Brahamaputra Basin

Brahamaputra Basin
4.3 Litholog well locations
A unit of rock containing a usable amount of water is termed as an aquifer. An aquifer is an
underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, or
silt) from which ground water can be extracted using water well. Related terms include aquitard,
which is a bed of low permeability along an aquifer and aquiclude (or aquifuge), which is a solid,
impermeable area underlying or overlying an aquifer. If the impermeable area overlies the aquifer
pressure could cause it to become a confined aquifer. Aquifers may occur at various depths. Those
closer to the surface are not only more likely to be used for water supply and irrigation, but are also
more likely to be topped up by the local rainfall.
The lithology well Locations in the basin are shown in the Map 16. There are 61 litholog well
locations exists in whole basin out of which 40 well locations falls in Brahmaputra lower sub basin
while 21 well locations fall in Brahmaputra Upper sub-basin. Hydrogeologically, the Brahmaputra
basin can be divided into two distinct categories : (a) dissected alluvial plain and (b) the inselberg
zone.The first category is represented in the flood plain extending from south of the sub-Himalayan
piedmont fan zone in the north to right up to the main rock promontory of the Garo hills and the
Shillong plateau.There are a large number of buried channels consisting of gravel and sands,natural
leaves of sands and silts,back swamp/bils of silts and clays.The inselberg zone is characterized by
fractured, jointed ,and weathered ancient crystalline rocks with inter-hill narrow valley
plains,consisting of thin to occasionally thick piles of assorted sediments. The detail Inventory of
litholog well locations of Brahmaputra Basin is listed in the Annexure V: A.
Geologically in the Brahmaputra basin the major lithology group found can be categorised in
Alluvium, Bouldary formation, Tertiary and Hardrock and major rock group is Un-Consolidated
Sediments (Quaternary - Recent) and Consolidated Sediments(Pre Cambrian-Devonian and other
ages). In the piedmont and alluvial fan deposits of the Brahmaputra Valley, ground water occurs
generally under water table conditions. The water table has, in general, a southerly slope
corresponding roughly to the surface topography. In the older and newer alluvium ground water
occurs both under water table, and confined condition. The artersian belts are known to occur in a
narrow strip between Dumni Chowki and Nalbari in Kamrup district, and within a small patch in
Borpathar area of Golaghat subdivision in the Sibsagar district. In the western fringes of Garo hills,
unconsolidated alluvial and semi consolidated sedimentary formations occured. The sandstones in
that region form potential aquifers locally. In the hilly regions of the basin, water yielding capabilities
vary considerably. In this region the consolidated formations comprises of the Archaean Gneissic
Complex, acid and basic intrusive, quartzite and phyllite of Shillong Group of rocks, carbonate and
non-carbonate sedimentary rocks. These rocks possess negligible primary porosity and the
movement and occurrence of ground water is controlled by the nature of the aquifer material and
the secondary porosity created by weathering and fractures.
In the districts of Darjiling, Jalpaiguri and KochBihar in West Bengal un-consolidated sediments
(Quaternary - Recent) as aquifer material occurs and depth of first aquifer encountered / tapped
varies from shallow (