The Aquifer System and Evaluation of its Hydraulic Parameters in Parts of South Ganga Plain, Bihar

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.