Supplying arsenic free water to rural areas

Some examples of Arsenicosis

It is believed that long ago, due to erosion in upper catchments of mountains and hills, arsenic might have been released into the transporting river water through weathering. Based on time and local hydrogeological structure prevailing in those times, it is found that these arsenic bearing sediments in river water were deposited in the alluvial floodplains at different depths. Arsenic is releasing from those deposited sediments to groundwater.

Studies at the author's laboratory have shown that arsenic is located in the form of a deposited layer in the shallow aquifer. The depth, thickness and size of this layer varies from area to area; in some areas, the layer may be narrow and thick, while in other areas, it may be wide and thin reflecting the variations in local hydrogeology. In the lower reduced zone of the shallow aquifers at low redox potential, arsenic previously adsorbed on finely divided hydroxy ferric oxide and manganese oxide particles is released to the interstatial water where Fe(III) is reduced to the Fe(II) and Mn(IV) is reduced to Mn(II). When reduction occurs, As(V) is reduced to As(III). This adsorbed and coprecipitated arsenic is released to the porewater and dominates the porewater.

The correlation between increasing concentration of arsenic and decreasing particle size is related to the coating properties of hydroxy ferric oxide and manganese oxide; the more clay and more oxide coatings there are in the sediments, the more arsenic is adsorbed. In the upper oxygenated zone of shallow aquifers and in the dry season (November to April) due to lowering of water table and non-recharge, air is penetrated in to the aquifer through the void fractions of soil grains and through the annular space of the shallow tubewells as well as the deep tubewells resulting in oxidation of As(III) to As(IV) species. During the rainy season (May to October), due to vertical recharge of the shallow aquifers, the void fraction of soil grains become filled-up with water and anaerobic condition (absence of free oxygen) prevails in the aquifers, as a result there is less arsenic in the extracted groundwater and besides, there may be dilution effect too.

The extracted groundwater therefore contains a mixture of As(III) and As(V) species. As(III) species are more toxic and more mobile than the As(V) species. Precise measurement of the forms of arsenic in human body from ingested water is extremely difficult due to variation in immunology and food habit which are variable from person to person, locality to locality, region to region. However it is generally agreed that when this arsenic contaminated groundwater is ingested, As(III) species are capable of forming complex with human enzymes and retains in the body leading to various sorts of arsenicosis while As(V) species are generally excreted with human urine and excreta as surface complex.

Works from the author's laboratory have shown that the deep aquifers below the sallow aquifers are found to be free of arsenic. Moreover the deep aquifers are composed of course grain soils and no question arises for deposition of arsenic there. In addition, in the deep aquifers water flows horizontally. For immediate solution of arsenic contamination in the rural areas, digging of tubewells in the deep aquifer will provide arsenic free water from a long period (70-80 years). It should be borne in mind that while digging tubewells in the deep aquifer, it will require sealing of annular apace around the tubewell at the juncture between the contaminated shallow aquifer and the deep aquifer; otherwise arsenic contaminated water from the shallow aquifer will penetrate in to the deep aquifer and will contaminate the water there.

The Department of Public Health Engineering (DPHE) as a representative of local government may take charge of proper digging of tubewells in the deep aquifers in a particular area based on depth and population density. Once the arsenic free water is extracted from the deep aquifer, for the time being, this extracted water may be stored in previously constructed cubic basins having tape on all four sides but free from contamination and corrosion.

For maintenance of these basins and proper utilization of the arsenic free water, DPHE may elect/select a local community who will look after the basins and supply of water to the local people only for cooking and drinking purposes. DPHE through the elected community may impose a token tax to the users of arsenic free water on a monthly basis for maintenance services. As regards the quality of water, DPHE itself through their own laboratories will quarterly monitor the bacteriological quality of water and countercheck the results in advanced laboratories at Dhaka. Any stagnant water leads to the formation of bacteria when it is not dis-infected periodically. When financial condition will permit, the local government may construct overhead tanks, store the extracted arsenic free water there and supply through pipe lines.

However, to meet the demand of agricultural irrigation water, during the rainy season, rain water be stored in sufficient number of previously constructed dams in government khash lands with proven solid walls/banks on all sides to avoid erosion and flooding. Stored water may be supplied from the dams through previously constructed and directed channels to agricultural lands on pre-paid token taxes. This should be done on short term basis (6-9 months) one in each division, then one in each district and finally, on a long term basis (5 years) all over the villages with imposition of token taxes for management. This will not only avoid arsenic contamination (through drinking, food and inhalation channel) but also satisfy the demand of water crisis in the dry season.

Dr. M A Hossain is Professor, School of Environmental Science and Management, Independent University, Bangladesh.