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Metal Toxicity and Water Quality Standards

Metals can enter aquatic environments in a chemical form, or species, which may or may not be bio-available and may or may not be toxic. And, once in the water column, metals can change into different forms depending on site-specific conditions. Whether metals in surface waters are actually bio-available and toxic depends not only on their form but also on interactions with biological receptor sites (“biotic ligands”) on sensitive aquatic animals such as fish and crustaceans, and on competition with natural substances in the water that may minimize or even eliminate bioavailability and thus toxicity of the metals.

Factors that increase the potential toxicity of metals:

  • pH – the lower the pH the greater the potential of toxicity. The solubility of metals is greater at lower pH and the toxicity of metals increases when the metals are in soluble form because it increases the bioavailability of the metal. Sediment composition (both substrate and suspended) – sediment composed of fine sand and silt will generally have higher l evels of adsorbed metal. Metals also have a high affinity for humic acids, organo-clays, and oxides coated with organic matter.
  • Temperature – increased water temperature will increase toxicity.
  • Dissolved oxygen – decreased dissolved oxygen will increase toxicity.
  • Alkalinity – alkalinity reduces toxicity by combining chemically with copper.
  • Hardness - Toxicity of all metals except Hg2+ decreases with increasing water hardness. The State Water Quality Standards for dissolved metals vary with hardness (Chapter 173-201A WAC).

Because predicting effects of metals on aquatic systems has been, at best, problematic, regulatory approaches have tended to be precautionary, generally resulting in a high level of protection. Water quality criteria have acute and chronic points of compliance. Chronic toxicity, according to the traditional meaning, refers to the long-term effects on the organ systems of exposed organisms that result in death, or impaired function. Acute toxicity refers to death due to short-term exposure.

In order to assess the toxic potential of the metals found in our samples, the data were compared to the criteria for the protection of freshwater life created by the USEPA and now adopted by the Washington State Department of Ecology. For most trace metals, the toxicity of a metal diminishes with hardness.