Correspondence Comment on “Limited Temporal Variability of Arsenic Concentrations in 20 Wells Monitored for 3 Years in Araihazar, Bangladesh” In a recent publication (1), on the basis of a 3-year study on 20 wells, in a 25-km2 area of the Araihazar Upazilla, Cheng et al. concluded, “The implications for mitigation and policy are significant because the proper installation of deep wells in thousands of affected villages in Bangladesh could provide a reliable source of safe water for years to come.” It is quite questionable whether (a) “deep wells” (>30 m in depth) always ensure a safe water source, (b) the study findings in the Araihazar Upazilla can be generalized for policy-making regarding arsenic-safe water all over Bangladesh, (c) a 3-year study suffices to judge the temporal variation in arsenic concentration properly, or (d) the conclusions drawn are commensurate with the results presented. The authors considered wells tapping deep aquifers (>30 m) as “deep tube wells”. The British Geological Survey and Department of Public Health Engineering of Bangladesh (BGS-DPHE) (2), on the basis of surveying 2022 samples, in five different depth segments (200 m) in Bangladesh, concluded, “deep wells are defined here to be those with total depths greater than 200 m below ground, and deep aquifers are exploited by such wells.” K. M. Ahmed, one of the coauthors of this paper (1), suggested a demarcation of 200 m between shallow and deeper aquifers (3). Notably, these selected 10 “deep wells” (>30 m) tapped “older aquifers of the presumed Pleistocene age that are orange brown in color”, and all except CW4 (30 µg/L; due to defective PVC pipe) and 1639 (17 µg/L) had As concentrations below 4 µg/L. Again, K. M. Ahmed, one of the coauthors of this paper (1), noted (3), “Most groundwater from the shallow alluvial aquifers (30 m) in the flood plain and deltaic regions of Bangladesh contained arsenic above 50 µg/L. BGSDPHE (2) reported that the maximum arsenic concentrations occurred at depths between 20 and 50 m. Harvey et al. (7) also showed that at their study site in the Munshiganj district, near Dhaka, Bangladesh, dissolved arsenic concentrations (>90% As(III)) peaked at a depth of 30-40 m. Importantly, as in their prior publications (8, 9), the authors here sought to generalize the observations on a 251714
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km2 area of Araihazar to the whole country of Bangladesh. The Araihazar Upazilla is located close to Dhaka, at the confluence of the Brahamaputra-Jamuna and the Meghna Rivers, in the southern fringes of the Madhupur Upland. Aftab Alam Khan, a noted geologist of Bangladesh, commented (10): The arsenic-contaminated region of the Bengal Delta is distinctly divided into two geologic domains: 1. The domain of arsenic-contaminated aquifers extending from Malda-Rajshahi down to Satkhira24 Parganas across Kustia, Meherpur, Jessor, and Khulna along Hoogly Basin bears a geologic character independent of its own. 2. While, the domain of arsenic-contaminated aquifers occupied by Jamuna Basin, Sylhet Basin, and Meghna Basin down to Noakhali-Patuakhali certainly bears a separate geologic character that would not fit with domain 1. Araihazar falls within the second domain. In fact, all of Bangladesh has been influenced by several tectonic episodes, and the distribution of the Pleistocene and the Holocene deposits varies widely from one region to the other. Out of 20 wells in the study area, 10 (50%) were chosen from Pleistocene age aquifers expected to be arsenic-safe. Leaving aside the deeper wells, 3 (30%) out of 10 shallow wells selected from the Holocene deposit area showed temporal variation within a 3-year span. We already have reported about the increase in arsenic concentration over time (11, 12). We observed that in 23 of the 100 villages studied from arsenic-affected areas in West Bengal, tube wells that we colored green (safe, less than 50 µg/L) became contaminated over time. The statistical analysis of our 10-year monitoring data revealed that the possibility of variation in As concentration increases significantly with time period of monitoring. The trend shows that 3 years may not be sufficient to comment on variability of arsenic concentration with time. BGS-DPHE also commented (2), “However evidence to prove or disprove the validity or otherwise of this relationship will take around ten years to collect, so that in the meantime the precautionary approach would be to assume that there is an increasing trend of arsenic concentrations in new wells.” Moreover, instead of absolute variability, relative variability (such as a coefficient of variation) would be a better measure to judge temporal change. Importantly, in “Implications for Mitigation and Policy” Cheng et al. wrote (1), “From a practical perspective, the most important conclusion to draw from the data presented in this study is that groundwater As concentrations typically do not vary over time.” In the same section they conclude, “Another important lesson to learn from the observations is that concentrations of As in some wells can vary significantly over time.”
