WMOD00467

• Title: Ecological Effects of Silver Iodide and Other Weather Modification Agents: A Review
• Date: 2/1/1970
• Weather Modification - Doc Type: Report

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Ecological Effects 95

elements previously concentrated by food or- reduction in growth rate of certain organisms
ganisms; (c) complexing of metals by coordi- than a dramatic lethal response. Growth reduc-
nate linkages with organic molecules; (d) in- tion would be detrimental if the affected micro-
corporation of metal ions into physiologically organisms serve as food for larger animals.
important systems; and (e) uptake by ex- More serious would be inhibition or slowing of
change, as for example onto mucous sheets of biological decomposition of bottom sediments,
the oyster. With appropriate modifications, most particularly in lakes and ponds. Decomposition
of these processes might also occur in fresh by bacteria and fungi is a vital link in the
water. cycle that returns essential nutrients to the
Some indication of the effect of silver com- water. Similar inhibition might affect sewage
pounds on fresh water ecosystems, and particu- treatment processes, but this is less likely be-
larly on aquatic invertebrates, might be ob- cause of the rapid turnover and close control
tained from a limnological survey of springs and in such systems.
streams in silver mining areas. Routine geo-
chemical surveys by the U. S. Geological Survey
have identified springs near Fairplay, Colorado,
having Ag concentrations as high as 5 X 10-0 g
ml-" without the s~lfur mineralization so com-
mon in mining regions. A comparative study of
the limnology of springs with high and low levels
of Ag in this and other areas could yield signifi-
cant information on adaptation of organisms to
this element.

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BIOLOGICAL El:FECTS ON ECOSYSTEMS

The data reviewed here indicate that there is
little likelihood tha:t silver from cloud seeding
will adversely affect terrestrial plant and animal
communities or marine environments, either
immediately or after some 20 years of AgI
application. Such a: statement is clearly risky;
a similar surveyor pesticide effects 20 years
ago would almost certainly not have anticipated
the recent discovery that DDT lowers the re-
productive rate of many bird species by reduc-
ing the thickness of their eggshells. Similar un-
foreseen effects of 'silver may appear, but we
believe that they are unlikely.
Direct lethal effects on fresh water fish are
also improbable, either as a result of detrimental
levels of silver in the water or of ingestion of
harmful silver compounds concentrated through
the aquatic food chain. There is a possibility,
however, that there may be sufficient silver in
some fresh waters, especially at the headwaters
of streams, to slow the growth of susceptible
fish or of the aquatic invertebrates upon which
they feed.
Perhaps the most likely possibility is that
adsorbed silver will inhibit the growth of algae,
fungi, and bacteria in fresh water. If such an
effect does occur, it is more likely to be selective

ORGANIC SEEDING AGENTS

Although AgI has been the principal nucleat-
ing agent used in weather modification experi-
ments so far, it is not the only such agent.
Other materials may largely replace it in the
future, owing to the high cost of silver and to
the demands that widespread application of
AgI might place on the silver market.
The advantage of AgI is not only its capacity
to induce ice crystal formation at relatively high
temperatures but also the ease with which it can
be finely subdivided. Minute particles of AgI
smoke are readily carried in updrafts to cloud
bases, and small quantities initiate crystalliza-
tion. No other known compound so well com-
bines these two properties.
Substantial progress in weather modification
will probably depend on more precise identifica-
tion of seed able clouds and on better systems
for delivery of the nucleating agents to the right
place at the right time. As more reliance is
placed on pyrotechnic devices, aircraft, and
other aerial delivery schemes, and less on ground
generators, AgI may lose its importance as a
seeding agent.
Several organic compounds are effective as
ice crystal nuclei at temperatures warmer than
the critical temperature for AgI. Phloroglucinol,
1,5-dihydroxynaphthalene, and metaldehyde
have been suggested, but so far they have been
used only experimentally. These compounds
cannot be subdivided as finely as AgI without
incurring unacceptable sublimation losses; hence
they cannot be efficiently dispensed from ground
generators.
Organic compounds would be acceptable seed-
ing agents only if they were degraded quickly
enough that they would not accumulate in the