Transboundary Aquifers with Special Reference to Indo-Bangladesh Border

INTRODUCTION

Transboundary aquifers are as important a component of global water resource systems as are transboundary rivers. For river system, the connection is visible and the mutuality is obvious. This is not the case for shared aquifers, since aquifers are invisible in their natural state. As in any transboundary resource, management of these aquifers may prove to be a daunting challenge, as it requires collaboration among various levels of water management institutions within a country and among the different countries involved. The challenge is compounded by the fact that there is no international convention, specially addressing transboundary aquifers.

Many aquifers today are being depleted, while others are being contaminated. Notwithstanding world’s considerable reliance on this resource, groundwater resources have long received secondary attention as compared to surface water, while there are hundreds of treaties governing transbound of rivers and lakes, existing international conventions and agreements barely address transboundary aquifers and their resources. Many of the regionally extensive aquifers, on which humanity so heavily relies, cross international borders. But there is a considerable gap in the sound management, allocation and protection of such resources. Many shared aquifers are under environmental threats caused by climate change, growing population pressure, over exploitation and human induced water pollution.

It is estimated that there are about 273 shared aquifers in the world (68 in America, 38 in Africa, 155 in Europe and 12 in Asia). One of the largest aquifers in the world is the Gurani Aquifer or GAS (Gurani Aquifer System), extending over 1.2 million square kilometer, shared by Brazile, Argentina, Paraguay and Uruguay. Nubian Aquifer System caters to the needs of Chad, Egypt, Libya and Sudan. Iullemedan Aquifer extends over 50, 000 sq. km. in the semi-arid and tropical Savanna ecoregion of West Africa, Niger, Nigeria and Mali.

In order to prevent future disputes over transboundary aquifers and to maximize the beneficial use of this resource, international law must be clarified as it applies to transboundary ground water resources. International Association of Hydrogeologists and UNESCO’s International Hydrological Programme have established the ‘Internationally Shared (transboundary) Aquifer Resource Management (ISARM) Programme’. This multiagency co-operative programme has launched a number of global and regional initiatives. These are designed to delineate and analyse transboundary aquifer systems and to encourage riparian states to work towards mutually beneficial and suitable aquifer development.

TRANSBOUNDARY AQUIFERS WITH RESPECT TO INDO-BANGLADESH BORDER

Ganga – Padma Interfluve in parts of Bangal Basin: Ganga – Padma interfluve area of West Bengal constituting a part of Bengal Basin (Fig-1) as well as a part of adjacent area to Bangladesh border where the shallow aquifers are being extensively used for irrigation purposes, has been considered for discussion in this paper. In the area the sediments comprising the aquifers system that crosses the international boundary between India and Bangladesh, geologically known as the ‘Bengal Basin’, consist of a succession of Quaternary Alluvium sediments of varying thickness, deposited by Ganga-Brahmaputra-Meghna rivers and their distributaries.

Tectonic history of Bengal Basin: The Bengal Basin is bounded on three sides - west, north and northeast by Pre-Cambrian crystalline rocks, the eastern side by Tertiary hill ranges of Assam-Burma arc and in the south by Bay of Bengal. The Archaean shield area in the western boundary of Bengal Basin which gradually disappears below a thin veneer of alluvium towards east and a row of enechelon faults marks the basin margin. The stable shelf zone of the basin is dipping gently to the southeast and is practically undisturbed tectonically except for a row of normal faults passing through Jalangi-Debagram-Burdwan-West Ghatal area . The stable shelf zone is occupied by sediments of Mesozoic and Tertiary age beneath the recent to sub-recent alluvium, the thickness of which increases uniformly from west to southeast from 900 m to 2700 m. After the formation of Garo-Rajmahal gap in the Pleistocene, the sedimentation pattern in Bengal Basin changed completely and a tremendous rate of growth started in the southern part of the basin. The edge of the stable shelf is marked by a zone of flexture (the hinge zone of the basin). The “deeper parts” of the basin which lie beyond the hinge-zone, occupy most of the eastern and southeastern parts of Bangladesh.

HYDROGEOLOGY OF GANGA/ PADMA INTERFLUVE OF WEST BENGAL ADJOINING TO THE INTERNATIONAL BORDER WITH BANGLADESH

Aquifer disposition: Ground water exploration, in the area lying to the east of the river Bhagirathi up to the international boundary falling in the districts of Murshidabad, Nadia and North 24 Parganas, reveals (Fig-2 a& 2b) that in general, three aquifer systems exist within the depth range of 100 mbgl, below 120 – 180 mbgl and 200 –250 mbgl. The individual aquifer system consisting of two to three aquifers (which vary from place to place), are separated by thin clay layers which are in lensoid form and are not regionally extensive. The material of the first aquifer system (10-40 m thick) is generally fine to medium grained sand. The aquifer material of the second aquifer system (5-30 m thick) is medium to coarse grained of Pleistocene age while that of the third aquifer system (5-20 m thick) coarse grained with gravel at places which is of Pleistocene to Tertiary age. The thickness (5-40 m) of the individual aquifer gradually decreases with the depth. In the northern part of the area, falling in Murshidabad and northern part of Nadia district the first aquifer gradually gets clubbed with the second aquifer without appreciable clay barrier.


