The freshwater environment differs from the marine in the greater
variety of its minerals, among other things. As sites for
radiobiological studies, rivers and lakes present problems of great
complexity, but conditions at river mouths or estuaries are even more
difficult because of the mixing by tidal action of fresh and salt water.
Rivers vary greatly in character and change radically from season to
season because of rainfall and other factors. General understanding of
their biological workings is difficult to formulate. But rivers are the
routes by which minerals and wastes are transported toward the sea, and
estuaries are significant because of the many forms of life that
flourish there.
Studies of radioactivity in rivers and estuaries usually have been made
in relation to the fate of effluents from nuclear plants. Among the
longest and most intensive studies are those near Hanford, Washington.
Observations were started in 1943, when the federal government was
preparing to build plutonium-producing reactors to be cooled by waters
of the Columbia River.
[Illustration: _Fisheries biologists studying hatchery fish reared in
water containing radioactivity from the Hanford plutonium reactors._]
For more than two decades, observations have been made of the physical
dispersion and biological disposition of low-level effluents in the
Columbia. Concentration factors have been established for significant
radionuclides in phytoplankton, algae, insects, and fish, and typical
patterns of dilution and dispersion have been plotted.
Similar programs, in an entirely different freshwater system, have been
conducted over a similar span of years near the Oak Ridge National
Laboratory in Tennessee. One area of interest has been the biological
disposition of trace amounts of strontium-90 released to the Tennessee
River via tributary streams.
Among the few broad estuarial studies yet undertaken is one started in
1961 to plot the dissemination in the lower Columbia River, and in the
Pacific Ocean, of radioisotopes transported by the river from the
Hanford plant. Radiobiologists are studying biological distribution.
Oceanographers are using the trace amounts of effluent radioactivity to
verify the patterns of dispersion of river waters in the ocean.