WMOD00470

• Project Name: Project Skywater
• Title: Environmental Impacts of Precipitation Management: Results and Inferences from Project Skywater
• Date: 6/6/1977
• Weather Modification - Doc Type: Report

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'~~IR ~dry deposition
AIR ~ I . agriculturol and industriol
I discharges to the environment
sweep-up~ 0 industrial pollution of the atmosphere
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Bulletin American Meteorological Society

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497

IMPACT
THRESHOLDS
FOR NATURAL
FOR MS OF
SILVER AND RELATED
PURITY STANDARDS
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c::> SILVER CONTENT, MEGAGRAMS (POWERS OF TEN)

. silver transport megagrams per year

FIG. 3. Generalized approximate content and concentration of silver in environmental com-
partments for the contiguous United States, rates of exchange between compartments, and
impact threshold concentrations. Representations of exchange rates relate only to the scale of
megagrams per year (bottom scale), not to scale of concentrations.

In the aquatic compartment, annual transports are of
the same order of magnitude as contents. Most transport
is thought to be by waterborne sediment.
The atmospheric domain receives silver in the form
of windblown dust, some of which returns to the soil
as dry deposition and some of which is swept up by
particles of precipitation (the label "Rain" includes
snow) and is carried with it to the ground, to lakes, or
to the sea. The silver content of the atmospheric com-
partment at any moment is small in comparison with
the annual transport through it.
Silver transports associated with cloud seeding and
with pollution are shown in Fig. 3 by open diamonds.
Atmospheric pollution causes local, temporary changes
within the atmospheric compartment that are very
large in comparison with natural amounts. Industrial
atmospheric emissions are variously estimated at 135
to >360 Mg (150--400 tons) per year. (Carson and Smith,
1975; Environmental Protection Agency, 1973), at con-
centrations ranging from ,---,10-10 in city air to an extreme
of 6 X 10-6 measured in the smog water within the down-
wind plume from a copper smelter (J. Fletcher, 1976,
personal communication). The other main source of
silver pollution affecting the soil and surface waters is
fertilizer, either Florida phosphates or processed sewage
sludge (Carson and Smith, 1975). Qualities and concen-
trations so produced are currently unknown.

Cloud seeding, if it became widespread, would result
in local, temporary concentrations in precipitation of
the same order of magnitude as the natural concentration
in surface waters. However, the rates of exchange would
remain more than an order of magnitude smaller than
the principal exchanges affecting the aquatic compart-
ment, and they would be many orders of magnitude
smaller than those affecting plants and soil, even in
localized areas of precipitation management. Widespread
and prolonged precipitation management, using silver
iodide as the cloud-seeding agent and assuming that all
the silver dispersed in the course of a century accumu-
lated in the top 2 em of soil, would not cause the silver
concentration there to exceed the normal background.
Silver from seeded rain follows pathways essentially
identical to those from natural rain, and the effect of
it on the silver transport by wet deposition (an increase
of <20%) is too small to be illustrated conveniently
in Fig. 3.
The threshold of silver concentration for observable
impacts on microbes and pollens in the laboratory
broadly overlaps concentrations in naturally enriched
regions and probably in sites of highest industrial pol-
lution and some areas of fertilizer use. Precipitation
management activities produce such concentrations only
within a few meters of ground-based smoke generators,
where some weak evidence of possible delay in de-