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

CHAPTER A

LAKE MICHIGAN RADIOLOGICAL SURVEY
John C. Golden, Jr., Phillip A. Plato and G. Hoyt Whipple*
SUMMARY
This chapter describes the history of radioactive materials in Lake
Michigan, reports on the present (1969-1970) radioactivity content of the
lake, and forecasts the situation in 1975. The primary radionuclides in
Lake Michigan are of natural origin and from fallout. The radioactivity
contributed so far by Big Rock Nuclear Power Plant is much less than one
percent of the total activity in the lake water. The most important
natural radionuclides in the water are potassium-40 as a dissolved salt and
carbon-14 as the inorganic carbonate ion. In 1970, tritium, cesium-137,
strontium-90, and yttrium-90 activity from fallout of nuclear detonations
exceeds the activity in water from natural or other man-made sources.
Natural radium, thorium, and potassium, and the cesium-137 from fallout are
the chief radioelements in the sediment.
The University of Michigan Great Lakes Research Division collected 370
samples of water, sediment, zooplankton, phytoplankton, benthic organisms,
and fish for analysis of their radioactivity content. Only 5 of 49 water
samples had cesium-137 concentrations greater than the minimum detectable
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level of 3.5x109 uCi/ml for a 2000 ml sample. Four of the five water samples
with cesium-137 were in the southern part of the lake. Zinc-65 was detected
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above the minimum detectable level of 8x10 uCi/ml in 8 water samples out of
49 scattered throughout the lake. The highest concentration of zinc-65,
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32x10-9 uCi/ml, was at the Big Rock Point sampling station. The average
cesium-137 activity in sediment was 1.4x10- uCi/g dried weight. There is no
significant trend of cesium-137 levels with depth of sampling although the
levels are somewhat higher at mid-depths (170' to 350') than in shallow or
deep regions. The average concentrations of radium-226 (with thorium-232) and
potassium-40 in sediment were 1.6x10 uCi/g and 15x10 uCi/g dried weight,
respectively. Cesium-137 and zinc-65 activities were detected in benthos and
*Department of Environmental & Industrial Health, Radiological Health Group,
The University of Michigan.
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