Groundwater Resource Assessment in India-Some Emerging Issues

Transcription

1 Journal of Groundwater Research, Vol./, December Groundwater Resource Assessment in India-Some Emerging Issues D. C. Singhal Professor, Emeritus Fellow, Department of Hydrology Indian Institute of Technology, Roorkee-, India Abstract India has an area of.x km with its population crossing the. billion mark recently. Due to regional imbalances in the supply and demand of water and due to human interventions, overexploitation of groundwater is taking place both in the alluvial tracts of north India and in the hard rock areas of Peninsular India. The present methodology of groundwater resources assessment in India as practiced by the apex groundwater agencies in the country is mainly based on the mass balance concept and is largely based on renewable groundwater recharge through the dynamic zone of water table fluctuation and is estimated to be about BCM. However, it excludes the static groundwater resources present in storage water within the deep aquifers. In the alluvial areas, these resources are renewable and may get replenished over long periods from recharge areas near the mountains. The tentative estimate of this static fresh groundwater is about, km. As of now, almost % of the groundwater assessment units (blocks/mandals/talukas/watersheds) of the country are categorized as overexploited or are in critical stage. However, the groundwater situation in alluvial areas of north India is not as disappointing as it may appear. Here, the deep in storage groundwater resources may allow enough groundwater development if the method of systematic aquifer mapping is resorted to in these areas for realistic assessment of groundwater resources present in the deeper aquifers. It is suggested that aphasewise development of deeper aquifers can be undertaken after proper aquifer mapping in the northern alluvial areas of the country in order to meet the increasing demand of groundwater. For accomplishing this, a network of deep observation piezometers needs to be established. It is also suggested that to begin with exploitation of deep aquifers may be commenced in the alluvial areas bordering mountains, especially in critical and overexploited units.. Introduction Groundwater is considered a strategic resource due to its usual high quality and perennial availability. However, groundwater management all over the world often lacks sustainability as evident by falling water tables, drying wetlands, increasing seawater intrusion and overall deterioration of water quality. As groundwater cannot be recharged naturally or artificially on a large scale, its sustainable management is vital. The widely used methodology of groundwater resources estimation in India practiced by Central Ground Water Board (CGWB, Ministry of Water Resources, Govt. of India) is largely based on the general mass balance approach involving safe yield. Yet, this approach does not account for groundwater in deep confined aquifers and thus, the static groundwater resources of the country are not yet accurately assessed. As per the present National Water policy of Govt. of India, all groundwater assessment is carried out in respect of dynamic resources pertaining to shallow aquifers tapped by the well hydrograph network. The common unit for such groundwater resource assessment is an administrative block, mandal or taluka. However, in some states like Maharashtra the unit for the groundwater assessment is now a watershed. This approach to assessment of groundwater resources is being recommended by the Govt. agencies for adoption in the other states of the country also (CGWB, ). However, recently, the Planning Commission of India (Planning Commission, ; Dhimanand Jain, ), have suggested some basic changes in the groundwater assessment methodology as aquifer mapping ought to form a basis for assessment of all groundwater resources, renewable and deep in sedimentary basis of the country. This approach, if complied in whole of the country will involve a visible shift in the norms for groundwater assessment and needs to be debated

2 Journal of Groundwater Research, Vol./, December elaborately for all its merits and demerits. Yet, it will ensure coverage of all groundwater present in the deeper aquifers below the zone of water level fluctuation. The deeper aquifers of passive recharge zone are believed to contain vast quantities of water which might have accumulated over past several million years (Romani, ). This calls for formulation of new approaches for a holistic development of shallow and deep (in storage) groundwater resources.. Groundwater Resources Assessment The hydrogeological set up in India varies widely. The multi-aquifer systems, explored down to m, promise extensive and productive ground water reservoirs. The peninsular shield in the south comprises discontinuous aquifers of limited potential in weathered and fissured consolidated sedimentaries, basalts and crystalline rocks. On the other hand, the coastal areas have a thick cover of Pleistocene to Recent alluvium with potential aquifers but are associated with the risk of sea water intrusion. The arid and semi-arid regions of Rajasthan and Gujarat with scanty rainfall and practically no recharge have restricted occurrence of deep aquifers tapping fossil water. Rainfall is the main source of recharge to ground water storage. Most of the groundwater development takes place from the dynamic zone of water level fluctuation in the unconfined aquifers where active recharge takes place. The annual replenishable ground water resource for the country has been estimated as Billion Cubic Metres (BCM). Out of this, BCM is available for utilization, leaving aside BCM for natural discharge. The total ground water draft is BCM of which % is for irrigation ( BCM) and % is for domestic and industrial use ( BCM). The overall stage of ground water development is %. However, there is a large variation in development of ground water in the country. Out of assessment units (blocks/mandals/talukas/watersheds), (%) are categorized as Over-Exploited where the stage of development exceeds annual replenishment and blocks/watersheds (%) are critical where groundwater development has reached a high level of development. The state wise categorizations of blocks/mandals/talukas/watersheds are given in Table.. Static Groundwater The Deep Groundwater Resource A huge potential of ground water also exists in the deeper confined aquifers found in the areas covered by alluvium in Ganga-Brahamputra plains, coastal and deltaic tracts with recharge zone in upper reaches of the basins. The ground water resources of the deeper aquifers beyond the zone of dynamic fluctuation are termed in storage ground water resources. In the alluvial areas, these resources are renewable and get replenished over long period from recharge areas near the mountains. The tentative estimate of the static fresh groundwater in the country is about, km (Table ). Table. Groundwater categorization of blocks, mandals, talukas in India (after Romani, ) S. N States/ Union Territories States Andhra Arunachal Assam Bihar Chattisgarh Delhi Goa Gujarat No.of assessed units Safe Nos. % Semicritical Nos %. Critical Nos. % Overexploited Nos. % Remarks Rest talukassaline

