Campbell presented a series of lectures during the two-week interregional UNESCO seminar on groundwater in hard rocks held in Coimbatore, India in 1979. He reported that the site-specific occurrence of ground water in igneous and metamorphic rocks is often difficult to predict. The ground-water production potential from such rocks is considered by many ground-water professionals to be generally low. Production can vary, however, from very low (or nil) to very high sustained production rates, that is, greater than 300 gpm (19 l/s) (Ellis, 1909; Stewart, 1962). Limited knowledge of the geological conditions that have combined to yield ground water at relatively high production rates have been major constraints to cost-effective and successful ground-water exploration and development of such rocks in the past.
The pressing need for obtaining even a limited water supply (less than 1 gpm or 0.06 l/s) has prompted random drilling, augmented occasionally by preliminary and often superficial geological evaluations. Drilling has been conducted in many areas underlain by crystalline rocks, and voluminous data resulting from such drilling also indicate a broad range in the productivity of such rocks. In Brazil, for example, approximately 63 percent of the country is underlain by igneous and metamorphic rocks. Reboucas ( 1976) reports that approximately 15,000 wells have been drilled in fractured crystalline rocks, of which 92 percent considered successful. Many of such wells have been in use for more than 30 years. The average well yield of the successful wells is approximately 0.03 liters per second per meter of aquifer penetrated (or 0.13 gpm/feet of aquifer), which is consistent with previous investigations conducted in other countries.
A sixty meter (197 feet) depth limitation has been suggested by many professionals as a maximum economic drilling depth, which has also been supported in other countries on the assumption that potential ground-water production decreases with increasing depth. For example, typical reported well yields in crystalline rock areas of Africa, India, Korea and parts of the Western Hemisphere are indicated in Tables 1 and 2. When an unusually high rate of production was encountered, the geological conditions (fractures, joints, faults, etc.) that permitted such productivity have been considered too complex to evaluate in any detail and were considered as an anomalous occurrence.
The general impression was that such productivity could not have been forecast because of the apparent complexity of the geological conditions. However, the type of systematic investigations discussed during this seminar demonstrates that the regional and local structural history of igneous and metamorphic rocks in selected regions, combined with appropriate surface geophysical and hydrochemical techniques, can serve to identify areas that may be capable of producing large, sustained ground-water supplies. The rigorous use of an integrated approach has resulted in an increased rate of exploration success and, consequently, the cost effectiveness of such programs.
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