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

Ra-226 reported in the Deerfield River but is equal to the combined total of
Ra-226 plus Th-232 at that location.(35) Radium-226 does not vary with depth
in Lake Michigan (Table A-20).
The average K-40 is 15x10 uCi/g and ranges from about 5x10 uCi/g to
30x10-6 uCi/g. There is an indication in Table A-20, that K-40 levels increase with depth.
On July 25, 1970 a survey was made near Big Rock Nuclear Point Plant with
an underwater gamma probe. Preliminary results of this survey substantiate the
findings here that Cs-137 is somewhat higher at mid-depths than in shallow
areas. Further, "the only man-made radioisotope seen in any of the spectra
was Cs-137, although some spectra were taken within a mile and a half of the
reactor."(36) Most spectra showed significant amounts of K-40 and small but
detectable amounts of Ra-226.
The sediment samples were classified into textural groups: sand, gravel,
silt, and clay. When the samples were arranged in order of increasing depth
of collection, the samples classified as sand and gravel came out in the
shallower areas, silt in the mid-depths, and clays from the deepest zones
(Table A-20). This is in good agreement with the observations by others of
sediment textural groups in Lake Michigan.(2)
Benthos
Both Cs-137 and Zn-65 were detected in environmental samples of benthic
origin. Refer to Tables A-21 and A-26, pages A-50 and A-58. Noticeable differences between benthos and zooplankton or phytoplankton are: a) the benthos
contain slightly more Cs-137, and b) there is approximately twice as much gross
beta activity in benthos as there is in zooplankton or phytoplankton.
Zooplankton
There is no indication in Tables A-22 and A-27, pages A-52 and A-60, that
Cs-137 or Zn-65 are concentrated in zooplankton. Gross beta levels are much lower
than those found by Risley in 1963 (Reference 32 and Figure A-3 of this paper).
Phytoplankton
The concentration of Zn-65 is much higher in phytoplankton (Tables A-23 and
A-28, pages A-54 and A-62) than in any of the other environmental medium sampled.
Gross beta radioactivity is the same as those for zooplankton.

A-23