The effects of various chemical manipulations of test water on acute toxicity of silver to fathead minnows (Pimephales promelas) were investigated. Increases in hardness and organic carbon substantially reduced toxicity. Toxicity was also inversely related to pH and alkalinity when these parameters were jointly changed by addition of strong acid or base. The addition of 2 meq/L sodium sulfate had no significant effects, but the addition of 0.2 meq/L sodium chloride increased toxicity, perhaps related to the formation of the dissolved AgCl0 complex. We also evaluated the effects of static versus flow‐through test conditions, feeding during exposure, and aging of test solutions before exposure on the acute toxicity of silver to fathead minnows and Daphnia magna. Static conditions and feeding reduced toxicity, likely as a result of accretion of organic carbon. Aging of test solutions had little effect. For both juvenile fathead minnows and D. magna, silver was much less toxic in water from the St. Louis River than in our normal laboratory water, presumably because of the much higher organic carbon content of the river water. This study identified some aspects of test conditions that are important in assessing the risk of silver to aquatic biota, but improved assessments will require information for more conditions, species, and endpoints. More importantly, if toxicity test results are to be extrapolated among waters with different chemistries, adequate characterization of the chemical speciation of silver and a better understanding of the mechanisms of silver toxicity and its relationship to silver speciation and other chemical factors are needed.

Abstract-The effects of various chemical manipulations of test water on acute toxicity of silver to fathead minnows (Pimephales promelas) were investigated. Increases in hardness and organic carbon substantially reduced toxicity. Toxicity was also inversely related to pH and alkalinity when these parameters were jointly changed by addition of strong acid or base. The addition of 2 meq/L sodium sulfate had no significant effects, but the addition of 0.2 meq/L sodium chloride increased toxicity, perhaps related to the formation of the dissolved AgCl 0 complex. We also evaluated the effects of static versus flow-through test conditions, feeding during exposure, and aging of test solutions before exposure on the acute toxicity of silver to fathead minnows and Daphnia magna. Static conditions and feeding reduced toxicity, likely as a result of accretion of organic carbon. Aging of test solutions had little effect. For both juvenile fathead minnows and D. magna, silver was much less toxic in water from the St. Louis River than in our normal laboratory water, presumably because of the much higher organic carbon content of the river water. This study identified some aspects of test conditions that are important in assessing the risk of silver to aquatic biota, but improved assessments will require information for more conditions, species, and endpoints. More importantly, if toxicity test results are to be extrapolated among waters with different chemistries, adequate characterization of the chemical speciation of silver and a better understanding of the mechanisms of silver toxicity and its relationship to silver speciation and other chemical factors are needed.

Mercury and copper inventories are low in central Lake Superior and increase markedly towards the Keweenaw Peninsula. Total copper flux to Lake Superior sediments averages 5.0 ± 2.5 μg cm −2  year −1 (mean ± 95% confidence limits), whereas mercury flux averages 7.5 ± 4.2 ng cm −2  year −1 . In the Keweenaw Peninsula region, copper, mercury and silver inventories are elevated and highly correlated. High copper, silver and mercury inventories can be traced back to shoreline stamp sand piles, the parent ores and to smelters. Mercury occurs in elemental form, probably as a natural amalgam, in native metal (copper, silver, gold) deposits and was liberated as volatile Hg 0 during on-site copper smelting. Stamp mills discharged at least 364 Mt of ‘stamp sand’ tailings, whereas smelters refined 5 Mt of native copper, liberating together at least 42 t of mercury. The Keweenaw situation is not unique, as mineral-bound mercury is commonplace in US and Canadian Greenstone Belts and is of worldwide occurrence in massive base metal ores.

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