HYDROGEOLOGICAL CONDITIONS AND OPTIMUM WELL DISCHARGES IN GRANITIC TERRAIN IN PARTS OF NALGONDA DISTRICT, ANDHRA-PRADESH, INDIA

Ground water in study area underlain by peninsular Gneissic complex oc curs in unconfined conditions in weathered zones and semi-confined to confined conditions in fractured zones. Productive fractured zones def ine narrow linear tracts closely corresponding to surface drainage. Fl uoride content of ground water is low mostly in topographic highs and to lesser extent in tank beds or tank commands and in areas where frac tures are not connected with fluoride bearing granites. For potability , most wells were drilled in topographic highs and some around tanks. The aquifer is highly complex (anisotropic and heterogeneous) needing critical analysis to assess performance of wells, as recently yields o f a large number of borewells drilled in granitic terrains of Andhra P radesh are reported to have declined. This is attributed mostly to dir ect interference with other wells, improper construction of wells lead ing to silting and general decline in water levels due to various reas ons. But rarely, it is realised that these aquifers are laterally limi ted. In this study pumping tests for 500 to 1000 minutes i.e., 2 1/2 t o 3 log cycles of time were carried out and the data was analysed usin g graphical methods of Cooper and Jacob and Theis recovery method. In four cases the test data was analysed by a micro processor based simul ation technique. Optimum discharges were calculated keeping in view th e effects of 'no-flow' boundaries, effective available drawdown and ot her field parameters. This has indicated that the optimum discharges o f the wells are in general between 60% to 80% of the yields recorded d uring drillng.