michael and voss 2009a
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Estimation of regional-scale groundwater ow propertiesin the Bengal Basin of India and Bangladesh
Holly A. Michael & Clifford I. Voss
Abstract Quantitative evaluation of management strate-gies for long-term supply of safe groundwater for drinkingfrom the Bengal Basin aquifer (India and Bangladesh)requires estimation of the large-scale hydrogeologicproperties that control ow. The Basin consists of astratied, heterogeneous sequence of sediments withaquitards that may separate aquifers locally, but evidencedoes not support existence of regional conning units.Considered at a large scale, the Basin may be aptlydescribed as a single aquifer with higher horizontal thanvertical hydraulic conductivity. Though data are sparse,estimation of regional-scale aquifer properties is possiblefrom three existing data types: hydraulic heads, 14Cconcentrations, and driller logs. Estimation is carried outwith inverse groundwater modeling using measuredheads, by model calibration using estimated water agesbased on 14C, and by statistical analysis of driller logs.Similar estimates of hydraulic conductivities result fromall three data types; a resulting typical value of verticalanisotropy (ratio of horizontal to vertical conductivity) is104. The vertical anisotropy estimate is supported bysimulation of ow through geostatistical elds consistentwith driller log data. The high estimated value of verticalanisotropy in hydraulic conductivity indicates that evendisconnected aquitards, if numerous, can strongly controlthe equivalent hydraulic parameters of an aquifer system.
Keywords Bangladesh . India . Inverse modeling .Parameter estimation . Geostatistics
Elevated levels of dissolved arsenic in groundwater ofthe Bengal Basin are causing widespread poisoning ofmillions of people in India and Bangladesh who rely onwells for drinking water supply. This crisis hasprompted numerous studies of the hydrogeochemicalprocesses that lead to arsenic release from sediments aswell as development of mitigation technologies such aslters, which, though effective, are not widely used.The distribution of dissolved arsenic in aquifer sedi-ments is not uniform; however, deep groundwater(greater than 150 m depth) is consistently low inarsenic (BGS and DPHE 2001; Van Geen et al. 2003;Ravenscroft et al. 2005). The deep low-arsenic ground-water may be a viable source of safe drinking water,but evaluation of sustainability requires analysis of theentire Bengal Basin aquifer system and large-scalegroundwater ow patterns.
Numerical modeling of groundwater ow can providequantitative insight into sustainability (e.g., Michael andVoss 2008), but requires input of hydrogeologic propertiesand aquifer structure that are not fully known throughoutthe Basin. Moreover, the very large scale of the system,which spans hundreds of kilometers, requires simplicationand estimation of equivalent (in the sense of Sanchez-Vilaet al. 2006) hydrogeologic properties that, when used in agroundwater ow model, would produce ow patterns thatclosely approximate the true ow patterns on the temporaland spatial scale of interest.
Accurate estimation of groundwater model parame-ters can be difcult, particularly for very large aquifersystems with limited data. Because of this, parametersare best estimated using all available data, includinggeologic knowledge and hydraulic, geochemical, andlithologic measurements. Each type of data providesinformation on different parts of the aquifer system andover different support volumes. Hydraulic heads reectdiffuse properties over a volume of inuence, ground-water age is inuenced by owpaths to measurementlocations, lithologic observations are point informationin space, and geologic knowledge informs understand-ing of trends, zones, and aquifer architecture. Differenttypes of data are used to estimate parameters indifferent ways. Inverse modeling incorporates hydraulic
Received: 20 May 2008 /Accepted: 2 February 2009Published online: 10 March 2009
* Springer-Verlag 2009
H. A. Michael ())Department of Geological Sciences,College of Marine and Earth Studies,University of Delaware, Newark, DE 19716, USAe-mail: email@example.com
H. A. Michael :C. I. VossUS Geological Survey,431 National Center, Reston, VA 20192, USA
Hydrogeology Journal (2009) 17: 13291346 DOI 10.1007/s10040-009-0443-1
and geochemical data into a groundwater modelingframework to infer parameters that result in the bestmatch between model results and data; such methodshave been widely used in groundwater modelingapplications (see e.g., Poeter and Hill 1997; Carrera etal. 2005; Sanz and Voss 2006). Properties can also beinferred based on knowledge of geology and lithology,the small-scale heterogeneity of which denes theaquifer fabric, which controls uid ow on a largerscale. Upscaling literature has addressed many methodsto infer equivalent or effective properties from geologicdata or knowledge of spatial correlation structures (seee.g., Wen and Gomez-Hernandez 1996; Renard and deMarsily 1997; de Marsily et al. 2005; Sanchez-Vila etal. 2006).
