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