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Geomorphic settings of an area provide valuable supplementary information regarding groundwater recharge, their occurrence and distribution. The geomorphic settings of the Lower Subansiri Basin can broadly be represented by three distinct geomorphic units viz., structural hills, piedmont zone and alluvial plain. While the elevation, slope, lithology, drainage pattern and various relevant morphometric parameters vary from one geomorphic unit to another, the conditions of recharge and discharge, occurrence and distribution of groundwater also differ in different units. The structural hills occupying only 4.5% of the area along the north-western boundary represent a high run-off zone characterised by steep slope and fairly dense parallel to sub-parallel drainage. The piedmont zone, built up by the coalescence of alluvial fan deposits, represents 7.7% of the area occurring in a long and narrow NE-SW trending steeply sloping belt along the foothills of Arunachal Pradesh. Owing to high permeability, this zone hardly retains any water and hence forms a high recharge zone with relatively deeper groundwater level. The alluvial plain, covering 87.8% of the basin area and characterised by a gentle slope, serves both as recharge and discharge areas where groundwater occurs relatively close to the ground surface. Panel diagrams prepared for the Lower Subansiri Basin showing the thickness and extent of granular zones display that the unconsolidated alluvial sediments are primarily composed of sands of various grade and gravel with minor amounts of silt and clay. The sand-gravel isolith maps showing the cumulative thickness of the granular zones down to the depth of 40 m reveal that the subsurface formations in a major part of the alluvial plain of the Lower Subansiri Basin are entirely represented by granular zones. The granular zone in most parts of the area forms one single aquifer system where groundwater mostly occurs under unconfined to semi unconfined conditions. Due to the presence of thin clay and/or sandy clay lenses at shallow depths, however, the single aquifer system gets locally dissipated into multiple aquifer system where, barring the uppermost aquifer, groundwater mostly occur under semiconfined to confined conditions. The overall regional variation of depth to water level, from the piedmont zone in the north and north-west to the alluvial plain of the south and southeast, is controlled by the prevalent geomorphic settings of the area. The disposition of water table contours of the area indicates that the configuration of groundwater table closely controls to that of general topography of the area. The steeper hydraulic gradient observed to be present in the north and north-east indicates the possible control of the characteristically distinguished geomorphic setting, i.e., topography, relief and lithology.

The Brahmaputra Valley comprising a thick pile of unconsolidated alluvial deposits encompasses an area of about 63,450 km2 in the state of Assam, India. It is composed of unclassified Older and Newer alluvial deposits consisting of boulders, cobbles, pebbles, gravels as well as sand of various grades, silt, and clay. The area is occupied predominantly by Recent and Older floodplains represented by udifluvents (younger alluvial soils), and palenstalfs and haplaquents (older alluvial soils). The Piedmont zone fringing the northern foothill region of the valley is represented by usthorthents (Bhabar Soils) and haplaquolls (Terai Soils). Hydrogeologically, the northern and southern banks of the river Brahmaputra differ considerably. The northern bank is generally characterised by a single aquifer system of great thickness and varied composition down to a depth of about 150 m, where mostly an unconfined aquifer is found. Semiconfined aquifers are also seen at places where the sediment s are stratified. Shallow tube wells yield 30 to 60 ml/hr for a drawdown of 1- 2 m. The yield of deep tube wells is found to range from I00 to 250 ml/hr for a drawdown of 4- 10 m. The depth to water table in the piedmont zone varies from 5 to 40 m with an average fluctuation of 3-10 m. The average hydraulic conductivity of the aquifer system is found to range from 30 to 200 m/day. The southern bank is characterised by the presence of unconfined and semiconfined aquifers. Some perfectly confined aquifers as well as some perched aquifers are also encountered locally. Shallow tube wells can yield 25- 35 ml/hr for a drawdown of 2 m while deep tube wells are capable of yielding 100-250 ml/hr for a drawdown of 4-8 m. The water table rests within a depth of 2-6 m from the land surface. The average hydraulic conductivity varies from 30 to 200 m/day. The decadal variation of water table in the alluvial aquifer of the Brahmaputra Valley indicates a rising trend of water level. Dynamic groundwater potential of the shallow aquifer zone is within the range of 15,440 mcm. Hydrochemically, the groundwater of the Brahmaputra Valley is found to be suitable for domestic and irrigation purposes, except for higher concentration of iron. The pH value ranges from 6.8 to8.7, indicating neutral to slightly alkaline nature of groundwater. Low value of electrical conductivity and total dissolved solids implies fresh nature of groundwater. Piper trilinear plot reveals the preponderance of alkaline earths (Ca, Mg) and weak acids (HCO3 + CO3) over alkalis (Na, K) and strong acids (SO4, Cl) in the groundwater of the majority of the study area indicating Ca + Mg- HCO3   type.