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76 )ournal of Weather Modification . To determine if the use of Agl would re- suft in a si9nfficant adverse impact on the environment; and ?m and envi- . Design a monitoring pi'a9 roriate ronmental mana? g en ?Pe?v ronmental to the proj `? ? risks identfied• in this paper we present the key elements of the filerature review and its relevance to the SPERP, and provide a brief overview of the SPERP ecotoxicity monitoring pro9ram and brief interpre- tation of the monitoring results to date. 2. pROPER ES OF SILVER A D SSLVER COMPOUNDS The element silver ocwrs naturatty in the earth's crust at a COn?ntratiO fPhysics) and ?s (Handbook of ChemistrY and found in the free state as the metal, and as ores such as the sulfide, arsenidalhis not (Cotton and Wlkinson, 1962). The met soluble, but salts such as silver nitrate (A9N03) are very soluble. Other silver salts? such as the sulfiid??l?le in water (PurcelCl'and PetIodide ers 1998 `) are Insoluble or complexed silver compounds were found to be much less toxic or essentlallY non- toxic to a renge of terresMal and aquadc verte- brates (Ratte, 1999)• Silver thiosulfate was found to be 15,000 times less toxic, and AgCI 1?1?,?) times less toxic than silver nitrate (Ratte, Particulate AgCt is described as virtually non- toxic (Bury et aL 1999; Rodgers et al. 1997). The importance of speciation of trace metals such as silver is well understood (Al1en and Hansen 1996)• The silver cation (A9+) binds strongly with li?ai d?s found in natural waters, hence its toxicity Tab{e 1: Sotubflity of Seleded Silver Salts Salt Silver nitrate AgN03 Silver ch4oride AgCI SiNer iodida Agl silver suKide AgsS Volume 41 reduced by the presence of substances with which it can associate or fo" c°valent bonds. Some silver complexes and silver ions are readily adsorbed to particulate matter to the extent that oniy some 25% of total silver is estima ied tWbe dissolved as either ion, cdloid or con?P ( et al. 1997). Free silver ion is known to be fun9iadal, algiadal and bacteriddal at relativey low doses. Typi- catiy, bactericidal concentrations are of the order of 0.01 to 1.0 mg/l-, which are well beloa+ human health hazard levels. Soluble silver compou?ble are used in medicine and for sterilisongba?cterial water, in park due to the sensi?vity metabolism to Ag' inhibition of the thioi function- ality in enzymes. 2.1 Solubilitv of Siiver Salts Silver is known to be a particular hazard in aquatic environments, however research over the tast decade and more has shown that the tox?onY is essentially a function of silver speciarather than total dissolved silver ion concentra- tion (gowles et al. 2002, Shafer et al. 1998, and il- others). Nevertheless, a inni anr assessmentof ity is a useful starting po its likely bioavailability. The solubility in water of some sefected silver compounds is shown in Table 1 below• In solutiort, the silver ion (Ag`) has been snown to be toxic to aquatic plants and a range of ani- mals (see for example Table 3), however the tox- icity has been clearty demonsVated to be de' pendent on the dhemical fortn, and has been shown to coRelate with free ionic silver (Ag`). For silver salts such as silver nitrate (whid'? dis- sociate strongly) the concentration of silver ion equates to the concentratlon of the silver salt. Solubllity (In g1100 ml) Cold water Hot waWr 122 952 0.000089 0.0021 insoluble insoluble insoluble _L ?nsoluble - Scientific Papers -