In India, groundwater has played the pivotal role in fulfilling the demands of domestic, industrial and agriculture sectors. At present the groundwater in India contributes more than 58% drinking water, 52% for agriculture production and 50% for urban and industrial sectors. Indiscriminate development and unscientific management of this resource has led to multiple problems of decline in groundwater level, sea water ingress, in- land salinity, groundwater pollution, land subsidence etc. (Ramachandra, 2006b). The measures that need to be adopted in the country to meet the increased water demand in the new millennium would include exploration of deeper aquifers, groundwater recharge, development of aquifers in flood plains, direct  use of saline / brackish water, conjunctive use of surface and groundwater in canal command area, creation of groundwater sanctuaries and regulation of groundwater development. During the past five decades, there has been phenomenal increase in growth of groundwater abstraction structures in India. Their number has increased from 4 million in 1951 to about 18 million in 1997-99, while in the same period irrigation potential created from Groundwater has increased from 6 to 30 million hectares.

The word groundwater should be considered as a complex medium composed of ground and water. While both the components are complimentary and interactive they influence in the resulting characteristics of groundwater. Groundwater is an increasingly important resource all over the world. The term groundwater is usually reserved for the subsurface water that occurs beneath the water-table in soils and geologic formation that are fully saturated. Geo-chemically, groundwater is aqueous solution of bicarbonates, chlorides and sulphates of alkaline earth and alkali metals. It supports drinking water supply; livestock needs irrigation, industrial and many commercial activities. While the degree of reliance on groundwater varies significantly, the need of groundwater as a dependable resource of fresh water has been unquestionable. In the overall geo-strategic scenario groundwater has also offered a medium for many environmental solutions. It is key to under standing a wide variety of geologic processes for e.g. the generation of earthquakes, the migration and accumulation of petroleum and the genesis of certain type of ore deposits, soil type and land forms.

Apart from the use of groundwater, which require specific characteristics, the role of groundwater chemistry is important in determining suitability of well. The chemistry of groundwater has an unusual characteristics in terms of  its ability to dissolve a greater range of substances than any other liquid by virtue of its occurrence in different environmental conditions controlled by specific pressure and temperature different from on-land conditions. Chemical process in groundwater zone can influence the strength of geologic materials and in situations where they are not recognized, can cause failure of artificial slopes, dams, mining excavations etc. Geologically 88% of the State is underlain by crystalline rocks of Archaean age comprising schistose formations, charnockites, khondalites and gneisses. All these formations are intruded by dykes of younger age.  The sedimentary formations of Tertiary age occurring along the western parts of the State comprise four distinct beds viz. Alleppey, Vaikom, Quilon and Warkali.  The crystalline and the Tertiary formations are lateritised along the midland area.  Alluvial deposits of recent origin are seen along the coastal plains.  The general stratigraphic sequence is given in Table 6.

Table 6: The general stratigraphic sequence of Kerala State

Ground water occurs under phreatic, semi-confined and confined conditions in the above formations. The weathered crystallines, laterites and the alluvial formations form the major phreatic aquifers, whereas the deep fractures in the crystallines and the granular zones in the Tertiary sedimentary formations form the potential confined to semi-confined aquifers.

Different Geological forms and ground water of Kerala

The Crystalline aquifers: The shallow aquifers of the crystalline rocks are made up of the highly decomposed weathered zone or partly weathered and fractured rock.  Thick weathered zone is seen along the midland area either beneath the laterites or exposed.  In the hill ranges, thin weathered zone is seen along topographic lows, area with lesser elevation and gentle slope.  In areas along the hill ranges generally rock exposures are seen.  The depth to water level in this aquifer varies from 2 to 16 metres below ground level (mbgl) and the yield of the well ranges between 2 to 10 m3 per day. The exploratory drilling carried out by Central Ground Water Board in the State in the crystalline formations has indicated that the potential fractures are encountered at depths ranging between 60 to 175 mbgl.  With yield varying from less than 1 to as much as 35 litres per second (lps).   In Charnockites, more than 40% of the wells have yielded more than 10 lps or above.

