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

Reported Concentrations in Tributaries and Biota

Tributaries
A paper by Risley(12) constitutes the only available data on radioactivity
in the lake's tributaries. His data, reproduced in Table A-6, show that the
total average gross beta radioactivity (suspended plus dissolved solids) in 24
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major tributaries in 1963 was 1.5x10-8 uCi/ml. This is the average of the last
column in Table A-6. Typically, seasonal variations in fallout and water drainage led to a range of concentrations, the highest level often times being 2-3
times the lowest. However, the average concentration of total gross beta ac-8
tivity, 1.5x10-8 uCi/ml, compares favorably with the gross beta concentration
found in Lake Michigan in 1963 (refer to Table A-l).
Risley also monitored gross alpha radioactivity, reproduced in Table A-7,
(12)
in the tributaries. Except for seven rivers on the Michigan side of the
lake, between Traverse City (Boardman River) and Muskegon (Muskegon River),
the gross alpha activity in tributaries reflected lake concentrations (refer to
Section II.A of this paper). Risley was not able to explain the higher gross
alpha levels in the seven Michigan rivers. He did, however, attribute all the
gross alpha activity to natural sources.
Mortimer(31) reports the outflow of water from Lake Michigan is approximately the same as the runoff from its drainage basin. It appears therefore,
that the quantity of fallout and natural radioactive materials lost from the
lake to Lake Huron is roughly balanced by the gain in fallout and natural activity from the tributaries.
Plankton
Radioactivity levels in plankton were also measured by Risley.(32) In
1962-1964 samples were collected from throughout the lake by towing a 20-mesh
plankton net from near the bottom to the surface. His data indicate that most
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plankton had gross beta levels of less than 2x10-4 uCi/g of ash. However, at
the entrance to Green Bay and the shoreline of Michigan from Ludington to
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Frankfort, the gross beta levels in plankton went as high as 5x104 uCi/g of
(32)
ash. Isoconcentration contours from Risley for gross beta in plankton are
reproduced in Figure A-3.
Fish
The State of Michigan Water Resources Commission, Water Quality Division(28)
has measured radioactivity in fish collected near Big Rock Nuclear Power Plant.

A-14