Silver in freshwater and marine water

​Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Description of chemical

Silver is among the less common but most widely distributed elements in the Earth’s crust (CCREM 1987). Silver is commonly used in photographic materials as well as in coins, jewellery, silver plating, mirrors, dental materials and electronic equipment. It is usually found as a by-product of mining for lead, zinc, copper and gold (CCREM 1987).

Primary sources of anthropogenic silver in surface waters include industrial and smelting wastes, and wastes from jewellery manufacture and the production and disposal of photographic materials (USEPA 1987a). Silver exists in aqueous systems primarily in the univalent state Ag (I). Background levels of silver in pristine unpolluted waters are around 0.01 µg/L (Ratte 1999). The current analytical practical quantitation limit (PQL) for silver is 0.01 µg/L in fresh water and 1 µg/L in marine water (NSW EPA 2000).

Silver is one of the most toxic metals to aquatic life in laboratory experiments. Silver nitrate and silver iodide are particularly toxic, whereas silver chloride is 300 times less acutely toxic (CCREM 1987). Silver thiosulfate, a common waste from photoprocessors, has a very low toxicity, > 15,000 times that of silver nitrate (Ratte 1999). It is important to note that in the natural environment, silver is often found in less bioavailable complexes with chloride, dissolved organic carbon and sulfur-containing ligands and hence laboratory data may overestimate the toxicity of silver (Gorsuch & Purcell 1999). Hence, site-specific assessments where silver levels exceed the trigger value assume greater importance. Erickson et al. (1998b) demonstrated that silver was markedly less toxic to fathead minnows (10-fold) and D. magna (60-fold) in St Louis River water than in laboratory water, presumably due to a 10-fold higher organic carbon content in the river.

The ecotoxicology of silver has been extensively discussed in recent issues of Environmental Toxicology and Chemistry (Volume 17 no. 4 1998 and Volume 18 no. 1 1999). The acute toxicity of silver is related to the water hardness; toxicity decreases as hardness increases. Galvez and Wood (1997) reported a hardness algorithm for maximum total recoverable silver:

Ag (µg/L) = e(1.72[ln hardness] – 6.52)

This was not adopted for these guidelines, as Hogstrand and Wood (1998) reanalysed the data and found that chlorine was a much more significant modifier, while calcium had modest effect. A 100-fold increase in calcium concentration increased the LT50 (time to 50% mortality) to rainbow trout Oncorhynchus mykiss by around 10-fold (Galvez & Wood 1997). A 100-fold increase in chloride concentration reduced silver toxicity to rainbow trout by at least 100-fold (Galvez & Wood 1997). These authors considered that silver toxicity can be correlated with the free Ag+ ion and that any factors affecting availability of this free ion will modify acute toxicity. Other forms of silver in water do not appear to contribute to its high toxicity (Hogstrand & Wood 1998), despite their bioavailability.

In freshwater fish, silver appears to damage the gills and toxicity appears to be unrelated to bioaccumulation (Hogstrand & Wood 1998). The toxic mechanism does not appear to change at sublethal levels, down to 5% of the 144-hour LC50 (Hogstrand & Wood 1998). Algae can bioaccumulate silver significantly but invertebrates and fish show much less propensity to accumulate silver (Ratte 1999). Bioconcentration factors in fresh waters ranged from not detectable to 150 (USEPA 1987a).

Acute toxicity values for both freshwater macroinvertebrates and fish ranged from 0.9 µg/L to 29 µg/L for the most sensitive species (USEPA 1987a). The most sensitive organisms are small aquatic invertebrates, particularly embryonic and larval stages (Ratte 1999). Toxicity varies markedly with water conditions, but Ratte (1999) reported 48-hour LC50 values for silver nitrate to D. magna as low as 0.9 µg/L and up to 12.5 µg/L. LC50 (96 hours) values as low as 6.5 µg/L were reported for fish. The lowest LC50 reported for freshwater fish by Hogstrand and Wood (1998) was 5 µg/L.

Chronic toxicity concentrations for silver in fresh waters are reported below and these data illustrate the very high toxicity of silver. Sorption and precipitation predominate in removing silver from the water column (CCREM 1987).

