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Transfer of Fallout Strontium-90 to Cows' Milk

Health and Safery Laboratory, U. S. Atomic Energy Commission, New York, New York

ABSTRACT

From 1962 through 1966, the monthly ⁹⁰Sr fallout rate, the accumulated deposit, and the concomitant ⁹⁰Sr levels in animal forage and milk were measured in samples collected at the Colorado State University dairy farm in Fort Collins. During the growing period, 42% of the total ⁹⁰Sr deposit was retained on alfalfa hay at the first cutting, 29% at the seeond cutting, and 23% at the third cutting. The difference between the first cutting and the two following was significant. These deposition coefficients combined with values for the area of forage utilized by the animal can be used to predict the contamination level of alfalfa hay from the measured total deposit.

The total intake of ⁹⁰Sr and the amount secreted in milk were measured in several groups of dairy cows. The transfer coefficients of ⁹⁰Sr from diet to milk ranged from 0.05–0.09% per liter of milk per day and were in agreement with tracer and field experiments reported by other investigators. By using the appropriate deposition coefficient, forage yield, forage intake, and diet to milk transfer coefficient, the amount of ⁹⁰Sr in milk was calculated from the measured total ⁹⁰Sr deposit over the growing period. The calculated ⁹⁰Sr in milk levels agreed reasonably well with measured values. For example, during the 1963 growing season—a period of relatively high fallout rate–the calculated amount of ⁹⁰Sr in milk was 18 pCi per liter based on a measured deposit of 2.66 nCi ⁹⁰Sr per m² Milk samples collected during the same period and analyzed for ⁹⁰Sr showed 20 pCi per liter.