Bangladesh has significant amount of good quality bituminous coal resources and has the potential to provide long-term sustainable solution for the alternative energy source, which the country is seeking desperately. So far, five coalfields have been discovered with an estimated reserve of about 2,500-3,000 million tonnes. Only Barapukuria and Phulbari coalfields have confidently estimated the coal reserves of some 1,000 million tonnes reserves for the others are inferred only from limited number of boreholes. 

Bangladesh entered the mining era relatively recently through development of the Barapukuria underground coalmine. However, coal mining has a very long history and importance in terms of world development. While other coalfields remain at the early exploration stage the Phulbari coal basin has been extensively explored and a scheme of development has been submitted to the Government for Phulbari Coal Project. Based on an extensive analysis of environmental and social issues, depth and thickness of the coal deposit, and the nature of the water bearing aquifer overlying the coal, it was concluded that open pit mining was safest, most reliable and most economic way to extract the coal and to ensure abundant affordable coal for the Bangladesh market. 

With the proven gas reserve of the Country being quoted by the Government as being 8.93 TCF, enough for only another 6 years, there is in urgent need to source alternative energy and the proven coal reserve of Phulbari is the only really immediate hope for the country to get access to a new energy source that can be used for significant power generation. 

There are debates over mining methods and also a growing concern for groundwater dewatering impacts for open pit mining method as groundwater is required to be extracted extensively during the mining operation. Is this really a problem or rather an opportunity to gain access to good quality water? Are there any mitigation measures? What are the practices in other open pit mines in the world? Are those measures are suitable for our geological conditions?

Groundwater management is a critical issue for the successful operation of any open pit mine. It is a challenge for underground mining also. The water level needs to be lowered as the floor of the mine is dug deeper to maintain dry and safe working condition. Bangladesh should take comfort in the fact that there is plenty of international experience with groundwater management built up from hundreds of open pit mines in the world operating that are successfully managing groundwater under a wide variety of climatic and geologic complexities.

Hydrogeology of the north-western region

The hydro-geological conditions of the coalfields will play an important role in mine planning and design. As shown in the Conceptual Hydrological Model for the Phulbari coal basin, there are three aquifer systems in the regional setting: Upper Dupi Tila sand unit (UDT), sand layers near the base of the Lower Dupi Tila (LDTs) and fractured coal sequence. The Madhupur clay, Lower Dupi Tila clay and the Upper and Lower Gondwana sequence are considered as impervious layers or aquitards. The flow direction in all aquifers is inferred from north to south direction. 

Conceptual hydro-geological model for the Phulbari coal basin

Upper Dupi Tila aquifer is the principal aquifer in the region. It is high yielding, porous, very permeable and regionally extensive unconfined to semi-confined aquifer. The aquifer is approximately 100m thick with groundwater levels varying seasonally from 2 to 9m. The UDT is primarily recharged through rivers and streams that have cut through the impervious Madhupur clay which restricts recharge to this aquifer. In fact it is the Madhupur clay that plays host for agriculture and forests and for most of the year water to support these activities comes from direct rainfall and inherent soil moisture and not from the aquifer. The aquifer is routinely tapped to supply clean water supply for households and for irrigation water during the dry season. Although this UDT aquifer will require dewatering for open pit mining, the actual area affected can be restricted to a few kilometers from the mine by injecting some of the water back into the aquifer. 

Pumping the groundwater from the aquifer to permit mining actually becomes a benefit for the community because the water is of high quality and could be used for reticulated village and town water supply and piped to nearby farmers for irrigation. At the same time some water could be released to rivers and wetlands to generally improve the environment. Incidentally the water in the Phulbari coal basin area has been tested and key parameters were found to be well within the acceptable limit for drinking water, eg; it has low salinity, neutral pH and low turbidity, and arsenic very low and generally less than the detection limit. 

Why mine dewatering?

The groundwater level must be pulled down to create dry conditions in the mining area so that the generally low strength aquifer sequence materials (sands, gravel and clays) can be safely excavated. Mine dewatering is usually undertaken for a variety of geo-technical (material strength considerations), mining and safety reasons:

Geo-technical Issues

• Increased material strength;
• Reduction in seepage forces;
• Decreased hydrostatic pressure;
• Reduce chance of piping failure;
• Increased wall and bench stability;
• Reduced weight of porous rocks; and
• Prevention of liquefaction.

