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Investigations involving satellite imagery studies, hydrogeological and geophysical surveys, fracture pattern studies, drilling, long duration pumping tests, water level monitoring, etc. provide an insight into the character of hard rock fracture aquifers, which are generally anisotropic and heterogeneous. Such investigations have helped ill delineating a fracture zone (Kalleppadanl-Oralassery fracture zone) near Pazhayannur in the Thrissur District of Kerala. The area forms part of the Precambrian metamorphic terrain with charnockite as the chief rock type. This fracture zone is one of the several NW-SE lineaments which represent groundwater-potential zones in Kerala State. Basic dykes intruding this zone are fractured. Although the phreatic zone consisting of laterite has low potential, the underlying two-layered fracture aquifers provide good yields through borewells of less than 80 111 depth. The aquifers are marked by steeply dipping fractures and dyke bodies. Pumping tests in a well-field have revealed the presence of poorly connected minor fractures within the major fracture zone. Test data from seven borewells have brought out features like similarity to a double-porosity model with a discontinuous fracture network, quick response of aquifers to pumping, possibility of temporary pressure build-ups in the aquifer during pumping. greater hydraulic connectivity along lineaments, a near-flat cone of influence etc. The double porosity model could consist of a few major fractures with associated network of minor fractures. The water table and the piezometric surface in the area show some similarity in the pattern of seasonal fluctuation.

Keywords

Groundwater, Fracture Aquifer, Hard Rock Terrain, Pumping Test, Water Level Fluctuation, Thrissur, Kerala.

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