The Tertiary aquifers: Ground water occurs under phreatic condition in the shallow zone and under semi-confined to confined conditions in the deeper aquifers. The Tertiary formation of Kerala coast is divided into four distinct beds viz. Alleppey, Vaikom, Quilon and Warkali.  These formations except the Alleppey beds are seen as outcrops and lateritised wherever they are exposed.  The maximum thickness of Tertiary sediments is found between Karunagapally and Kattoor and all the four beds are encountered in this area. Ground water is commonly developed through dug wells tapping the sandy zones at shallow depth in the Tertiary sediments.  The depth to water level in this shallow zone ranges from 3.0 to 27 mbgl and the yield of the well ranges from 500 lpd to 10 m3 per day.  

The Vaikom and Warkali beds form the most potential aquifers in the Tertiary group.  The Alleppey beds have been encountered at deeper levels in the bore holes drilled in the coastal tract of Alleppey district and the formation water is found to be saline and hence, no tube well has been constructed by tapping this formation.  In the Vaikom aquifers, the piezometeric level is between 2 m and 20 m above msl. The yield of the tube wells constructed in this formations ranges from 1 to 57 lps.  This bed forms 'auto flow' zones along the coast between Karunagapally in Quilon district and Nattika in Trichur district. 

Warkali aquifers are the most developed aquifer system among the Tertiary group.  The urban and rural water supply in the coastal area between Quilon and Shertalai is mostly dependent on this.  The piezometric head is about 3 m. above msl along the eastern part of the sedimentary basin whereas it is 10 m. below msl in and around Alleppey.  The yield of the wells tapping this formation ranges from 3 to 14 lps. The hydrogeological information on the Quilon beds is very limited.  The formation is considered to be a poor aquifer compared to Vaikom and Warkali beds.

Laterites: Laterites are the most widely distributed lithological unit in the State and the thickness of this formation varies from a few meters to about 30m. The depth to water level in the formation ranges from less than a meter to 25 mbgl.  Laterite forms potential aquifers along valleys and can sustain medium duty irrigation wells with the yields in the range of 0.5 - 6 m3 per day.  The occurrence and movement of ground water in the laterites are mainly controlled by the topography.  Laterite is a highly porous rock formation, which can form potential aquifers along topographic lows.  However, due to this same porous nature, groundwater is drained from elevated places and slopes at shortest duration after monsoon due to which scarcity is experienced in the elevated places and slopes.  This is the most extensive hydrogeologic unit in the State.  The thickness varies generally from less than a meter to above 20m and thicker zones are seen along Malappuram and other northern districts.

Alluvium: The alluvium forms potential aquifer along the coastal plains and ground water occurs under phreatic and semi-confined conditions in this aquifer. The thickness of this formation varies from few meters to above 100 m. and the depth to water level ranges from less than a meter to 6 mbgl.  Filter point wells are feasible wherever the saturated thickness exceeds 5 m. This potential aquifer is extensively developed by dug wells and filter point wells throughout the state and the yield ranges from 5 to 35 m³ per day.

In the state groundwater occurs in all the geological formations from Archaean crystalline to recent alluvium. Groundwater occurs in phreatic condition in the laterite, alluvium and in weathered crystallines. It is in semi-confined to confined condition in the deeper fractured rocks. However, in the eastern part of Palghat District (study area), both the phreatic and deeper aquifers tap only the crystallines. The phreatic zone comprises weathered to partly weathered crystallines, and the deeper aquifer is formed by the fractured crystallines. laterization is limited and alluvium is restricted to river beds and no dug well taps these zones. Moreover, this area has been subjected to a high degree of deformation, which has given rise to network/intersection fractures; this can be traced surficially. This intersecting nature of lineaments suggests that the water-yielding zones for phreatic and deeper aquifers are interconnected in hydraulic continuity. Thus the mechanism of fluoride release for both the aquifers is the same since the aquifer is one and the same, i.e. country rock, hornblende biotite gneiss. In this area the depth of the well ranges from 6 to 18 mbgl. The diameter ranges from 1 to 5 m and water level ranges from 3 to 15.0 mbgl.