As safe as water
Anwar Zahid

In the southern coastal Delta, multi-layered aquifer conditions exist, with arsenic contamination at shallow depth and high iron and brackish groundwater occurring mainly in the deeper layers, that forms a constraint to groundwater use in the area

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    Photo- COURTESY

With the combined efforts of government agencies and NGOs, though significant progress has been achieved for fresh and safe water supply in Bangladesh, about 15% of the country’s population have yet to receive this service.

The sufferers are obviously the poor people in the community. The detection of arsenic in shallow groundwater is still a big threat to groundwater-based water supply in Bangladesh. Seawater encroachment in coastal aquifers (sand formation in the sub-surface from where water can be withdrawn) is another major problem.

On the other hand, surface water source is not suitable for drinking purposes and harvesting rainwater is mainly seasonal and available during the monsoon.

Therefore, use of deep groundwater is becoming an important issue in the country, mainly in areas of high arsenic and salinity, but installation of deep tubewells (200-250m deep) for household use is expensive and out of the reach of low-income families.

As a consequence, coastal people suffer from acute storages of safe water supply. Still, to expand and improve the water supply services in order to satisfy the basic needs in these areas is a necessity.

In the southern coastal Delta, multi-layered aquifer conditions exist, with arsenic contamination at shallow depth and high iron and brackish groundwater occurring mainly in the deeper layers, that forms a constraint to groundwater use in the area.

The occurrence of brackish and saline water in the coastal aquifers does not follow any regular pattern spatially or vertically. Various studies and investigations show that all the different depth-levels of aquifer units down to the investigated depths of 350m have been affected by salinity in many areas. The change from potable water to very saline water is sharp and occurs over a relatively short distance.

A recent study by the Bangladesh Water Development Board in 19 coastal districts shows that in the shallow groundwater fresh water is noticed in areas of Patuakhali, Barishal, Satkhira, Jessore, and Narail districts, while brackish water was found at Shariatpur, Chandpur, and Gopalganj areas.

In shallow groundwater, the salinity is extremely variable, overlain by very shallow fresh water pockets in many areas recharged from recent rainfall.

However, to some extent, salinity increases with time during the dry period and over short distances. Studies show that, in many areas like Paikgacha and Tala, very shallow groundwater has salinity levels within the drinkable limit of 600mg/l of chloride, both in wet and dry seasons.

The arsenic concentration of this very shallow groundwater is also within the allowable limit of 10g/l for drinking water. Deeper water shows higher salinity and arsenic values.

The design of affordable, low-cost recharge technologies such as recharge tanks has been proposed for the poor to augment the storage of fresh rainwater to shallow sub-surface, in order to increase fresh water discharge by reducing salinity level of nearby very shallow hand tubewell.

This is a feasible and affordable option for low-income families. Where a significant aquitard, ie clay layer, exist in the surface, the very shallow groundwater is less vulnerable to surface contamination such as the movement of bacteria from the surface or nearby pit-latrine.

A recharge tank will have a cover that can be closed when there is no rain, and will not allow any debris or pollution. From rooftop, rainwater, through pipes, can be poured directly in the recharge tank.

In many areas, local inhabitants are already using very shallow hand-tubewells, but without knowing the concentration levels of arsenic and salinity.

In all cases, care must be taken to avoid any surface contamination including bacterial pollution, salinity intrusion due to shrimp cultivation and storm surges, and to monitor arsenic contamination level.

It needs to maintain a periodic maintenance of artificial recharge structures to avoid rapid reduction of infiltration capacity due to clogging, silting, chemical precipitation, and accumulation of organic matter.

As the rain water will be used directly for recharge, the possibility of clogging will be less and maintenance will be easier.

This small scale, very shallow tubewell, combined with a recharge tank, is a good idea in areas where the thickness of the surface clay is within the excavated range, groundwater salinity level is low, and depth to groundwater table is also very shallow and remains within 5-8m from the ground surface. 

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Anwar Zahid

Anwar Zahid is a groundwater researcher working in Bangladesh Water Development Board and is involved as a part-time teacher in the Department of Geology and Department of Disaster Science and Management, University of Dhaka.