3 Journal of Groundwater Research, Vol./, December Haryana Himachal Jammu & Kashmir Jharkhand Karnataka Kerala Madhya Maharashtra Manipur Meghalaya Mizoram Nagaland Orissa Punjab Rajasthan Sikkim Tamil Nadu Tripura Uttar Uttaranchal West Bengal States Union Territories Andaman & Nicobar Chandigarh Dadra & Nagar Hevili Daman & Diu Lakshdweep Pondichery Uts Grand Rest blockssaline Rest blocksaline Rest blocksaline Rest RegionSaline

4 Journal of Groundwater Research, Vol./, December Table. Estimates of Static Fresh Groundwater Resources of India (after Planning Commission, ) S.No State Note: Andhra Assam Bihar Gujarat Haryana Himachal Jammu & Kashmir Karnataka Kerala Madhya Maharashtra Orissa Punjab Rajasthan Tamil Nadu Tripura Uttar West Bengal Delhi Chandigarh Fresh Groundwater Resources Alluvium/Unconsolidated Hard Rocks rocks (mham) (mham) (mham) the estimates are for the aquifer zones below the zone of water table.. In-storage ground water resource=volume of aquifer zones x specific yield. It is clear from Table- that there is little scope for additional groundwater development in Punjab, Rajasthan, Delhi and Haryana while it is limited in Tamil Nadu, and Gujarat. There are local areas of over-exploitation and falling water tables in these states. There is considerable scope for development in the remaining states. Particular efforts are needed in Madhya, Orissa, West Bengal and Assam. However, as will be seen from the following discussion the situation in some of the so called over-exploited regions of north India is not as desperate as would appear.. Status of Groundwater Use The trend of block categorization increase in the numbers of unsafe (semi critical, critical and over exploited) blocks over the last years (- ) is indicated in Table, which shows the fast depletion of resources (Kurien and Sinha, ) Table. Block Categorization based on Ground Water Use in India Block Position as in categorization within January April August the unsafe category Over-exploited Critical/Dark Subtotal, Semi-critical/grey,

5 Journal of Groundwater Research, Vol./, December. Need for Development of Deep Aquifers It is clear that the stage of ground water development is very high in the States of Haryana, Punjab, and Rajasthan and in majority of the dark and over-exploited blocks fall in these States. Yet, it has to be noted that there is ample scope of ground water development from deeper aquifers in Punjab, Haryana and U.P. The underutilization of the ground water from deeper aquifers has resulted in near stagnant conditions at depth and so it gets the necessary time for the deterioration in the quality of ground water. It was observed that calcium bicarbonate type water occurs in quality of ground water. This water gradually deteriorates to sodium bicarbonate type with depth, indicating a base exchange between the cations of ground water and the sub-surface clays. The inferior quality water seems to progress upwards as well as laterally to deteriorate the quality of water in shallow aquifers of downstream areas (Romani, ). There is a great scope of ground water development of deeper aquifers in alluvial areas of Punjab, Haryana and Western U.P. where the confined aquifers have good quality water. The flood plains in the vicinity of rivers are good repositories of ground water. A planned management of ground water in the flood plain aquifers offers an excellent scope of its development to meet the additional requirements of water. There is a need for decision making by the planners for arranging exploitation of the deep in storage groundwater resources. This author does not foresee any adverse implications in exploitation of this deep component in phases as this step is likely to result in maintaining the overall quality of the deep groundwater resources by making this precious resource available for users in times of dire need. For effective groundwater management, a network of deep piezometers should be constructed in the country for regular monitoring of the deep water levels and the quality of the in storage groundwater resources. Such piezometers have also been referred as sanctuary wells (Gupta and Sinha, ) in which pumps should also be provided for collecting sample for effective water quality monitoring. The phased programme of development of deep in storage groundwater can be commenced from alluvial areas in the critical and overexploited administrative units. Such development should also avoid the semi-confined aquifers which are indirectly connected to the rechargeable aquifers in the areas receiving direct recharge as in case of Doon Valley (Rai, ). Reportedly, the sizable task of deep aquifer mapping in the country has been recently initiated under HP-II by NGRI, Hyderabad as a pilot project.. Conclusions The recent studies by CGWB have indicated that in spite of a large increase in the number of unsafe blocks over last years, the deep aquifers in the northern states of the country have not been exploited fully and there is ample scope for phased ground water development from such aquifers in Punjab, Haryana and U.P. where the confined aquifers have good quality ground water. This is all the more essential due to the apprehension of deterioration in quality of large groundwater over long periods by processes like transportation of calcium bicarbonate rich groundwater to sodium bicarbonate type over long periods. Yet, the sustainability aspects of water resources development need to be kept in mind during the efforts being made for development of water resources and irrigated agriculture. The fact that current restriction on the ground water use to the annually rechargeable dynamic resources are presently practiced during routine ground water development is an important step towards meeting the needs of present generation without harming prospects of the future generations for which sustainability stands.

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