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

Since a gross beta count does not determine the radionuclides present in a
sample, the data are of little use in defining health hazards. Self-absorption further complicates the interpretation of gross beta data. The calibration curve used to convert gross beta counts to activity is given in Figure
A-6, page A-38, where the method used to generate this curve is also explained.
RESULTS AND DISCUSSIONS
Water
The measured concentrations of Cs-137, Zn-65 and gross beta radioactivity
in Lake Michigan surface waters are presented in Table A-17, page A-39.
The data are arranged alphabetically by stations selected by the Great Lakes
Research Division. Five of 49 samples had Cs-137 concentrations greater than
-9
the MDL of 3.5x10-9 uCi/ml. All but one of the five samples with Cs-137 were
in the South Basin. Zinc-65 was found in 8 samples out of 49 scattered throughout the lake. The highest levels of Zn-65 were at the Big Rock Point (BRK)
-9
sampling station. The average gross beta radioactivity was 3.3x10-9 uCi/ml
(range 1.3 - 5.9x10 9 uCi/ml).
Sediment
One hundred and thirty-eight sediment samples were collected from throughout the lake, 85 samples in 1969, 53 in 1970. The only identifiable radionuclides were Cs-137, K-40, and Ra-226. We do not doubt that Th-232 and its
daughters were in the activities reported as Ra-226. Radioactive members of
the U-238 and Th-232 series are found extensively in nature.(5) The results of
this portion of the study are included in Tables A-18 and A-19, pages A-41 and A-44.
The average Cs-137 activity in sediment was 1.4x10 uCi/g of dried weight.
There was no significant difference between the activities of samples taken in
1969 and those in 1970. The average Cs-137 in Lake Michigan sediment is approximately the same as the lxlO uCi/g reported by Eisenbud( for the Hudson
River and by Kahn(35) for the Deerfield River. There is no significant trend
of Cs-137 levels with depth of sampling although the levels are somewhat higher
at mid-depths (170' to 350') than in shallow or deep regions. This is shown in
Table A-20, page A-46.
The average Ra-226 was 1.6xlO uCi/g. This is approximately twice the

A-22