ABSTRACT

Ground water, once in abundance, is now becoming a critical resource with fast development and ballooning population. Thus the precise estimation of this resource becomes imperative for sustainable development of the country in future. Patna urban agglomerate lies in the Lower Ganga plains and depends completely on ground water resources. On an average about 76 mcm of water is being withdrawn annually.

It is seen that there is a large gap between ground water recharge of 45 mcm/ year calculated on water level fluctuation basis and the draft of over 76 mcm/ year. But there is no appreciable decline in water levels. Thus, it is obvious that calculated resource on the basis of water level fluctuation method does not reflect the true potential of deeper aquifer as it is laterally being recharged from distant sources. Aquifers of the Patna urban area are under semi-confined condition with storativity value as high as 0.008 i.e. almost on the threshold of the unconfined aquifer and yet yielding copious volume of water without any significantdecline in water table over the decades. Thus, it is apparent that ground water resources of 124 mcm of deep aquifers calculated on the basis of storativity value is sufficient enough to sustain that massive withdrawal. It is thus, safe enough to calculate the ground water resources of deep aquifers in general by considering the storativity value of 0.0099, of the aquifers lying under semi-confined to confined conditions, and without causing any appreciable decline in water level over the time.

Key Word: Evaluation, Aquifer, Resource, Ganga Plains.

INTRODUCTION

Ground water, once in abundance, is now becoming a critical resource with a fast development and urbanization, thus the precise estimation of this resource becomes imperative for its planning, development and conservation. Patna urban agglomerate having population of over 1.9 million and lying on the southern bank of the river Gangaof Lower Ganga Plain depends completely on ground water resources for all its water requirements. There are about 93deep tube wells besides innumerable shallow tube wells dug in almost every house and tapping the same aquifers. A very large volume of water to the tune of 76 million cubic meters (mcm) is being withdrawn annually within the Patna urban area to cater its requirement. Thus, the ground water comes under stress and there issome decline in water table which may aggravate in future. This paper deals to establish a mass balance relation in ground water recharge & draft by estimating the resources with storativity of the aquifers.

HYDROGEOLOGY

Patna urban agglomerate, lying in the lower Ganga plain, is underlain by the typical sediments of Sone-Ganga flood plains. It is a monotonous plain with subdued relief feature other than the marked levees on the bank of Ganga river. Exploratory drilling carried out down to a depth of over 300 m in Patna town by Central Ground Water Board revealed that there is thick sequence of alluvium sediments having the basement over 600 m deep as estimated by Geological Survey of India (GSI). Alluvial sediments are cyclic in nature and generally it is fining upward. The sediments lying below the Ganges sediments are in contrast and it is very coarse sand to gravely in nature. There are three sets of aquifers (fig-I) within the explored depth of 300 m, namely (i) The Ganga Aquifer, (ii) The SoneAquifer and (iii) The infra Sone Aquifer. The infra Sone aquifer lying below about 120 m comprises highly angular, medium to very coarse grained to gravely, grey coloured sediments attains the thickness of 60-90 m. Lying above the infra Sone aquifer is the Sone aquifer and comprises well rounded, medium to very coarse grained of typicalyellow colour sediments attains the thickness of over 60 m in general and large thickness in palaeocourses of the Sone river. Lying on the top and separated from the Son aquifer by thick clay is the Ganga aquifer comprising medium to fine grained platy sediments of typical grey colour which attains the small thickness of about 15-20 m. The Sone aquifer system is very prolific aquifers. It is apparent from the geological section (fig-II) across the riverGanges from Karbigahia (near Patna Railway Station) to Golghar to Konhraghat on the northern bank in Vaishali district that the aquifers of Patna area are abutted against a 80 m thick clay in the river bed and there is no hydraulic continuity with the river water for an appreciable length. Thus, they are not receiving any water as affluent recharge from the Ganga river.

AQUIFER CHARACTERISTICS

Deep aquifers of the study area are under semi confined condition. Individual aquifer characteristics are not known. However, tube wells constructed under exploratory drilling program of Central Ground Water Board (CGWB) have tapped number of aquifers within the depth range of 75-200 m below ground level (m bgl). Thus, a combined aquifer properties of about 125 m thick aquifers of this area are known.