The acute toxicity of silver to marine fish (96-hour LC50 of 330 to 2700 µg/L) is considerably lower than for freshwater fish (5 to 70 µg/L) (Hogstrand & Wood 1998). Toxicity to most species increases with decreasing salinity (Hogstrand & Wood 1998). The intestine, rather than the gills, seems to be the main site of toxic action in marine fish. Ammonia accelerates silver toxicity in marine environments (Hogstrand & Wood 1998).

Freshwater guideline

Screened chronic freshwater data (40 points) were available for silver for seven taxonomic groups, as follows (data expressed as NOEC equivalents after adjustment using the procedure adopted from van de Plassche et al. 1993) (pH range was 6.64 to 8.39):

Fish: three species, 0.07 µg/L (O. mykiss; from 548-d LOEC, mortality) to 22 µg/L (Micropterus salmoides; from 8-day LC50). Most geometric means were below 2 µg/L.

Amphibian: one species, Ambystoma opacum, 48 µg/L (from 8-day LC50).

Crustaceans: three species, 0.11 µg/L (D. magna; from 21-day MATC, mortality and reproduction) to 1.3 µg/L (C. dubia; from 7-day LC50).

Insects: two species, 3.1 to 6.6 µg/L (from 7 to 14 day LOEC & NOEC, mortality).

Molluscs: one species, Corbicula fluminea, 2.6 µg/L (21-day NOEC, growth) to 31 µg/L (from 8-day LC50).

Rotifer: one species, Philodina sp, 280 µg/L (from 4-day EC50, immobilisation).

Algae: one species, Scenedesmus sp., 1.6 µg/L (from 6-day EC50).

Although this figure is close to the lowest calculated NOEC for O. mykiss (0.07 µg/L), the experimental LOEC figure from which it was derived was 0.17 µg/L. As most natural water parameters should ameliorate toxicity, the 95% figure is considered sufficiently protective.

Marine guideline

Chronic data (42 points) were available for silver in marine water on eight species, belonging to five taxonomic groups, as follows (some NOEC figures were derived from other end-points):

Crustacean: one species, Mysidopsis bahia, 28 to 38-day NOEC, 2.5 to 42.0 µg/L (from MATC, reproduction and mortality).

Mollusc: three species, 8 to 28-day NOEC, 5 to 42 µg/L (from LC50 and MATC, growth, reproduction). All species were similar in sensitivity.

Annelid: one species, Neanthes sp., 5 to 28-day NOEC, 21 to 71 µg/L (from LC50).

Cnidarian: one species, Phymactis sp., 28-day NOEC, 6 to 42 µg/L (from MATC, growth, reproduction and mortality).

Algae: two species, Champia parvula (red macroalgae) and Ditylum (diatom), 5 to 14-day NOEC, 0.8 to 3.5 µg/L (from MATC, reproduction and LC50, growth).

References

ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.

CCREM 1987. Canadian water quality guidelines. Canadian Council of Resource and Environment Ministers, Ontario.

Erickson RJ, Brooke LT, Kahl MD, Venter FV, Harting SL, Markee TP & Spehar RL 1998b. Effects of laboratory test conditions on the toxicity of silver to aquatic organisms. Environmental Toxicology and Chemistry 17, 572–578.

Galvez F & Wood C M 1997. The relative importance of water hardness and chloride levels in modifying the acute toxicity of silver to rainbow trout (Oncorhynchus mykiss). Environmental Toxicology and Chemistry 16, 2363–2368.

Gorsuch JW & Purcell TW 1999. Eight years of silver research: What have we learned and how may it influence silver regulation? SETAC News 19(4), 19–21.

Hogstrand C & Wood C M 1998. Toward a better understanding of the bioavailability, physiology and toxicity of silver in fish: Implications for water quality criteria. Environmental Toxicology and Chemistry 17, 547–561.

NSW EPA 2000. Analytical Chemistry Section, Table of Trigger Values 20 March 2000, LD33/11, Lidcombe, NSW.

Ratte HT 1999. Bioaccumulation and toxicity of silver compounds: A review. Environmental Toxicology and Chemistry 18, 89–108.

USEPA 1987a. Ambient aquatic life water quality criteria for silver. Environment Research Laboratories, US Environmental Protection Agency, Duluth, Minnesota.

van de Plassche EJ, Polder MD & Canton JH 1993. Derivation of maximum permissible concentrations for several volatile compounds for water and soil. National Institute of Public Health and Environmental Protection, Report 679101 008, Bilthoven, The Netherlands.