methods can thus often be based on the measurement of trace quantities of the metal itself in waters, soils, sedi• ments or rocks. In Canada, for example, centre-lake sedi• ments and lake waters have been widely used during the National Geochemical Reconnaissance survey (Cameron A new technique for the determination of trace IAvP.ls of and Hornbrook, 1976). Hydrogeochemical and stream• uranium in solution has been developed. A compact laser sediment reconnaissance surveys also form an important emits short, but intense, ultraviolet pulses• to excite the part of the National Uranium Reconnaissance Exploration
green fluorescence of a uranyl-complex formed by the addl- program in the U.S.A. (Shannon, 1977). tion of a proprietary reagent to ti.e sample. Organic species Where the distribution of lakes is sufficient to permit an present in most natural waters also fluoresce quite intensely, adequate grid to be established, the collection of surface but their emission does not last appreciably longer than the waters is the most economical method of sampling large excitation pulse, in contrast to the relatively long persist- areas. This application has, however, been limited by the ence of the uranyl fluorescence. Electronic 'gating' is em- analytical problems and difficulties of interpretation ployed to separate the long-lived contribution, rejecting sub- associated with the very low levels of uranium found. stantially the organic emission. 'Background' levels may be less than 0.0l ppb U in many Results using the laser fluorescence method on many sets areas and, although surface waters associated. with nearof natural waters of various origins have been compared surface mineralization may contain 1-10 ppb U or even against corresponding values generated by independent higher concentrations, it seems certain that 'anomalies' of
laboratories.

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