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Tritium in Arabian rainfall has followed the trend observed in North America with peaks in 1958 and the spring of 1959. These measurements will be useful for future hydrologic studies. Water from wadi gravels averages 10 yr old. Carbon-14 measurements of deep waters indicate ages of several thousand years.

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This paper begins by providing a brief overview of the development of methods for the determination of selenium. Results of a study by Cheng in 1956 found that at a pH greater than 5, piazselenol could be quantitatively extracted with toluene, benzene, or xylene, butyl, and amyl alcohols and that ethylenediaminetetraacetic acid tetrasodium salt (EDTA) can be used to minimize the interference of polyvalent metals. The objective of the study described in detail in this paper was concerned essentially with reproducing Cheng's results and adapting his method to the field of water chemistry. The modified method presented here can be used for 0‐50 μg selenium with good results. When the single‐extraction technique is used for simplicity, approximately 60 per cent of the selenium is extracted. The results are, therefore, largely empiric, and care must be taken to insure that operating conditions are kept as nearly constant as possible. Experimental apparatus is described, along with reagents used in the study, the procedure, experimental results, calculations, and precision and accuracy. Conclusions indicate that the removal of water from the toluene layer can be accomplished by filtering through 100 mg anhydrous sodium sulfate on No. 40 filter paper. This method is quicker than the centrifuging recommended by Cheng, and a comparison of both methods shows no difference. The diaminobenzidine reagent can be added either in the solid form or as a solution. It is easier to use the reagent in solution, provided only enough is made up for a few days at a time.