Literature Cited (1) Cheng, Z.; van Geen, A.; Seddique, A. A.; Ahmed, K. M. Limited temporal variability of arsenic concentrations in 20 wells monitored for 3 years in Araihazar, Bangladesh. Environ. Sci. Technol. 2005, 39, 4759-4766. (2) British Geological Survey and Department of Public Health Engineering, Bangladesh. In Arsenic Contamination of Groundwater in Bangladesh; Kinniburgh, D. G., Smedley, P. L., Eds.; 10.1021/es051492o CCC: $33.50
2006 American Chemical Society Published on Web 01/21/2006
(3)
(4) (5)
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Final Report, BGS Technical Report WC/00/19; British Geological Survey: Keyworth, U. K., 2001. Ahmed, K. M.; Bhattarcharya, P.; Hassan, M. A.; Akhtar, S. H.; Mahbub Alam, M. A., Hossain Bhuyian, M. A.; Imam, M. B.; Khan, A. A.; Sracek, O. Arsenic enrichment in groundwater of the alluvial aquifers in Bangaldesh: An overview. Appl Geochem. 2004, 19, 181-200. Abdullah, S. K. M. Retired Chairman of Petro Bangla. Personal communication, 2005. Chakraborti, D.; Biswas, B. K.; Basu, G. K.; Chowdhury, U. K.; Roy Chowdhury, T.; Lodh, D.; Chanda, C. R.; Mandal, B. K.; Samanta, G.; Chakraborti, A. K.; Rahaman, M. M.; Roy, S.; Kabir, S.; Ahmed, B.; Das, R.; Salim, M.; Quamruzzaman, Q. Possible arsenic contamination free groundwater source in Bangladesh. J. Surf. Sci. Technol. 1999, 15, 180-188. Roy Chowdhury, T.; Basu, G. K.; Mandal, B. K.; Biswas, B. K.; Chowdhury, U. K.; Chanda, C. R.; Lodh, D.; Roy, S. L.; Saha, K. C.; Roy, S. Kabir, S.; Quamruzzaman, Q.; Chakraborti; D. Arsenic poisoning in the Ganges Delta. Nature 1999, 401, 545546. Harvey, C. F.; Swartz, C. H.; Badruzzaman, A. B. M.; KeonBlute, N.; Yu, W.; Ali, M. A.; Jay, J.; Beckie, R.; Niedan, V.; Brabander, D.; Oates, P. M.; Ashfaque, K. N.; Islam, S.; Hemond, H. F.; Ahmed, M. F. Arsenic mobility and groundwater extraction in Bangladesh. Science 2002, 298, 1602-1606. Van Geen, A.; Ahsan, H.; Horneman, A. H.; Dhar, R. K.; Zheng, Y.; Hussein, I..; Ahmed, K. M.; Gelman, A.; Stute, M.; Simpson, H. J.; Wallace, S.; Small, C.; Parvez, F.; Slavkovich, V.; Lolacono, N. J.; Becker, M.; Cheng, Z.; Momotaj, H.; Shahnewaz, M.; Seddique, A. A.; Graziano, J. H. Promotion of well-switching to mitigate the current arsenic crisis in Bangladesh. Bull. W.H.O. 2002, 39 (5), 732-737. Van Geen, A.; Cheng, Z.; Seddique, A. A.; Hoque, A.; Gelman, A.; Small, C.; Graziano, J. H.; Ahsan, H.; Parvez, F.; Ahmed, K. M. Reliability of a commercial kit to test groundwater for arsenic in Bangladesh. Environ. Sci. Technol. 2005, 39 (13), 299-303.
(10) Khan, A. A. Professor, Department of Geology, University of Dhaka. Personal communication, 2005. (11) Chakraborti, D.; Basu, G. K.; Biswas, B. K.; Chowdhury, U. K.; Rahman, M. M.; Paul, K.; Chowdhury, T. R.; Chanda, C. R.; Lodh, D. Characterization of arsenic bearing sediments in Gangetic Delta of West BengalsIndia. In Arsenic Exposure and Health Effects; Chappell, W. R., Abernathy, C. O., Calderon, R. L., Eds.; Elsevier Science: New York, 2001; pp 27-52. (12) Chakraborti, D.; Mukherjee, A.; Sengupta, M. K.; Hossian, M. A.; Ahamed, S.; Lodh, D.; Das, B.; Nayak, B.; Rahman, M. M.; Saha K. C.; Mukherjee, S. C.; Pati, S.; Dutta, R. N.; Chatterjee, G. Chronic arsenic poisoning in the Asian countries: Technical issues to be addressed to combat the situation. In Proceedings of the Inter-Regional Conference on Water Quality and Arsenic Mitigation, Taiyuan, Shanxi, China, Nov 23-26, 2004. (Jointly organized by MOH-PRC, MWR-PRC, UNESCAP, UNICEF, UNN, and WHO.)
Mrinal Kumar Sengupta, Amitava Mukherjee, Sad Ahamed, Md. Amir Hossain, Bhaskar Das, Bishwajit Nayak, and Dipankar Chakraborti* School of Environmental Studies Jadavpur University Kolkata 700 032, India
A. B. Goswami Retired Director Geological Survey of India ES051492O
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