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

April 2009 Williams and Denhoim 89
Marchant also notes that "... it is well known that
water quality problems (e.g. low water tempera-
tures, low oxygen concentrations and high con-
centrations of toxins such as hydrogen sulfide)
can occur below the deep dams that release bot-
tom water'. This indicates, qualitatively at least,
that sulfur-containing ligands are present in the
ecosystem.
The significance and relevance of each of these
ameliorating factors in regulating the bioavailabil-
ity of silver ion in the present triai cannot be over-
stated.
7.4 Silver Levels in Snow
Cloud seeding operations make very small addi-
tions of the insoluble silver iodide to the KNP.
For silver iodide to mobilise in the environment, it
must first dissotve and dissociate, leading to ex-
tremely low levels of silver ions. The silver ions
produced will then be subject to the factors de-
scribed below, which in tum act to ameliorate
their bioavailability.
The proportion of this ultra-trace concentration of
added silver ion that wouid become bioavailable
will be determined by the relative importance of
each of the amefiorating factors described in de-
tail below. These factors affect the bioavailability
of silver ions in both soil and water environments.
example having occurred in the summer of 2003.
During the early planning stages for the SPERP,
some stakeholders raised concems with respect
to likely environmental fate of any silver iodide as
a result of bushfire.
It is reasonable to expect that any silver iodide
associated with cloud seeding reaching the
ground would be incorporated into the top or-
ganic-rich soil layer (in the first instance). The
miniscule number of siiver ions arising from dis-
sociation of the solid siiver iodide wouid then be-
come bound to humic and other components as
described above. While combustion would pre-
sumably destroy the organic matter to which the
silver iodide is bound, the trace leveis of silver
iodide would remain in the ash or be volatilised
depending on fire temperature. '
The melting point of silver iodide is recorded as
558°C (Handbook of Chemistry and Physics) and
its boiling point as 1506°C and it has a negligible
vapour pressure (MSDS). At the levels this com-
pound is being used in snow augmentation, it is
difficult to see any enviranmental issue arising
from the impact of fire. It is possible that at ex-
tremely high flame temperatures there could be
some dissociation of silver iodide but the leveis of
iodine atoms formed would be insigniticant.
8. MONITORING PROGRAM
8.1 Objectives
Warburton and co-workers (1985) have shown
that if silver iodide aerosols are released at rates
of 1013 to 10t5 nuclei per second, the concentra-
tion of silver found in the precipitation is in the
range 10"12 to 10"9 g/ml (corresponding to one
part per triliion (ppt) to one part per billion (ppb)
in snow). This concentration was similar to that
found in a much earlier study by Warburton and
Young (1972), where snow in seeded areas had
a median concentration of the order of 500"9 M
silver.
In separate studies associated with cloud seed-
ing experiments around Lake Aimanor (US),
Warburton and co-workers (1995a, 1995b), found
the amount of silver deposited in snow ranged
from about 8 x10"12 to 245 x10''Z g/cm2.
7.5 Other Relevant Factors Affectina the Fate of
Silver
The study area has been subjected to extensive
bushfires in the past, the most recent notable
The specific objectives of the SPERP ecotoxicity
monitoring program (Snowy Hydro Limited
2008b) are:
• To be able to detect any increases in the
concentrations of silver in environmental
matrices, compared to baseline concen-
trations and compared to natural variabil-
itY;
• To be able to assess concentrations of
silver against the relevant GN for further
investigation if required; and
. To provide an eariy waming of any ad-
verse trend in silver concentration, allow-
ing for timely and effective management
intervention.
8.2 Ovenriew of Monitorina
As described earlier in this paper, the following
work was undertaken prior to commencement of
the SPERP