-6th August 2000, Table-4 shows the results.

It appears from Table-4 that 82 numbers of samples out of 304 we had analyzed have arsenic between 10 & 49 m g/l. Only one sample has arsenic 129 m g/l and the depth is 242 meters. This is only tube-well out of 304 containing high arsenic and the water is saline in nature. We collected the same sample at 2 different date and found the arsenic concentration quite similar.

From Table 4 it appears that above depth 227 meters except one, other tubewells have arsenic below Bangladesh recommended value of arsenic in drinking water. Finally, we expect Bangladesh may have arsenic in groundwater below 50 m g/l at depth above 200 meters. However, the success will depend on aquifer composition as well as design, construction and development of tube-well.

Our analytical result for arsenic from some selected geological units developed by surface erosion from Himalayan terrain and transportation by the Ganga, Bhrahmaputram Meghna river systems during Holocene period in Bangladesh and stratigraphic correlation of these soil sedimentary formations (Table 5) indicate a possibility of an environment of arsenic contamination free huge reservoir of usable water in some areas of Bangladesh. Stratigraphically the Barind Tract, Madhupur Garh, Lalmai Hills of Comilla are same. The Barind Tract, Madhupur Garh and Lalmai Hills of comilla are known as Tableland consists of Pleistocene red clay, silt and sand. Barind Tract and Madhupur Garh are marginally overlain by arsenic bearing Holocene sediments^2,6. Arsenic analysis report in some geographical locations in the districts like Gazipur, Bogra, Tangail, which are in Tableland but partially filled up by Holocene Flood Plain deposit show presence of considerable amount of arsenic in its groundwater. The possible reason of such contamination is due to Flood Plain deposition on the eroded surface of Tableland. An unique example is Kapasia police station of Gazipur district, where out of 5 police stations of Gazipur, 4 are in Tableland and free from arsenic contamination while Kapasia the eroded Tableland of Gazipur district filled up by Holocene Flood Plain deposit is found arsenic contaminated. In the Eastern hills of Chittagong the Holocene Flood plain deposit is absent. All the above areas are underlain by a thick medium to coarse sand and gravel bed of Pliocene epoch known as Dupi Tila^2,7-13. The Dupi Tila Sandstone Formation extends all over Bangladesh excepting probably the western two third of the Delta^2,14. The Pliocene sandstone gravel beds remains practically unexposed in these regions and forms the main aquifer beneath these areas. The notable geological control on arsenic occurrence in groundwater is in the oldest unit, where the Pleistocene Barind clay, Madhupur clay and Lalmai Hills and East and northeast of hilly areas overlies the Dupi Tila sands are found to contain arsenic free aquifer water. The total area occupied by the Dupi Tila measured roughly 91,000 sq. km. and the total thickness at places attains more than 300 m¹⁴. Pliocene Dupi Tila present undisputed evidence as a basement rock of Barind Tract, Madhupur Garh, Lalmai Hills of Comilla having considerable arsenic free water resource.

1. WHO, Arsenic, Environmental Health Criteria 18, World Health Organization, Geneva, 1981.

2. Groundwater studies for arsenic contamination in Bangladesh, Phase I: Rapid Investigation Phase, Final Report (MAIN REPORT), British Geology Survey, Mott MacDonald Ltd (UK), 1999.

3. Bangladesh Physical Map; source: Bangladesh in Maps, Dhaka University, World Graphosman World Atlas, 1981, Dhaka, Bangladesh January 1996, pp 7.

4. Fact Sheet 12 on Arsenic (A Compilation of Information of Arsenic Related issues), Disaster Forum (Source: CARE , Bangladesh), Dhaka, Bangladesh, March 2000, pp 5.

5. T. Roy Chowdhury, G.K. Basu, B.K. Mandal, B.K.Biswas, U.K. Chowdhury, C.R.Chanda, D. Lodh, S.L. Roy, K.C.Saha, S. Roy, S.Kabir, Q. Quamruzzaman, & D. Chakraborti,. Nature, 401, 545, (1999).

6. Arsenic pollution in groundwater in West Bengal, June 1991, Steering Committee, Arsenic Mitigation project. Final report PHED, Govt. of west Bengal 1991.

7. M.K.Alam, A. K. M. S. Hassan, M. R. Khan & J. W. Whitney. Geological Map of Bangladesh, Geological Survey of Bangladesh, 1990.

8. Monsur Hussain. Stratigraphical and palaeomagnetic studies some Quaternary deposits of the Bengal Basin, Bangladesh. Ph. D. Thesis. Free University of Brussels, Belgium, 1990.

9. Monsur Hussain. An Introduction to the Quaternary Geology of Bangladesh. International Geological Correlation Programme IGCP-347. Rehana Akhter, Dhaka, 1995.

10. J. Davies. Pilot study into Optimum Well Design: IDA 4000 Deep Tubewell II Project. Volume 2: The geology of the alluvial aquifers of Central Bangladesh. British Geology Survey Technical Report WD/89/9, 1989.

11. M.A.Bakr. Quaternary geomorphic evolution of the Brahmanbaria Noakhali area. Geol. Surv. Babgladesh Rec. V. I, 2, 1977.

12. Final report of the Deep tubewell II project; vol 2.1 Natural resources, vol 2.1/1 Groundwater resources, vol.2.1/3 Groundwater salinity study, (Mott MacDonald International in association with Hunting Technical services). Report prepared for the Bangladesh agricultural development Corporation under assignment to the Overseas development administration, UK, 1992a.

13. Feasibility study report for Jamuna River bridge construction project. Volume VI: geology and stone material (Japan International Co-operation Agency), 1976.

14. F. H. Khan, Geology of Bangladesh, University Press, Dhaka, 1991.

* Out of 28 samples: 17 samples were from Dhaka (arsenic range 10-29 m g/l); 2 samples from Jaipurhat (arsenic range 20 - 30 m g/l) and 9 samples from Bogura (7 samples arsenic ranges 20 - 30 m g/l; (2 samples arsenic range 34-40 m g/l)].

Table-2: 1611 Hand Tubewells water samples collected from 21 Police stations assured to be in Tableland in 11 districts

*Nilphamari, Sirajganj, Rajshahi and Nawabganj districts are in mixed geo-morphological region but Police Stations Godagari and Godagari & Nachole are in Tableland

*Depth information received from local people/owner.

Table 5 Simplified stratigraphic correlation of the study area^2,9

~ U.C ~ Unconformity