Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Dipankar Saha ¹, Y. R. Dhar ¹, P. K. Sikdar ²

Affiliations
1 Dept. of Applied Geology, Indian School of Mines, Dhanbad, IN
2 Dept. of Environment Management, Indian Institute of Social Welfare and Business Management, Kolkata, IN

Abstract

The study area is spread over 1950 sq. km and covers apart of Pleistocene deposists in the Ganga Plain. A two tier aquifer system made up of sands of various grades occurs in the area and caters to the entire water demand to the tune of 0.12 MCM/sq. km/year. The top 30 m of the alluvial deposits are dominated by clay, sandy clay, silt with thin lenses of sands. The latter constitutes the Shallow Aquifer, occurring under unconfined condition. The Deep Aquifer is made up of interconnected sand layers below 30 m depth, forming a potential zone, where groundwater occurs under semiconfined condition. Sluggish hydraulic conductivity in Shallow Aquifer results in higher mineralization of groundwater than In Deep Aquifer. Principal Component Analysis with 10 chemical constituents, and plots in Expanded Durov Diagram indicate distinctly different geochemical processes in Shallow and Deep Aquifers. In Shallow Aquifer the processes shaping up the chemical character of groundwater are non-exchange, sediments dissolution return seepage from irrigation water and rain-water lnfiltration. But the major processes in Deep Aquifer are leakage from Shallow Aquifer, followed by ion-exchange and weathering of silicate minerals. In the process of ion-exchange, Na⁺ from the aquifer matrix dominated by clays and sandy clays replaces Ca⁺² in groundwater, resulting in formation of patches of calcium carbonate nodules.

Keywords

Hydrochemistry, PCA, Ion-Exchange, Leakage, South Ganga Plain, Bihar.

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Dipankar Saha ¹, S. Upadhyay ¹, Y. R. Dhar ², R. Singh ³

Affiliations
1 Central Ground Water Board, 6th Floor, Lok Nayak Bhawan, Patna - 800 001, Bihar, IN
2 Department of Applied Geology, Indian School of Mines, Dhanbad, Jharkhand, IN
3 Department of Geology, Vinoba Bhave University, Hazaribagh, Jharkhand, IN

Abstract

The geometry and disposition of the Quaternary alluvial aquifers laid on northerly dipping Precambrian basement stretching between the southern margin of the Gangetic Plain bordering Precambrian highland and the present course of the Ganga river, stretching over 680 sq. km has been studied in detail. The three morphostratigraphic units of the alluvial deposits, viz., Nawada Formation (Upper Pliestocene to Lower Holocene), Fatwa Formation (Middle to Upper Holocene) and Diara Formation (Recent) have been investigated for the aquifer systems and variation of their aquifer hydraulic parameters. The unconsolidated sand layers of Upper Pleistocene to Recent age constitute the productive aquifers which are often inter-Layered with clay or sandy clay beds, particularly in Nawada Formation. In major part of the Nawada Formation, the cumulative thickness of sand layers varies from 20 to 40 m in Nawada Formation, and the basement occurs within 150 m below ground level. In Fatwa and Diara Formations the cumulative thickness of sand layers is more than 200 m within the drilled depth of 300 m. Regionally these sand layers behave as a single aquifer system, overlain by a regional clay blanket with a varying thickness of 20-50 m. The ground water within the sand layers occurs under semi-Confined to confined condition. Transmissivity ranges from 74.9 to more than 20,000 m²/day. The potentiality of the aquifers increases towards north but there is considerable improvement in aquifer parameters from Nawada to Fatwa Formation. In Nawada Formation, the hydraulic conductivity generally ranges from 10-20 m/day and the average yield factor has been found to be 0.99 m³3/hr/m/m respectively. Based on the hydrogeological properties and aquifer hydraulic parameters, the alluvial plains has been divided into three zones. The Zone-1, covering the Fatwa and northern part of the Nawada Formations, is characterised by highest ground water potential. The safe distance between deep tube wells in this unit worked out to be 20 km for 8 hours of pumping with 150 m³/hr discharge. The safe distance in Zone-2 covering the central part has been found as 6.5 km for 6 hours pumping with 100m³/hr discharge. The Zone-3 covering the southern part, the safe distance worked out as 1 km for 4 hours pumping with 50 m³/hr discharge.

Keywords

Aquifer, Transmissivity, Storativity, Formation, Specific Yield, Safe Distance, Ganga, Bihar.

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