Metals and metalloids are electropositive elements that occur in all ecosystems, although natural concentrations vary according to local geology. Land disturbance in metals-enriched areas can increase erosion and mobilize metals into streams. Human activities redistribute and concentrate metals in areas that are not naturally metals-enriched. These metals can reach water bodies when they are released into the air, water, and soil. Unlike sediment and nutrient impairments, there is often no visible evidence of metals contamination (Figure 1).

While some metals are essential as nutrients, all metals can be toxic at some level and some metals are toxic in minute amounts. Impairments result when metals are biologically available at toxic concentrations affecting the survival, reproduction, and behavior of aquatic organisms.

Checklist of sources, site evidence and biological effects

This module addresses water column contamination by metals and metalloids that commonly cause toxic effects. These include arsenic, cadmium, chromium, copper, lead, inorganic mercury, nickel, selenium, and zinc. Organic mercury and tributyl tins are special forms of metals that are beyond the scope of this module. Salts of abundant and relatively nontoxic metals are discussed in the Ionic Strength module, and metals in complex toxic mixtures are discussed in Unspecified Toxics module on toxic substances in the water column.

Metals should be included as a candidate cause when potential human sources and activities, site observations, or observed biological effects support portions of the source-to-impairment pathways in the conceptual diagram for metals (Figure 2). This diagram and some of the other information also may be useful in Step 3: Evaluate Data from the Case. The checklist below will help you identify key data and information useful for determining whether to include metals among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate metals as a candidate cause. For more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.

Consider listing metals as a candidate cause when the following sources and activities, site evidence, and biological effects are present:

Consider contributing, modifying, and related factors as candidate causes when listing metals as a candidate cause:

• Unspecified toxic chemicals
• Ionic strength

Consider other causes with similar evidence:

Colored water:	Nutrients (sulfate reducing bacteria can color water)
Spinal abnormalities:	Temperature
Aquatic life kills:	Other toxics, low dissolved oxygen, pH
Blackened tails:	Whirling disease
Gill damage/mucous:	pH, pathogens

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