Mine Production Considerations 

• Reduced drilling and blasting costs;
• Reduced wear on equipment;
• Reduced haulage costs for unsaturated material;
• Reduced corrosion of equipment; and
• Improved trafficability.

Safety Issues 

• Improved road conditions;
• Reduced risk of slope instability during and after excavation; and
• Reduced risk of water inrushes.

Mine dewatering can be achieved by pumping continuously from a ring of dewatering tube wells around the mine and on benches, and installing pumps in special sumps on the mine floor to remove any surface water. This is a well-proven way to maintain dry working conditions, prevent flooding and ensuring the stability of the mine walls and mine haulage road pavements. 


Dewatering impacts and mitigation measures 

Pumping to lower the groundwater level in an open pit mine in the Bangladesh situation will over the life of the mine produce an abundance of good quality water. If all of this water was simply discharged into rivers and streams, then nearby households and villages would have difficulty accessing water through shallow tube wells for drinking and irrigation. In fact the water pumped from the aquifer is of great benefit for the community and there is a range of well proven mitigation measures that could ensure the area affected by the water level lowering is restricted to a few kilometers from the mine area and that adequate water is delivered through pipes to the town and farming communities.


Aquifer injection to restrict area of water level lowering

Aquifer injection is a well-tested method to limit the area of water level draw-down. This involves injecting water from the dewatering operation into the aquifer at a certain distance (around 5 km) from the mine to hydro-logically isolate the mine dewatering area from the regional aquifer. Experience shows up to 30% of water from the mine dewatering operation can be injected into the aquifer with this system. Certain geological and hydro-geological criteria are required for successful operation of the injection system:

• A really extensive, thick, sedimentary sequence
• The target aquifer preferably overlain by a confining layer; and
• A simple geological structure

The overlying UDT layer of the coal bearing sequence of Bangladesh is much thicker and extensive throughout the region and suitable for such operation.

RWE Mine of Cologne Germany has been successfully using this method for many decades under Germany’s strict environmental rules and regulation, in the same geological and groundwater conditions to Bangladesh. RWE has been conducting its operation for last 50 years and currently produces more than 100 million tones/year of low quality lignite coal, which contributes 30% of Germany’s total power generation. The RWE’s mining experience is a demonstration that the aquifer injection technique can be used successfully in Bangladesh to minimize the impacts of water level lowering. It is known that Asia Energy Corporation has proposed this technique in the Phulbari Coal Project and is planning to involve RWE expertise to assist in its implementation. 

Water supply for irrigation and domestic uses and monitoring

Groundwater is the major source of irrigation water during the dry season and a substantial number of shallow tubewells used for irrigation purposes will not be functional within the influence area of dewatering operation. Availability of water must be ensured so that lowering of water layer doesn’t hamper the crop production. There will be an abundance of water from the mine dewatering operation and certain portion of it could be delivered to the farmers by large diameter pipelines and trenches/canal and if required in some areas deep tube wells can be installed to ensure the supply of water. Farmers have been affected by recent increasing fuel cost and shortage of electrical power, both making crop production and life in general difficult. Having a reliable supply of water throughout the year from the mine area will allow farmers to reliably produce three crops a year and will thus both increase crop production and reduce the production cost.

Groundwater is also the primary source of drinking water to the rural villages and township and safe drinking water must be ensured to the affected households. A reticulated water supply system could be developed to supply a portion of water from the mine dewatering operation to the affected households. Deep tube wells can be installed in isolated villages far off from the mine operation area to source water for reticulated water supply system. This will improve the water supply quality to the affected rural households.

Water may also need to discharge to surrounding water bodies from the dewatering operation to maintain current seasonal water levels and quality. Any surface water pumped from the floor of the mine must be treated to ensure it meets national water quality standards before releasing to the neighboring environment. A comprehensive monitoring system could be developed to monitor the impacts of dewatering, aquifer injection performance, water chemistry etc and appropriate mitigation measures to be implemented to rectify the impacts.

Bangladesh desperately needs alternative energy source and coal is the only source to meet the long-term energy demand of the country. But maximum extraction of this valuable resource must be ensured and Barapukuria experience suggests that underground method is not viable option for the coal deposits in this country. Open pit mining can ensure >90% extraction of the coal resource and impacts related to this type of operation can be mitigated using well-tested internationally practiced mitigation measures. To eliminate the concern of the people, an independent monitoring system can be developed with data reviewed by some of Bangladesh’s renowned hydro-geologists to ensure the impacts of water level lowering are being managed and the benefits of the abundant water supply are being delivered.