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

INTRODUCTION

Lake Michigan is the third largest in surface area of the Great Lakes
and is the only one that lies wholly in the United States. The bottom of
Lake Michigan is divided into five areas: South Basin, Divide, North Basin,
Straits Area, and Green Bay.()
The South Basin extends from a line connected Milwaukee, Wisconsin,
and Grand Haven, Michigan, to the southern tip of the lake.) Of the ten
reactors scheduled presently for the lake, six will be located within the
South Basin.(3'4) The five areas of Lake Michigan and the locations of the
ten nuclear power stations are shown in Figure A-1. The Kewaunee Nuclear
Power Plant and the two Point Beach units are to be located in the North
Basin, whose bottom is irregular in shape and is the deepest area of the
lake. Big Rock Nuclear Power Plant, the only operating reactor on the lake,
discharges heat and radioactivity into the Straits area of the lake.
The purposes of this chapter are: to describe the history of radioactive
materials in Lake Michigan, to report on the present (1969-1970) radioactivity
content of the lake, and to estimate what the situation will be by 1975.
RADIOACTIVITY IN LAKE MICHIGAN
Natural Sources
There are a number of natural radionuclides present in Lake Michigan.
Eisenbud(5) states that of the 340 natural isotopes, approximately 70 are
radioactive. The most abundant primordial radionuclides are K-40, Rb-87,
Th-232, U-235 and U-238.(6) Of their daughter products, Ra-226, Rn-222 and
Pb-210 have sufficiently long half-lives to have been found in the aquatic environment.(5-9) In addition to the primordial nuclides and their daughters,
an important group of natural radionuclides is produced by cosmic ray interaction with the stable nuclides N-14, 0-16, and Ar-40 of the atmosphere.(7'9)
The most fully studied of these natural activation products are H-3 and C-14.(5,6,7)
The average potassium content of Lake Michigan is 1.3 mg/1.(10) Of the
three potassium isotopes, only K-40 is unstable and decays with a half-life of
1.3x109 years.(5) The fractional content of K-40 in natural potassium is 0.012
percent.) From these figures one finds that the average concentration of
K-40 radioactivity in Lake Michigan is 1.1x10 9 uCi/ml.(11)

A-3