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80 )ournal of Weather Modification Volume 41 tained by multiplying the acute value by 0.5. Chronic tests are significantly influenced by the binding of siNer ion to the food used during the test period. Guadagnolo et al. (2001) showed that silver con- centratlons in different compartments of rainbow trout eggs were greatest just before hatch. How- ever, they noted that the silver burden was not correlated with mortality. 5. FACTORS AFFECTING THE BIOAVAIL- ABILITY AND TOXICITY OF SIWER 5.1 General Discussion lt is now dear from a large number of studies, that the toxicity of silver ion in water is signifi- cantly ameliorated by the presence in water of species such as chloride ion, sulfide ion, carbon- ate and dissolved organic carbon. In addition, silver is strongiy adsorbed onto particulate mat- ter. Where the number of moles of each of these ameliorating fadors in total, is greater than the number of moles of silver added there is a negli- gibie risk of boxicity due to silver occumng. The explanation for this is discussed below. Bell and Kramer (1999) found that in the aqueous phase, silver at the lowest concenVations exists either as a simple sutfydrate (AgSH) or as a sim- ple polymer HS-Ag-S-Ag-SH. Many studies have shown that because of the possibility of binding to colloids or through the formation of covalent and other complexes, the toxicity of dissolved silver ion in the environment is generally less than that found in laboratory tests. and local water quality characteristics are further confounding factors in the application of labora- tory results to field situations. It is worth reiterating that the source of the silver ion used in these studies was generaily derived from the readily solubie silver nitrate (AgNO3) which dissociates completely and gives essen- tially free siiver ion. In contrast to the situation found in most labora- tory studies, Hogstrand and Wood (1998) note that only a sma41 proportion of total dissolved sil- ver in natural waters, <40%, exists in the free form, and very often wiil be much less. In studies where the silver ion was derived from silver salts that are insoluble, such as silver thi- osulfate A92(S203) and silver chloride (AgCI), the toxicity decreased significantly as the solubility decreased. No studies have been reported for silver iodide, presumably due to its insolubility. For silver chloride, for example, the 96-hr LC5o was reported as > 100 Ng/L (Wood et al. 1996). LeBlanc et al. (1984) reported that for fathead minnows, silver ion was 300 times more toxic than silver chloride, 15,000 Emes more toxic than silver suifide and 17,500 fimes more toxic that the silver thiosulfate compiex. Similar results were found by Hogstrand et al. (1996), who also reported that toxicity decreased as hardness was increased from 50 to 250 mg CaCO31L. Increasing concentrations of humic acid were also found to decrease silver ion toxic- ity, as noted elsewhere in this review. It has recentiy been shown by Bianchini et al. (2002), Bowles et a/. (2002) and others, that re- active suMdes occur at concentrations ranging from picorrwiar to nanomolar concentrations in natural oxygenated waters. These concentra- tions are sufident to strongly bind soft metals auch as Ap(I) (BovAes et aL 2002). Bianchini et al. (2002) have observed fhat these reactive sulfirdes probabty aa:ount for the fact that waterbome chronic silver toxicity has never been shown for natural fleki situations. Attempts to extrapolate current laboratory results to fleld sites where siNer is found have resulted in extremely low reyulatory limits (Karen et al. 1999). Differences in laboratory test methods From time to time silver toxicity studies have been attempted using the insoluble siNer saits, but the researchers had to resort to indirect methods to achieve the desired silver salYs con- centration, see for example, Wood et al. (1996). A number of researchers investigatlng the toxicity of insoluble silver salts have attempted to circum- veM the problem of the very low solubility by pre- paring tlhe required silver salt in solutlon by chemicat reaction prior to toxicity testing. For ex- ample, adding silver nitrate to a solutwn of so- dium chloride to produce silver chloride. This oveiiooks the fact that these anions, often in some excess, will aet to reduce the available stl- ver ion concentration in solution (Ksp) leading to AgCI (precipitate) and or may produce a silver - Scientifk Papers -