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    3. WATER SUPPLY OPTIONS

    INTRODUCTION

    Groundwater extracted from arsenic contaminated aquifers in worst affected areas of Bangladesh by shallow tubewells can no longer be considered safe

    for drinking and cooking. Although 27 % of shallow tubewells are known to be contaminated in the national scale, in many areas more than 90% of

    shallow tubewells are contaminated. The problem has been magnified due to the fact that the tubewells with high levels of arsenic are also located in the

    areas where percentage of contaminated tubewells is high. In the absence of an alternative source, people in acute arsenic problem areas are drinking

    arsenic contaminated water without paying much attention to possible consequences. On the other hand, people with arsenic phobia are likely to use

    unprotected surface water to avoid arsenic poisoning and get sick by water borne/related diseases. Arsenic toxicity has no known effective treatment, butdrinking of arsenic free water can help the arsenic affected people to get rid of the symptoms of arsenic toxicity. Hence, provision of arsenic free water is

    urgently needed to mitigate arsenic toxicity and protect health and well being of rural people living in acute arsenic problem areas of Bangladesh.

    The options available for water supply in the arsenic affected areas can be brought into two major categories:

    i. alternative arsenic-safe water source, and

    ii. Treatment of arsenic contaminated water.

    Groundwater from deep aquifers and dug wells, surface water and rain water can be potential sources of water supply to avoid arsenic ingestion through

    shallow tubewell water. On the other hand, there are several treatment methods available to reduce arsenic concentration to acceptable levels for watersupply.

    ALTERNATIVE ARSENIC-SAFE WATER SOURCES

    Groundwater

    Technological options

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    The type of handpump technology suitable for a particular area depends on the groundwater level, water quality and hydrogeological conditions. Arsenic

    safe groundwater is generally found in shallow aquifers in north-western region, and in pockets/strata in arsenic contaminated areas where conventional

    shallow tubewells are producing arsenic-safe water. Deep aquifers separated from shallow contaminated aquifers by impermeable layers can be a

    dependable source of arsenic safe water. The deeper aquifers without any separating aquiclude/clay layer may initially produce arsenic safe water but

    vulnerable to contamination. The important alternative water supply technologies include:

    Shallow shrouded tubewell (SST) and Very shallow shrouded tubewell (VSST)

    Deep Tubewell;

    Dug Well; and

    Infiltration galley

    Shallow Shrouded Tubewell (SST) and Very Shallow Shrouded Tubewell (VSST)

    In many areas, groundwater with low arsenic content is available in shallow aquifers composed of fine sand at shallow depth. This may be due to

    accumulation of rainwater in the topmost aquifer or dilution of arsenic contaminated groundwater by fresh water recharging each year by surface and rain

    waters. However, the particle size of soil and the depth of the aquifer are not suitable for installing a normal tubewell. To get water through these very fine-

    grained aquifers, an artificial sand packing is required around the screen of the tubewell. This artificial sand packing, called shrouding, increases the yield ofthe tubewell and prevents entry of fine sand into the screen.

    These low-cost handpump tubewell technologies have been designed and installed in the coastal areas to collect water from very shallow aquifers formed

    by displacement of saline water by fresh water. The SST/VSSTs can be convenient methods for withdrawal of fresh water in limited quantities. Over-

    pumping may yield contaminated water. Installation of low capacity pumps may prevent over exploitation of shallow aquifers. The systems may be

    considered suitable for drinking water supply for small settlements where water demand is low. A shallow/Very shallow tubewell is shown in Figure 3.1.

    The depatment of Public Health Engineering has sunk a total of 5,904 VSST/SST to provide water to 0.44 million people in coastal areas (DPHE, 2000).

    Deep Tubewell

    The deep aquifers in Bangladesh have been found to be relatively free from arsenic contamination. The aquifers in Bangladesh are stratified and in some

    places the aquifers are separated by relatively impermeable strata as shown in Figure 2.4. In Bangladesh two types of deep tubewells as shown in Figure

    2.4 are constructed, manually operated small diameter tubewell similar to shallow tubewells and large diameter power operated tubewells called

    production well. Deep tubewells installed in those protected deeper aquifers are producing arsenic safe water. The BGS and DPHE study has shown that

    only about 1% deep tubewell having depth greater than 150 m are contaminated with arsenic higher than 50m g/L and 5% tubewell have arsenic content

    above 10m g/L (BGS and DPHE, 2001). Sinking of deep tubewells in arsenic affected areas can provide safe drinking water but replacement of existing

    shallow tubewells by deep tubewells involves huge cost. Some of the deep tubewells installed in acute arsenic problem areas have been found to produce

    water with increasing arsenic content. Post-construction analysis shows that arsenic contaminated water could rapidly percolate through shrouded materials

    to produce elevated levels of arsenic in deep tubewell water. Experimentation by sealing the borehole at the level of impermeable layer is yet to be

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    conducted to draw conclusions.

    Fig. 3.1 Shallow and Very Shallow Shrouded Tubewell

    However, there are many areas where the separating impermeable layers are absent and aquifers are formed by stratified layers of silt and medium sand.

    The deep tubewells in those areas may yield arsenic safe water initially but likely to increase arsenic content of water with time due to mixing ofcontaminated and uncontaminated waters. Again the possibility of contamination of deep aquifer by inter-layer movement of large quantity of groundwater

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    cannot be ignored. If the deep aquifer is mainly recharged by vertical percolation of contaminated water from the shallow aquifer above, the deep aquifer is

    likely to be soon contaminated with arsenic. However, recharge of deep aquifer by infiltration through coarse media and replenishment by horizontal

    movement of water are likely to keep the aquifer arsenic free even after prolong water abstraction. Since many people in the rural area still use surface

    water for cooking, installation of deep tubewell in an area can be a source of drinking water supply for a large number of people.

    In general, permeability, specific storage capacity and specific yield usually increase with depth because of the increase in the size of aquifer materials.

    Experience in the design and installation of tubewells shows that reddish sand produces best quality water in respect of dissolved iron and arsenic. The

    reddish colour of sand is produced by oxidation of iron on sand grains to ferric form. Which will not release arsenic or iron in groundwater, rather ferriciron coated sand will adsorb arsenic from ground water. Dhaka water supply, in spite of arsenic contamination around is probably protected by its red

    coloured soil. Hence, installation of tubewell in reddish sand, if available, should be safe from arsenic contamination.

    Some areas of the coastal region of Bangladesh is very suitable for construction of deep tubewell. Department of Public Health Engineering has sunk a total

    of 81,384 deep tubewell mainly in the coastal area to provide safe water to 8.2 million people (DPHE,2000). The identification of areas having suitable

    deep aquifers and a clear understanding about the mechanism of recharge of these aquifers are needed to develop deep tubewell based water supply

    systems in Bangladesh.

    Dug Well

    Dug well is the oldest method of groundwater withdrawal for water supplies. The water of the dug well has been found to be free from dissolved arsenic

    and iron even in locations where tubewells are contaminated. The mechanism of producing water of low arsenic and other dissolved minerals concentration

    by dug wells are not fully known. The following explanations may be attributed to the low arsenic content of dug well water:

    The oxidation of dug well water due to its exposure to open air and agitation during water withdrawal can cause precipitation of dissolved arsenic

    and iron.

    Dug wells accumulates groundwater from top layer of a water table which is replenished each year by arsenic safe rain and surface waters by

    percolation through aerated zone of the soil. The fresh recharges also have diluting effects on contaminated groundwater.

    The presence of air and aerated water in well can oxidize the soils around dug wells and infiltration of water into w

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