ToxicityOfSilver

• Title: An Assessment of the Environmental Toxicity of Silver Iodide
• Prepared By: Bruce D. Williams and John A. Denholm
• Date: 4/1/2009
• Country: Australia
• Weather Modification - Doc Type: Scientific Study

90 lournal of Weather Modification Volume 41
• M assessment of factors known to ame-
liorate toxicity of silver ion; and
. Detenninatlon of background levels for
total siNer and bio-available silver.
This data was analysed to confiRn that the pres-
ence of the ameliorating factors discussed earlier
in this paper, and to verify that GTV for siiver in
each environmental maMx to be monitored was
appropriate.
Annual monitoring of total silver cancentretions in
soils, sediments, potable water supplies and peat
and moss is undertaken at a targe number of lo-
cations across and downwind of the study area.
These include:
• Generator sites (sites from which silver
iodide is dispensed);
• Potential points of accumulation in the
landscape (for example sediments in
glacial alpine lakes, peat bogs, stream
sediments);
• Potable water supplies; and
• Randomiy selected locations across the
study area.
The key outcomes of these investigations include
the following:
. Measurable concentrations of silver oc-
cuRed at all locations and in all matrices
sampled prior to commencement of the
doud seeding trial. Importantiy, there
was a measurable baseline of silver pre-
sent in the local environment prior to
commencement of the SPERP;
. The sampling design had sufficient
power to detect any statistically signfi-
cant increase well before the GN was
reached. This provides an important
levei of confidence to stakeholders that
management intervention could be im-
plemented in a timely manner if ever re-
quired;
. The analysis of the silver data showed
mean concentrations and upper 95%
confidence limits of silver to all be well
below the GN in all matrices, and all
locations and for atl periods of sampiing;
• Generally, mean concentrations of silver
were less than 20% of the relevant GN,
and in many cases less than 10% of the
GN.
Monitoring of potable water supplies is under-
taken on a number of oocasions throughout the
winter, typically foliowing significarrt snowfall and
run-off events. Monitoring of other matrices com-
mences following the cessaflon of daud seeding
operaaons for the season, usualiy in October
each year (Snowy Hydro LimRed 2008b).
8.3 Preliminarv Interoretation of Results
A statstical analysis of the siiver data for various
environmental matrices being monitored was un-
dertaken for the perbd 2004 throuyh 2007. The
data were oompared to 1he reisvant GNs for
each matrix, and also between periods end loca-
tbns of samplinp.
q statisbcai power analysis undertaken for gen-
erator sites (the locatlons from where silver io-
dide is dispensed, and the most Ifkely to show
any change) was used to asseas the likelihood of
being able to detect an increase in concentra-
tbns of sllver before the GN was reached
(Snawy Hydm Lfmited 2008a).
Detailed results and interpretaflons from the
monitoring pr+ogram for the period 2004 through
2009 (prior to expansion of the SPERP target
area) are presently being prepared for pubilca-
don.
9. CONCLUSIONS
The Act mandates that the SPERP must use sil-
ver iodide as the seeding agent. We consider
the risk of an adverse ec:otoxicologicai impact for
the project to be negligibly smali for the foliowing
reasons:
. Although sifver ions from soluble silver
salts have been shown to be toxlc to
aquaticc spedes, this fa not the case for
tlhe insoluble silvsr bdide;
. Since water adubNity osnerally cantrols
bioavailability, sifver oompounds that are
not readily soluble or insoluble are of
much less environmerrtaf ooncem;
. Where siNer toxicity studies have been
attempted using the insoluble siiver satts,
researc.hers have had to resart to indirect
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