STRONTIUM-90 ON THE EARTH’S SURFACE. Ill ROBERT J. LIST,* LESTER MACHTA,* LYLE T. ALEXANDER,t JAMES S. ALLEN,t MILTON W. MEYER,t+ V. T. VALASSIS,+ and EDWARDP, HARDY, JR.} *Weather Bureau, U. 5S. Department of Commerce, Washington, D. C., 7Soil Conservation Service, U. 8. Department of Agriculture, Beltsville, Maryland, tHealth and Safety Laboratory, New York Operations Office, U. S. Atomic En- ergy Commission, New York, New York. ABSTRACT A worldwide soil-sampling program was conducted in late 1963 and early 1964. This program was a continuation of earlier programs carried out in 1956, 1958, 1959, and 1960 to study thetotal deposition and distribution of *Sr over the surface of the earth. It is estimated on the basis of the data obtained that 7.6 Mc of Sr were on the surface of the earth in late 1963. Some possible sources of error in this estimate are discussed. INTRODUCTION Since 1955 the U. S. Department of Agriculture has been conducting a soil-sampling program at the request of the U.S. Atomic Energy Commission to ascertain the worldwide distribution and total amount of Sr fallout. Previous reported surveys!.2 on a worldwide basis were conducted in 1956, 1958, 1959, and 1960. A fifth worldwide survey was conducted in late 1963 and early 1964; the results of this survey’ are discussed here. Samples have been collected from more than 100 sampling sites chosen to measure the total amount of *Sr fallout at particular loca- tions. These sites are level or nearly so, have a complete vegetative cover, and are sufficiently permeable to absorb all the precipitation that falls on them. The sites receive no water from higher ground. 359