The authors welcome Dr Wurzel's contribution concerning radioactive tracers and look forward to reading the paper he is preparing on their use in crystalline rocks in Rhodesia. A guide book on “Nuclear Well Logging in Hydrology”, shortly to be published by the International Atomic Energy Agency in association with the International Hydrological Decade, with which one of us (DAG) is associated, reviews the techniques comprehensively. However, the author's paper was concerned with the recognition of fissure flow using non-radiometric techniques which are of general application and do not suffer from the somewhat restrictive attitude of some territories to the use of radio tracers in water supply wells, even in the small quantities which are necessary; this has been recognised as a problem in the U.S.A. where a brine injector was developed as an alternative (Scott Keys 1968). The technical advantages of a tracer free from density effects are fully recognised both at high and low velocities, especially at the low end, near the diffusion rate. One of us has been associated with the United Kingdom Atomic Energy Authority in the development of a prototype, single-point radio tracer injection probe for use in boreholes (Anon. 1968). Severe calibration problems were encountered which resulted in part from the density effect, although Scott Keys (1967) commented that such effects may not be significant. Certainly other errors may be more significant, including those due to the position of the injector in relation to the centre of the borehole, to variations in borehole diameter and

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