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Canadian Water Quality Guidelines for the Protection of Aquatic Life - Aluminium (Withdrawn)

Review of Toxicity to Freshwater Life

Aluminium in the aquatic environment can have a major impact on aquatic life. Acid precipitation has been implicated in the loss of fish populations from acidsensitive waters through the mobilisation of aluminium (Buckler et al. 1995; Wilson et al. 1994; DeLonay et al. 1993; Palmer et al. 1988; Sadler and Lynam 1988; Baker and Schofield 1982). Heavy rain and snowmelt can also impact on the concentrations of aluminium in the aquatic environment by leaching large quantities of aluminium from watershed soils (Mount et al. 1990; Gunn and Noakes 1987). The short-term changes in aluminium concentrations caused by these events can have severe effects on aquatic biota.

Aquatic plants are able to tolerate higher levels of aluminium than aquatic invertebrates, amphibians, or fish (Sparling and Lowe 1996; Butcher 1987). As pH decreases, Ca2+, metal ions (e.g., Fe) and phosphorus may interact with Al thereby negatively impacting plant growth (Hörnström et al. 1984). Sprenger and McIntosh (1989) observed that aqueous Al concentrations are negatively correlated with pH, thus plants that grow in acidic conditions are often aluminium tolerant. Aluminium toxicity in invertebrates is primarily due to the disruption of ion regulation and the loss of Na+. Aluminium is known to be more toxic to invertebrates at pH 5.1 to 5.8 than at higher pH (Hörnström et al. 1984). Freda (1991) reviewed the toxicity of aluminium to nine species of amphibians at different life stages. Three possible amphibian toxicity types were suggested based on the interaction of aluminium with pH. Type I indicated that toxicity is negatively related to pH; Type II toxicity is positively related to pH; and, Type III, aluminium ameliorates acid toxicity. Amphibians may be at elevated risk from Al under acidic conditions, but H+ is also toxic and it can be difficult to distinguish between these two factors. Fish species are sensitive to aluminium concentrations. Key factors in aluminium sensitivity for fish are, species, life stage, and form of aluminium. Salmonids are more sensitive to the toxic effects of aluminium than are warm water fish species (Spry and Wiener 1991; Roy 1998; Valcin 1998). Several studies have reported that juveniles tend to be the most sensitive life stage while embryos are the least sensitive in fish (Roy 1998; Rosseland et al. 1992; Baker and Schofield 1982). Fish species have been found to be equally sensitive to three forms of aqueous monomeric Al (i.e., Al3+, Al(OH)2+, and Al(OH)2+) (Sparling and Lowe 1996). The mechanism of aluminium toxicity in fish tends to revolve around ionoregulatory and osmoregulatory dysfunction and various respiratory problems related to aluminium precipitation on the gills (Phippen and Horvath 1998). Aluminium binding to the gill epithelium and intracellularly within lamellar epithelial cells is a prerequisite to acute toxicity (Phippen and Horvath 1998). Visible signs of aluminium toxicity in fish include coughing response, hyperventilation, and excessive mucous clogging of the gills (Lewis et al. 1990).

When critically reviewing the scientific literature on aluminium toxicity, over-saturated aluminium solutions can be a major hurdle in the appropriate interpretation of laboratory toxicity tests (Roy 1998; Spry and Wiener 1991). Over-saturated solutions can be very toxic if the aluminium precipitates and adheres to gill tissues potentially causing asphyxiation (Spry and Wiener 1991). Lower toxicity may also be exhibited if the solution has been allowed to age thereby causing the formation of large polymers of aluminium and amorphous aluminium that do not interact with gill tissue (Freda 1991; Spry and Wiener 1991). Based on the solubility product for microcrystalline gibbsite, media containing more than 18.9 μg-L-1 of inorganic Al at pH 6 and 20°C are considered over-saturated (Parent and Campbell 1994). The aqueous chemistry of aluminium becomes very complex and unpredictable under these conditions. In the natural environment, over-saturated solutions of aluminium rarely occur, particularly in low pH waters (Hutchinson et al. 1987, 1989; Tipping et al. 1988; LaZerte 1984). Situations where over-saturated aluminium solutions could occur in the environment include neutral pH watersheds receiving inputs of aluminium sulphate from municipal wastewater treatment or from the production of pulp and paper products (Roy 1998). Studies that report toxic effects based on oversaturation of the exposure solution were not considered acceptable for guideline derivation. Similarly, studies that did not use proper controls, statistics and standardised methodologies were not selected for guideline derivation.

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