Current annual withdrawal of groundwater in the city is in excess of annual recharge of the aquifers. Withdrawal rate in the city should be, in fact, reduced to prevent further lowering of the water table/piezometric level. So, new sources for extra water need to be identified for exploitation.

Possible sources for augmenting supply
Rainwater: In many countries of the world rainwater is effectively harvested from roof catchments to supplement water supply. In Bangladesh, where huge amount of rain pours during the monsoon months, roof water harvest may partially meet the city water requirement in the rainy season. An investigation, carried out in Bangladesh Agricultural University, Mymensingh, showed that about 160 thousand liters of water might be available from a roof area of 100 sqm, in an average year in Dhaka city. This is equivalent to the amount of water required by an 8-person family for about 130 days. Rainwater harvest may also play an important role in reducing city flood, caused by local rainfall. In a roof water harvest system, roof top runoff contributing to city flood will be stored in domestic water tanks and thus reduce the extent of flooding.

While the potential of rainwater harvest deserves exploration, availability of such water is seasonal. So, other substantial sources need to be explored to ensure adequate water supply round the year.

Surface water sources: Waters of the rivers around the city are heavily polluted and are not in general suitable for treatment. On the face of groundwater shortage, the government initiated a surface water treatment project at Sayedabad for treatment of the Sitalakkha river water to augment the city supply. The project was supposed to be implemented in three phases. The first phase was commissioned in 2002 at a cost of Taka 580 crore. The treatment capacity was 22 crore liters of water per day. In 2004, the project was reported to be facing problems due to increased pollution load of the Sitalakkha river water. The report also added that the authority was considering to move the water intake point upstream at a cost of Taka 430 crore to reduce the pollution level of the intake water. Some people believed that shifting of intake point would give only temporary results. They added, in a situation of continued increase of pollution level, all parts of the river would become heavily polluted in the near future (Prothom Alo, 20 March, 2004).

The Sitalakkha serves as one of the main drainage systems that remove the waste pollutant of the city. For many reasons, the pollution level of the river is on an increase and the flow rate reduced. In the last dry season, there were reports of mass deaths of fish and problems in navigation in the river (The Ittefaq, 6 Dec 2004). Any diversion of water from this river for treatment would further deteriorate its navigability, and reduce its capacity to transport pollutant load, resulting in an aggravated aquatic environment. So, any step towards further expansion of the Sayedabad project should be taken with utmost care in consideration of cost, sustainability and environmental impact.

Water import: In a situation where withdrawal of groundwater cannot be enhanced and surface water in the local rivers so polluted, water import from neighbouring areas could be a justifiable alternative. Particularly, when plenty of water is available in the areas around the city.

Surface water import: Some people suggest that water from the Padma river at Maoa may be treated instead of the Sitalakkha river water. It was reported that, a plan was under consideration to use the Padma river water for treatment at Pagla. But that plan did not proceed any further (Prothom Alo, 20 Mar 2004). However, if implemented, such a project would be sustainable, environment friendly but unduly expensive due to requirement of water treatment.

Ground water import: An alternative to importing SW for treatment may be to import underground water from the neighbouring areas. It should be noted that, the problem of arsenic contamination of groundwater (GW), as reported in many areas of the country, way not be a matter of concern in this case. Such contamination is reported, so far, to be limited to shallow depths (within 100 ft or so from the ground surface). GW pumped from deeper aquifers, as is practiced in Dhaka city, is generally of goods quality.

Groundwater projects for Dhaka city supply should not be undertaken to the north of the city (viz Gazipur areas). Considering the natural flow of groundwater from the north to the south, any project in the north will intercept groundwater flowing towards the city, and thus reduce its subsurface water availability.

Proposition of a GW project
A viable GW project for this purpose may be considered in the areas between the Buriganga and Dhaleswari rivers to the south-west of the city (greater Keraniganj area). The project area may even be extended to the other side of the Dhaleswari river. A supply rate 22 CLD, the same as being currently received from the Sayedabad project, may be obtained from about 50 standard capacity (2 cusec) deep tubewells from the area. Possible layout of such a project is shown in Fig. 1. The technology, required in a GW project, is simple and familiar to local experts. The project may be implemented in phases, if needed, to obtain phased outputs. So, the entire capital investment may not be required at a time.

The proposed GW project will also be sustainable, but may interfere other water users, particularly irrigators, due to possible lowering of the water table in the future. A complementary SW irrigation project may be undertaken in the area using the Dhaleswari river water. The combined project would make the whole system environment friendly. The cost of the GW project (including a SW irrigation component) will be significantly less than that of a SW treatment project.

Although both of the SW treatment and GW projects (having equal capacity, says 22 CLD) for importing water for the city would be sustainable and environment friendly, the latter would have the advantages of low cost, simple technology and possibilities of phased investments.

From these considerations, a GW project appears to be a better choice. SW treatment is an expensive method for urban water supply, and is used in many countries where good quality groundwater is scarce. For a country like Bangladesh, where groundwater is so abundant, SW treatment plants for urban supply are inappropriate.

Conclusion
The basic policy for water resources utilisation in Bangladesh should be: Use clean groundwater for urban supply and unclean river water (and/or groundwater where convenient) for irrigation, on the contrary to using clean groundwater for irrigation and expensively treating unclean river water for urban supply. In selecting projects for augmentation of water supply for Dhaka city, GW resources in the neighbouring areas should be given due consideration.

Before getting more involved in SW treatment type of projects, the feasibility of a GW project in the Keraniganj area should be studied as an alternative case for comparison. Failing to do so may exclude a more viable project from consideration.

Necessary steps should also be taken to start utilising rainwater in the city, at least on an experimental basis. Secondary benefits of rainwater harvest in the form of flood reduction should not be overlooked.

Dr MH Khan is Professor, Dept. of Irrigation and Water Management, BAU, Mymensingh.