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82 Journal of Weather Modification Volume 41 Because of the importance of DOC in ameliorat- ing silver toxicity, it is relevant to note an impor- tant example of a so-called "hot ma ment" (McClain et al. 2003) which could be ex- pected to occur in the KNP. A"hot moment" is an isolated zone of enhanced biogeochemical cycling (refeRed to as a"hotspot"), which is itself "hot" in a temporal dimension (referred to as a "hot moment"). In this specific case, the "hot moment" is the pulse of DOC that leaches from near-stream soils dudng snowmelt in aipine areas and which would be expected to play a key role in binding any sil- ver ions arising from the silver iodide associated with the snowpack. Boyer et a/. (2000) reported that in Deer Creek, Colorado, DOC increased rapidly from 1 to more than 4 mg/L on initiation of snowmelt, remained high for about one month then decreased quickly as n.inoff peaked. The effect of snowmelt is to flush DOC accumulated under the snowpack, and this DOC then binds to silver. 5.3 Chloride lon and other comolexes In a study on rainbow trout (Oncofiynchus myklss), Galvez and Wood (1997) showed that increasing calcium by 100-fold increased the me- dian lethal time by a factor of 10. However, in- creasing chloride ion levels by 100-fold increased the medlan lethal time by a factor of at least 100- fold. As they observed in their paper, "... com- plexation processes are expected to reduce sil- ver ion concentrations to weli below acute toxicity concentrations". The importance of complexes in reducing silver toxicity is well illustrated in the study of silver thi- osulfate toxicity to freshwater rainbow trout (Wood et al. 1996). In this study, rainbow trout were ab{e to tolerate a 3000-fold higher concen- tration of silver where it was complexed as Ag (S203)'" compared with free Ag+ from silver ni- trate. During exposure to silver thiosulfate there was a doubling of silver concentrations compared with that found for silver nitrate. This may reflect the increased octanol-water partition coefficient of these neutral species (Ratte 1999, Reinfelder and Chang 1998, and Fortin and Campbell 2000). In their review, Hogstrand and Wood (1998) point out that the ability of native chiodde and sulflde to significantly reduce the toxicity of silver by pre- cipitating it out of solution in natural waters should not be overlooked. Silver ion forms com- plexes with chloride ion including AgCl2, AgC132' and AgC43'. Increasing the chloride ion concentrations will increase the concentration of these chloro- complexes. There is some evidence that the neu- tral AgCI may represent the most bioavailable form as suggested by Bryan and Langston (1992) and others. It has been suggested by some researchers that chloride levels >35 mg/L will affect silver solubil- ity in fresh watera. However, as we now show lrnnrer concerttrations can aiso be eAective. In their study examining the toxicity of silver to seawater-acclimated rainbow trout, Ferguson and Hogstrand (1998) note that in brackish wa- ter, with a typical chloride concentration of 50 mM NaCI (1775 mg/L Cr), total silver at a con- centration of 0.1 g/L was not toxic over 168 hr. These silver ooncentratona however are well outside the ranges expected in the planned triai. 5.4 Sulfldes and Sulfur C n inin Li ands There is a growing recognition that because re- active suifldes are found in oxic as well eis anoxic environments, silver = sulfide" complexes may have a greater impact in reducing acute siiver toxicity than many, if not most, of the factors de- scribed above. Probably the most important chemical fact to note is the strength of the silver - sulfur bond. The presence in the environment of tiny concen- trations of inorganic suifides and organic mercap- tans plays a major role in the environmental be- haviour of siiver. As mentioned earlier, silver binds strongty to the suifide ion (see KsP data), which results in nanogram per litre concentra- dons of aqueoua dissdved siNer. This outcome is aiso found for sulfide ion associated with inor- ganic and organic species. Occluded mercaptans as well as HS' or H2S trapped within colloids or particulate matter witi act as reaction sftes for Ag . Silver sorbs rapidty onto amorphous FeS giving an aqueous silver ion concentration similar to that for AgzS solubil- ity. - Scientific Papers -