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Arsenic in drinking water




Dr. Shah Rukh

Arsenic in drinking water is a serious issue worldwide. Long-term drinking from the resource containing arsenic concentration beyond the permissible limit poses a cancer risk to human. Even moderately elevated levels cause high blood pressure and diabetes. Here, we review the general perception of arsenic contamination in drinking water, the flaws and merits of recent assessment studies on arsenic contamination and highlight what is yet needed to be done. It would be pertinent to share some simple technologies for removing arsenic from drinking water.

Arsenic in the drinking water extracted from deep groundwater reservoirs comes from the weathering of primary minerals besides that the anthropogenic inputs including industrial waste, mining, sewage sludge, and agriculture may be a serious problem of surface water bodies. The discussion mainly focuses the drinking water extracted from deep groundwater reservoirs hosted in Quaternary sediments of alluvial and deltaic origins that form large parts of the Indus Basin where the thickness of those sediments occasionally reaches several hundred meters.

The sediments consist of coarse sand containing iron bearing minerals (arsenic sulfides or arsenopyrite in which arsenic substitute for iron); and arsenic release and mobility is facilitated through competitive adsorption, complexation, and redox reactions of the natural organic matter in the sediments. These sediments have many similarities with the arsenic affected aquifers of Bangladesh and West Bengal (Quaternary alluvial-deltaic sediments from Himalayan driven by Gangetic River system) where arsenic concentration so high that it is considered as the greatest mass-poising in the human history.

The Indus Basin differs in having a more arid climate, greater prevalence of older (Pleistocene) deposits and prevalence of unconfined aquifer conditions with greater relative connectivity between the river systems and the aquifers. Under these conditions, the aquifers are likely to be less reducing than those found in the Bengal Basin, and hence arsenic problem is less severe.

Arsenic contamination in groundwater is primarily in the form of inorganic arsenic forms i.e. arsenite and arsenate; and among the arsenic forms, arsenite is 25 to 60 times more toxic then arsenates. Some studies revealed high levels of arsenic in groundwater in Punjab and Sindh. Professor Podgorski of Swiss Federal Institute of Aquatic Science and Technology in Dübendorf sampled 1200 wells and found higher arsenic concentrations in sediments younger than 10,000 years and estimated 50 to 60 million people might consume water having arsenic above the permissible limits. Pakistan Council of Research in Water Resources (PCRWR) also reported high arsenic in drinking water in rural areas of Gujrat, Lahore and Kasur districts. These studies measured total arsenic while the dissolved arsenic ions species differ in their mobility and level of potential hazard necessitating measuring the distribution of arsenic ions species rather than total arsenic for effective and efficient removal of arsenic from drinking water.

Further, the total arsenical concentration has mostly measured using the field test kits, which have low reliability and high detection limit. Use of an atomic absorption spectrophotometer for measuring arsenic around the permissible limits in the drinking water would not be possible due to obvious technical reasons. We have reviewed arsenic estimation techniques and suggest that an atomic absorption spectrophotometer equipped with hydride generation assembly is needed for measuring arsenic forms to sub-parts per billion levels. PMAS-Arid Agriculture University has this facility in working order where an accurate analysis for arsenic can be carried out cost effectively.

PCRWR reported data which was obtained using field kits is not very reliable in our opinion though it has been widely cited. Groundwater drinking water from the same locations where PCRWR had reported arsenic 50 ppb (micrograms per liter) or, we found almost 90 % samples had total arsenic greater than 10 ppb, the maximum permissible limit for arsenic in drinking water set by the US-Environmental Protection Agency (US EPA) and World Health Organization (WHO). Forty percent of samples had total arsenic above the 50-ppb limit set by Pakistan Environmental Protection Agency (Pak EPA).

Further, the dissolved arsenic ionic forms differ in their potential hazard: arsenite being 25 to 60 times more toxic than arsenates. When the safe limit for total arsenic is 10 ppb (US EPA and WHO) the projected safe limit for arsenite would be 0.5 ppb. Taking projected safe limit of 0.5 micrograms per liter almost all the drinking water samples had values above the safe limit and were unfit for drinking. However, Pak EPA has set a safe limit of arsenic 50 microgram per liter which sets projected safe limit for arsenite to 2.5 micrograms per liter following the same logic. With 2.5 micrograms per liter safe limit, again almost all drinking water samples fall in the unfit category. Therefore, study suggests that further extensive work be carried out in the area including health parameter.

Finally, the least expensive and easy to use arsenic removing method that can be installed at the point-of-use are the use of iron oxide/ hydroxides, the sand coated with iron oxide/hydroxides. Some of the alternative treatments include distillation, use titanium oxy/hydroxide, anion exchange resins, manganese greensand, and reverse osmosis. The effectiveness of these techniques needs further testing.

The article is based on MS and Ph.D. research of the author carried at PMAS-Arid Agriculture University Rawalpindi under the supervision of Professor Dr. Mohammad Saleem Akhtar, Department of Soil Science.

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