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Thrivikramji.K.P. & Rajan, A.N.
Department of Geology, University of Kerala
Kariavattom campus 695 581

Kerala (Area = 38,863 Km2; Population = 30Million) is endowed with a total of 44 (41 west flowing and 3 east flowing – all tributaries of Cauvery) minor and medium rivers following the scaling of rivers by Rao (1972). Among these only 5 rivers (>2000 Km 2 of basin area) fall under the group of medium sized rivers. Sahyadris (a.k.a. Western Gahts) form the eastern border of the state, where as the Laccadive sea shoreline (length = 560 Km) demarcates the western border. Despite the relatively heavy rain fall (annual av.= 400 cm) received in the state out of the SW and NE monsoons, geomorphology and subsurface geology dictate a very low residence time for the surface and phreatic water. The annual runoff of Kerala’s rivers is estimated at 70,323 Mm3 out of which 42,722 Mm3 is utilizable.

The state (in the highland region; >75.0 m a.m.s.l.; extent = 21777 Km2) is mostly underlain by Pre-Cambrian crystalline rocks like gneisses (Hornblende or garnetiferrous-Biotite-gneisses, Khondalites) and charnockite. Dominant surface envelop in the midland (7.5 m – 75.0 m. a.m.s.l.; extent = 13476 Km2) tract is mostly laterite – a derivative of the crystallines. The coastal land (
Scrutiny of the extent of lithological cover provides an interesting insight on the nature of abundance. The dominant cover rock is undoubtedly of the Precambrian age (area = 27955 Km2), where as the laterite has a spread of only 5116 Km2, followed by recent alluvium (area = 4672 Km2) and lastly by Laterite capped sedimentary sequence of Tertiary age (area = 1120 Km2).

In comparison with the dominant cover of Pre-Cambrian rocks, the cover of laterite and alluvium (though secondary in dominance) possess exceedingly commendable water bearing properties and recharge characteristics. This belt also covers the entire low land and most of the midland tracts. Hence, ground water potential and resource of the state are positively dictated by the hydro-geological properties of the crystalline rocks.

The surface water potential on the other hand is a function of total rain fall on the one hand and the topographic characteristics and structural aspects like discontinuities of lithological types. Surface water, due to the very steep gradient of the land surface, rushes down through most part of the channels in the drainage net, and slows down only after reaching the tracts of lower elevations in the lower part of the midland and low land.

Access to satellite imageries (especially the LANDSAT) provided a synoptic view of the surface lithological cover as well as the gross structural make up of the basement rocks. The data gathered on lineaments traversing the terrain (like multiple generations, attitudes, extents as well as their influence on the geometry of the stream (valley) net or disposition of the ridges), added newer dimensions to knowledge base of the practicing hydrogeologist.

The role of the topographic lows falling along or coinciding with some of the major or minor lineaments (= now mostly stream courses or part of the stream net) in recharging the phreatic reservoir gained acceptance. In fact later, litho-structural analysis of the rock suites led the workers to believe that some of these lineaments truly coincide with certain leading shear zones (e.g. Bavani and Achankovil shear zones).

The cumulative length of the major lineaments mapped from satellite imageries (the LANDSAT) is placed at 1824 Km. (or let say 1800 Km.). It has been possible to identify two sets based on their orientation in plan view, viz., a minor NE-SW set (498 Km. or say 500 Km.) and a major NW-SE set (1325 or let us say 1300 Km.). One may consider this 1800 Km long lineament, as a slab of very highly fractured and sheared rock (hence more porous and permeable), in comparison with the rock slabs, and as an excellent phreatic aquifer. Assuming an average width of say 100 m., a depth of 100 m. and a porosity of say 30%, then ideally, total volume of water borne in this body of rock, at any point in time, will be of the order of (1800 Km. x 0.1 Km. x0.1Km. x 0.3) 5.4 Km3.or 5400 Mm3

Assuming an average water yield of 10 lps, this phreatic aquifer shall support
The CGWB and SGWD have promptly and adequately recognized the role of the lineaments as major conduits of recharge and/or discharge. The width of the lineaments has been identified to range between a few tens of meters to several tens of meters. The lineaments are important conduits of water flow like the other weak planes like the schistosity and fracture sets.

Some of the deep bore wells constructed as part of a SIDA-CGWB project in the late seventies or early 80’s have tapped sources as deep as 200 to 300 meters. The results also indicate and affirm the poor water bearing properties of charnockite which is mostly massive without any major fractures, joint sets or cleavages. In Kerala, the gneisses are by far more water bearing than charnockites as a consequence of the presence of fractures, joints, foliations etc. In fact, the chemical weathering process dominant in this tropical environment, adequately enlarged these discontinuities in otherwise massive rock, facilitating through passage of recharge.
The water demand by a teeming population of 30 million is also on the rise in comparison with the demand in the pre- or immediate post-independence days. Though an equitable water distribution for this large population calls for (30×106 x 40 x 365 = 438000.0×106 lit. or 438.0 Mm3) annually, most of the water supplied by the rains is lost to the ocean either through the surface flow or by the sub-surface flow.7>

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