The goals of this paper are: (1) to synthesizegeologic knowledge of the Bengal Basin in order tocreate a simplied conceptual model of the aquifersystem architecture, (2) to estimate equivalent hydraulicparameters for this model using all hydrogeologic andsedimentologic data available when this study wasbeing conducted, and (3) to identify particular needsfor future data collection that would improve parameterestimates and reduce uncertainty. The paper begins witha discussion of Bengal Basin physiography, sedimenta-tion, and hydrogeology that forms the basis of aconceptual model of homogeneous anisotropy that mayvary among regions of the Basin. Three methods forparameter estimation are used, with varying degrees ofsuccess, illustrating the effectiveness of each methodunder data constraints and the utility of types ofhydrogeologic data. Lastly, spatial distributions ofhydraulic conductivity (K-elds), having the sameequivalent regional horizontal and vertical hydraulicconductivity values as estimated from data, are pre-sented in order to corroborate the conceptual model andto better understand the functioning of highly hetero-geneous, anisotropic aquifer systems.
PhysiographyThe Bengal Basin lies primarily in Bangladesh and thewestern portion in West Bengal, India (inset, Fig. 1); themajor physiographic and geographic features of the Basinare shown in Fig. 1a. The northern boundary of the Basinis formed by the Himalayan Mountains, the source areafor the Ganges and Brahmaputra rivers. On the west, theBasin is bounded by the hard rock Rajmahal Hills, whichrise out of the oodplains west of the Hooghly River. Inthe northeast, the Precambrian Shillong Plateau controlsthe eastern migration of the Brahmaputra River andsupplies sediment to the surrounding oodplains, includ-ing the subsiding Sylhet Basin to the south. The majorsedimentary geomorphological units of the Basin areTertiary Hills, Pleistocene Uplands (Barind Tract andMadhupur Terrace), and Holocene Plains, which includethe northern piedmont plains, meander oodplains, andthe tidal plains of the Southern Delta (MPO 1987, Ahmedet al. 2004).
The Tertiary Hills, a fold belt consisting of a seriesof NS trending anticlines and synclines, form theeastern boundary of the Bengal Basin. The sedimentarysuccession, composed of Quaternary sediments under-lain by sequences of shale and sandstone, can be morethan 6 km thick and sedimentary units as old asMiocene age have been identied in outcrops (Sikderand Alam 2003). The two major areas of Pleistoceneuplands are the Barind Tract in the northwest part of theBasin and the Madhupur Terrace in the central region(Fig. 1). These older alluvial deposits are similar ingrain size to recent Basin sediment (Morgan andMcIntire 1959) and a thick clay layer covers the surfaceof both blocks (Ahmed et al. 2004). There are manydistinct Holocene plain areas within the Bengal Basin(Ahmed et al. 2004); in general these uvio-deltaicdeposits are characterized by complex patterns of
Fig. 1 Major physiographicregions and geographicalfeatures of the Bengal Basin(adapted from Goodbred et al.2003, EROS 2002; and Persitzet al. 2001). Dashed line is theborder between India andBangladesh. a Grayscalerepresents elevation of landsurface (EROS 2002) and bluerepresents major surface-waterbodies. b Physiographicregions used in spatialstatistical analysis. Colorsdifferentiate four regionsfound to have hydraulicparameter values that are sig-nicantly different from theother regions. Inset givesgeographic location
Hydrogeology Journal (2009) 17: 13291346 DOI 10.1007/s10040-009-0443-1
unconnected layers of clay, silt, sand, and gravel,creating a highly heterogeneous aquifer architecture.
GeologyThe Bengal Basin is one of the largest uvio-deltaicsystems in the world; its sediments extend over an area of200,000 km2 (Uddin and Lundberg 1999) and to depths ofmore than 20 km (Shamsuddin et al. 2002). The Basin islled with orogenic sediments deposited by the Ganges-Brahmaputra-Meghna River system that drains an area ofmore than 1.7 million km2 (Allison 1998) and carries anestimated 1109 tons of suspended sediment per year(Coleman 1969). Consideration of stratigraphy and sedi-mentation give insight into the likely structure of both thesmall-scale aquifer fabric and the large-scale geometry oflayers.
Sediment deposition into the Bengal Basin has oc-curred since the Late Cretaceous (Johnson and Alam1990), and sedimentary successions since the Permianhave been identied in the western part (Khan 1991; Alamet al. 2003). However, the bulk of sedimentation withrelevance to present-day hydrogeology began in themiddle Miocene (Uddin and Lundberg 1999; Allison1998). A generalized description of the vertical strati-graphic s