Rainfall :

Climate A & N Islands is humid tropical. Extreme winter & summer are practically absent. The mean maximum and minimum temparature varies from 20.4°C to 30.8°C. The mean relative humidity varies from 78% to 82%.

Soil :

The soils of A & N Islands are classified into seven distinct series based on various morphological characteristics in soil profile. The soils are acidic.

Geology & Hydrogeology :

The Andaman & Nicobar group of Islands are underlain by Late Cretaceous igneous rocks -- the 'Ophiolite Suite', marine sedimentary group of Palaeocene to Oligocene age and Recent to Subrecent beach sand, mangrove clay, alluvium and coralline formations. The Ophiolite suite comprises a wide variety of acidic to ultrabasic plutonic rocks and their serpentinised equivalents and essentially basic volcanicrocks occur in South, Middle & North Andaman, in sporadic patches of Little Andaman , Kamorta and Bampooka Islands in Nicobar group. The marine sedimantaries comprise greywakes, graded sandstones, siltsones, black shales and volcanogenic sediments. These fine grained deep see deposits are generally having predominantly clayey particles. The coralline formations are observable in coral islands (atolls) and they also occur in the fringe areas of the sedimentary and igneous formations and form the narrow beaches encircling the islands. The ophiolite and marine sedimentaries have undergone different phases of folding, faulting and upliftment which is responsible for the development of fractures and fissures as a result infiltrated rainwater percolates downwards.Tectonically the zone is highly active.

Marine sedimentary rocks are located to an anomalous admixure of sand and clay where clayeyresidue are predominating since clayey product is poor water yielder, these sedimentary rocks in majority of the cases do not form well developed potential aquifer system both in shallow and deeper horizons. Bore wells are not feasible in these formations and the yield of dugwells tapping the weathered mantle varies from 3000 to 5000 lpd with a dia of 4.5 to 5m and 6m depth. Depth of water level in these formations varies from 0.2 to 4.4 mbgl. All the borehole drilled in these formations were unsuccessful except one borehole at Prothrapore which is yielding brackish water (5500 EC ) and discharge 17000 ltr./hr.

The igneous rocks also do not form potential aquifers but in comparison to the sedimentaries they yield moderately. These rocks form aquifer both in weathered mantle and fractured basement rocks which are generally developed through dugwell, ponds and private borewells. Out of six successful borewells of CGWB 5 nos. were constructed in igneous suite of rocks. The success may be due to partial clogging of fractures with clayey residue where it is fully sealing the fracture conduits in sedimentaries. The calicut well constructed by CGWB is now the only supplying well of APWD, A & N
Admn. The fracture conduit is observed upto 45m depth. The discharge of Calicut well is 45000 lit/hr.

The coralline limestone formations also form potential aquifer in shadow horizon and it is developed by dugwells in many of the islands like Carnicobar, Katehal, Neil, Havelock etc. and the yield of dugwells (having 5m dia, 6m depth) varies from 15000 to 1 lakh lpd. The recent studies of CGWB have revealed that although the vast impermeable sedimentary formations do not form potential aquifers. The alluvial deposits compared to boulders , pebbles, cobbles, sand and occasional clay lain in the stream channels 0carry lot of water through out the year along the subsurface and form this observation the idea of implementation of Artificial Recharge and conservation was contemplated by CGWB and it is proved highly successful. Such valley fills form potential aquifers in A & N Islands. One such valley fill at Beadnabad was tapped through CGWB exploratory well having depth of only 17m and yield 72000lit/hr.

Activities of CGWB in Andaman & Nicobar Islands :

	Since its inception in 1972 CGWB is carrying out systemetic hydrogeological surveys and geophysical studies in the Islands.
	Ground water exploration was carried out in A & N Islands during 1984-91 and during that period 47 exploratory bore wells were drilled.
	Short term water supply investigation is carried out in the Islands as per the request from various A & N Administrative Departments, Central Departments and Defence (MES).
	Recently from 1997 CGWB is carrying out artificial recharge and conservation studies for sus tainable water supply in the Islands. 76 such schemes have been demarcated. 3 schemes of these schemes are being funded from Central Sector Funds through CGWB and 20 ongoing schemes are being funded by A & N Administration.
	CGWB also has carried out surveys to monitor environmental degradation in Carnicobar Islands following request from Andaman Harbour Works, Govt. of India as per the directive of Ministry of Environment & Forest.
	CGWB also has carried out intensive studies specially to solve the permanent water supply problem to Portblair city through Interisland transfer of spring water from Rutland as also the study in Portblair Municipal Council area as per the request of Hon'ble Member of Parliament, A & N Isalnds has been carried out.
	CGWB has surveyed all the streams in North, Middle and South Andaman to find out the suitable measures to conserve surface ground water and surplus monsoon run off for development of irrigated agriculture.
	CGWB also has carried out studies in remote Kamorta, Nancowry and Katehal Islands of Nicobar district to find out the drinking water sources.
	CGWB is financing in a big way to A & N Administration during Xth 5 year plan period to develop artificial recharge and conservation practice including rain water harvesting from Central Sector Funds.

Important finding by CGWB in Andaman and Nicobar Islands :

	The studies carried out by CGWB in the find of artificial recharge and conservation and rain water harvesting to solve the water supply problems and to develop minor irrigation have been proved highly feasible and successful and being implemented by A & N Administration.
	To solve the water supply problem in Port blair town the study by CGWB in Rutland Islands has proved that everyday 180 lakh litres of spring water is lost into the sea, which may be conserved and brought to Portblair crossing sea. This may from a dependable basis of solution of drinking water crisis of Portblair. The detailed feasibility work is in progress. This was done at the behest of Hon'ble Lt. Governor of A & B Islands in peak crisis period of 2002.
	The marine sedimentaries are devoid of water in the subsurface where as the bore wells in igneous suit of rocks may be successful upto 60 m depth .However, there fractured aquifers are to be developed construction as recent drilling of private bore hole have significantly brought down the available water sources with an alarming cut down of discharge of the supply well drilled by CGWB.

Findings of CGWB in Andaman and Nicobar Islands :