Lake Michigan environmental survey : final report / [edited] by John C. Ayers.

summarizes the gross beta radioactivity (excluding tritium) for the surface
water of Lakes Superior, Michigan, Huron, Ontario and Erie. The gross beta
concentration of natural origin is less than 2xl09 uCi/ml, thus most of the
gross beta activity in Table A-1 is from fallout.
One noticeable trend in gross beta radioactivity in the lakes is the increase in concentration as one moves from Lake Superior to Lake Ontario.
Generally, the concentrations in Lake Superior are the lowest, Lakes Ontario
and Erie are the highest, and Lakes Michigan and Huron are somewhere in the
middle. The concentrations in the lakes are highest in 1963, one year after
the United States ceased atmospheric testing.
Estimates of Sr-90 deposition in the Great Lakes region were obtained
from soil samples and precipitation collections at various collection stations.( 17'18)
Table A-2 gives the data for Argonne, Illinois, which were used to estimate Sr-90
deposition in Lake Michigan. In Table A-3 the Sr-90 deposited before 1956 was
estimated by subtracting the deposition for 1956-1962 given in Table A-2, from
the 43 mCi/km "probable" total deposition to December 31, 1962 reported by the
Federal Radiation Council.(24)
The Sr-90 activity in Column 2 of Table A-3 was corrected for decay to
1970 and to 1975. The sums of Columns 3 and 4, respectively, are estimates of
the total Sr-90 in Lake Michigan in 1970 (3570 Ci) and 1975 (3330 Ci).
Machta ) reports that the Sr-90 concentration in the water of Lake
Michigan in 1964 and 1965 was approximately 9x10-10 uCi/ml and 8x10-10 uCi/ml,
respectively. Table A-4 shows that the concentration of Sr-90 in lake Michigan in 1965, 8x10 uCi/ml, accounts for most of the Sr-90 deposited upon the
surface of Lake Michigan to that time. Machta also concluded that the tributaries contributed little Sr-90 to the total amount ir the lake and that the
sediment held little Sr-90. His data also show that thermal stratification of
the lake may influence the surface water concentration of Sr-90. (Figure A-2)
In secular equilibrium with Sr-90 in Lake Michigan is its daughter product, Y-90. Both Y-90 and Sr-90 emit beta particles, but no gamma photons.
Therefore, the contribution to gross beta activity in the lake from Sr-90 is
doubled when one considers its daughter Y-90.
The estimates of Cs-137 deposition were taken from reference 19 for the
years 1956 through 1966. In Table A-5, cesium-137 deposition in the years before 1956 and after 1966 were estimated by multiplying the Sr-90 deposition
(Table A-3, Column 2) by 1.6, the measured Cs-137/Sr-90 ratio in air.(18) On

A-7