In the district of South 24 Parganas, aquifers at different depth levels are composed predominantly of fine to very fine sand. It has also been found that towards the south in the active delta part, the aquifers are separated by thick clay layers. These clay layers control hydrochemical properties of the different group of aquifers. The lower group of aquifers relatively fresh water bearing and generally occurs between the depths of 180 - 350 m. The clay blanket here ranges in thickness between 30 - 70 m and overlies a succession of sand and gravel sequence.

Ground water Condition: The depth to water level in the area under consideration varies from 2.11 to 6.70m bgl during post-monsoon period and 2.35 to 9.46 m bgl during pre-monsoon period (2008). Average seasonal fluctuation of water level varies from 1.47 to 4.14 m from April to November. The upper aquifer is developed mainly through Low duty to heavy duty tubewells used mainly for irrigation and are capable of discharging 30 to 150 m³/hr. Deep tubewells are mainly constructed for drinking water supply and are capable of discharging 50 to 100 m³/hr. Transmissivity ranges from 1000 to 4500 m²/day in the upper aquifer and 758 to 2000 m²/day in the deeper aquifer. Storativity of the aquifers ranges from 1.1x10^-2 to 3.5x10^-2.

Ground water quality : In general, ground water in shallow aquifers within the depth of 100 mbgl, except in South 24 Parganas and parts of North 24 Parganas districts (coastal area), is potable and within the permissible limit of drinking water standard as referred by BIS, except the occurrence of arsenic in sporadic manner. High arsenic in ground water (>0.05mg/l) has been detected in a sporadic manner in 79 blocks of west Bengal fringing the Bangladesh mainly within the depth of 80m bgl and most of the blocks are in the eastern part of the river Bhagirathi. It has been established that arsenic in ground water in this area is geogenic and mainly restricted within the recent to sub-recent alluvium deposits. Ground water exploration in the arsenic infested area reveals the existence of deeper arsenic free aquifer and tubewells constructed adopting cement sealing techniques are capable of yielding 10 to 30 lps of arsenic free water which are being supplied for drinking water purposes. High arsenic in ground water within the upper aquifer is also common in adjoining Bangladesh. In the coastal tract of West Bengal the top aquifers down to 150 mbgl is brackish which extends eastwardly into Bangladesh. Existence of deeper potable aquifer has also been Identified in West Bengal and suitably designed tubewells are capable of yielding fresh potable water in coastal tract of West Bengal.

Ground Water Resources: In the area under discussion, ground water occurs under unconfined condition in 52 blocks of the districts of Murshidabad, Nadia and North 24 Parganas and the dynamic ground water resources (as on March, 2004) of the blocks has been estimated based on the GEC 1997 methodology and 3rd M.I. Census, 2000-01, projected upto March, 2004 and reconciled with the concerned State departments. In the rest of the areas, falling in South 24 Parganas district and in 5 blocks of North 24 Parganas district, ground water occurs under confined condition and the ground water resource estimation for the area has not been done. The net ground water availability of the 52 blocks has been estimated as 491876 ham/ year. Large scale development has been done in the area, specially for irrigation through 1411 deep tube wells, 218842 shallow tube wells and 2 dug wells (as per 3rd Minor Irrigation Census, 2000-2001), which is reflected by 89% stage of ground water development figure in the area. The estimated gross ground water draft for all uses is 438490 ham, of which 95.55% (418969 ham) is for irrigation only. Based on the stage of ground water development and long term pre and post monsoon water level trend, 17 nos of blocks (11 in Murshidabad and 6 in Nadia districts) have been categorized as Semi-critical and the rest of the blocks (35 nos.) are under ‘Safe’ category.

Shared Aquifer : It is observed from the available literature that the first aquifer within the depth of about 100 mbgl as observed in the N-S profile of holocene sediments across Ganga delta (Fig-3) is continuing in Bangladesh (Bahadulabad-Gabargaon-Sirajgang—Sibaloya-Faridpur-Khulna-Mongla tract) (Fig-4) The information regarding deeper aquifers in Bangladesh is not available. However from the sub-surface lithological correlation diagram (Fig-5), it appears that the shallow as well as deeper aquifers continue beyond the international boundary. This is also corroborated by the fact that:

- Depositional history of the transboundary aquifers is the same being the part of the Bengal Basin.

- Regional ground water flow, which follows the master land slope, is towards south-east on the either side of the boundary. However, pattern of the water table configuration may behave differently in localized patches based on the draft component.

- In coastal tract of West Bengal, salinity in ground water is common which is also continuing in Bangladesh.