Depth to water level of deep aquifer varies from 9.00 to 16.00 m bgl during pre-monsoon period. The specific capacities of 18 tube wells of this area vary from 74 to 94 m³/ hr/ m. The transmissivity of aquifer system has been found to vary from 5900 to 9900 m²/ day with storativity value varying from 0.5 10^-2 to 0.77 10^-2.

RESOURCE ESTIMATION

Patna urban agglomerate is spread over an area of about 100 sq km along the Ganga river with a stretch of about 20 km from Danapur in the west to Patna city in the east, having average width of 5 km. Considering the average annual ground water fluctuation of 2.50 m over the area and the specific yield of 18%, the resource as per norm of the Ground Water Estimation Committee-1997 (GEC-1997) would be as follows:

GROUND WATER WITHDRAWAL

A total of 93 deep tube wells, within the depth of 200 metres and tapping the aquifer in the depth range of 70- 200 m bgl, are catering the entire water requirements of the Patna urban area apart from a large number of shallow tube wells dug in individual houses and tapping the same aquifers. The discharge of deep tube wells on an average is 225 m3/ hr and each one is being run on average for 10 hrs/ day during the year. Thus, the total annual groundwater draft would be as follows:

Now, it is seen that there is a large gap between ground water recharge of 45 mcm/ year and the draft of 76.40 mcm/ year. It is, thus, apparent from this fact that aquifers would have been desiccated drastically and water level would have gone down considerably whereby causing the failure of tube wells within few years. But this is not the fact. Then there should be an inflow of excess volume of water exceeding the draft component. Thus, it is obviousthat calculated resource on the basis of water level fluctuation method does not reflect the true potential of deeper aquifer as it is laterally being recharged from distant sources and having rich potential.

GROUND WATER RESOURCE OF DEEP AQUIFER

It is found that storativity of aquifer systems in Patna urban areas varies from 0.5 10^-2 to 0.77 10^-2 and the tube wells are yielding copious volume of water( 250m3/yr) with a meager drawdown of 3^-4 m. It is testified that with these storativities the peizometric head in the semi-confined aquifers would not fall appreciably unless it becomes unconfined i.e, storativity value increases above 0.0099. This value is slightly above the observed storativity value of 0.008. Thus, we can take this storativity value of 0.0099 safely for resource estimation of deep aquifers.

Thus, it is apparent that the ground water resource of 124 mcm of deep aquifers calculated on the basis of storativity is sufficient enough to sustain the massive withdrawal of ground water in Patna urban area.

CONCLUSION

The aquifers of the Patna urban area are under semi-confined condition with storativity value as high as 0.008 i.e. almost at the threshold of unconfined aquifer value and yet yielding copious volume of water with a meager drawdown and without any significant decline in water table over the decades. It is, thus, safe enough to calculate the ground water resources of deep aquifers lying under semi-confined to confined conditions by considering0.0099 as storativity value for resource estimation for safe use of deep aquifers over the time.

ACKNOWLEDGEMENT

The author expresses his deep gratitude to Shri Sushil Gupta, Chairman, Central Ground Water Board (CGWB) for encouragement and guidance in writing the paper. Shri R. S. Singh, Ex Director, CGWB, MER, Patna is also acknowledged duly for rendering technical support and encouragement. Sri S. S. Ganguli, Geophysicist (STA), CGWB, MER, Patna is also acknowledged thankfully for conducting geophysical survey in the Ganga riverbed andinterpretation of the geophysical data. My all colleagues who helped me directly or indirectly in preparation of this paper are also acknowledged thankfully.

REFERENCES
Roy, A. K. & Sinha, S., Geological Survey of India (1968), Ground water resources of Bihar with special reference to the problems of planning and development for irrigational purposes.
Ministry of Water Resources, government of India (1997), Ground Water Resource estimation Methodology-1997.

K. K. Singh & B.K.Mishra
Scientist-D, Central Ground Water Board, NR, Lucknow.
Professor, Department of Geology, Patna University, Patna