- The nature of crops grown by ground water is common in West Bengal and Bangladesh indicating same nature of chemical condition of soils.

- The ground water irrigation is common in both side of the boundary through low duty as well as heavy duty tube wells which indicates that the shared aquifers are potential and prolific in their water yielding capacities.

- The shallow aquifers (within 100 mbgl) in West Bengal are sporadically arsenic contaminated and the same scenario has been projected for Bangladesh also (Fig-6). Since the geogenic origin of the contamination has been established, the provenance of sediments in both the countries appears to be same.

RELATION BETWEEN TRANSBOUNDARY AQUIFERS AND TRANDSBOUNDARY RIVERS/STREAMS

Source of Water for both surface water and ground water is the annual rainfall. Surface water commonly is hydraulically connected to ground water but their interactions are difficult to observe and measure. Streams interact with ground water in three basic ways.

1. Streams gain water from inflow of ground water through this stream bed (gaining stream),
2. they loose water to ground water by outflow through this stream bed (loosing stream),
3. they do both, gaining in some reaches and loosing in other reaches

However, ground water contributes to streams in most physiographic and climatic settings. An analysis indicates that on an average (which varies in time & space), 52% of the stream flow is contributed by ground water, which ranges between as low as 14% as high as 90%. Withdrawing water from shallow aquifers that are directly connected with surface water bodies, can have a significant effect on movement of water between these two bodies, specially when in number of wells withdraw water from safe aquifer over large areas.

The subsurface zone where stream water flows through short segments of its adjacent beds and banks is referred to as “Hyporheic” zone. The size and geometry of hyporheic zone surrounding streams vary greatly in time and space. Because of mixing between ground water and surface water in the hyporheic zone, a chemical and biological character of the hyporheic zone may differ markedly from adjacent surface water and ground water. Contaminated aquifers that discharge to streams can result in long term contamination of surface water, conversely streams can be source of contamination of aquifers.

It is therefore evident that any development and management plan on transboundary aquifers can not be viewed in isolation without considering the impact that the shared aquifers may undergo due to different international agreements and treaties on sharing of water from transboundary rivers.

STEPS TO BE ADOPTED FOR DEVELOPMENT AND MANAGEMENT OF TRANSBOUNDARY AQUIFERS (INDIA & BANGLADESH):

- The dynamic ground water resources of upper unconfined aquifer (upto depth of 100m bgl) in Bengal Basin have to be assessed in totality and its component in small sectors (like block/ taluk/ administrative division) can be assessed separately. This will give the replenishable ground water recharge into the aquifer as a whole as well separately in the different small sectors. Also the existing ground water draft needs to be calculated in Bengal Basin and its component in different sectors as described above. This can give a clear picture for future ground water resources to be utilized in different sectors. Accordingly the scope of development of the aquifer can be planned. This study is required to be done jointly with the team of both the countries (India & Bangladesh).

- In the vicinity of the International boundary it needs to be monitored that any pattern of ground water trough in the transboundary aquifers does not get developed due to significant ground water abstraction by the individual country. If possible measures/ legislation may be imposed in such cases in the vicinity of international boundary.

- In case of deeper confined aquifer (in these case aquifers below 120 m bgl) the recharge area of the aquifer has to be clearly assessed by the individual country. If recharge area of an aquifer of a country lies in other country (like some deeper aquifer in Bangladesh may have recharge area in India) then judicial measures may be adopted jointly by both the countries, so that the deepening of piezometric level of the aquifer in the territory of one individual country may not pose hazardous environmental impact like possibilities of contamination, land subsidence etc. on the other country. In this respect it is better to evaluate the ground water flow in a particular area and its judicious use so that regional flow pattern does not get unduly changed.

- Constant monitoring of water level and water quality in shallower & deeper aquifers has to be assessed jointly by both the countries specially adjacent to the international boundary which can give the future implementation policies for further ground water development of the transboundary aquifers.

- Aquifers contributing water to the surface water bodies like gaining rivers bordering the international boundary or flowing from one country to other should be developed cautiously to maintain the surface water flow as well as to prevent the possibilities of contamination of ground water from the surface water flows. Similarly the rivers loosing water/ recharging water into ground water bodies can be judiciously used to avoid further contamination.

- Scope for artificial recharge in the depleted aquifer may be taken up by the individual country in consultation with the neighboring country to recharge water of desired quality.

- Modern management practice for irrigation may be adopted for judicious use of ground water. In this context crop water requirement should be taken into account and low water required crop may be adopted changing the cropping pattern in the water level depleted areas.

- Cautious approach requires to be taken up during the construction of wells so that, upper arsenic contaminated aquifer does not get hydrologically connected with the deeper arsenic free aquifer during the construction of tube well/bore well. In this context it may be mention that cement sealing techniques has yielded positive results to get arsenic free water from deeper aquifer.

Abhijit Ray
Regional Director( Retd), Central Ground Water Board