Surface waters contain natural organic matter and particles, both of which react with metals to reduce their bioavailability. Often, more than 90% of the metal present in the water is adsorbed to particles or complexed with organic matter. Environmental assessments have traditionally considered total recoverable metal as the basis for establishing water quality criteria and standards and in conducting risk assessments. It is not surprising, therefore, that water quality criteria frequently overestimate biological effects by a factor of 2- to 20-fold. More recently, it has been recognized that dissolved metal concentrations are more indicative of the bioavailable fraction of the total metal present in the water. Both approaches are protective, but conservative. A number of studies are underway to provide a better assessment of the impact of metals through the integration of environmental chemistry and toxicology. These are based on the understanding that the natural metal-binding agents in the water compete for metals with receptor sites on organisms, such as the gills of fish. This approach allows a better prediction of actual metal effects than does the present conservative approach.

In soils and sediments, concentrations of a metal may span three to four orders of magnitude. A low concentration at one site may have an adverse biological effect, while at another site a concentration a hundred-fold greater may not have any adverse effect. Clearly, an understanding of the chemical forms of the metal present in these systems, and the relationship between these forms, which are often called chemical species, and the bioavailability of the metal is necessary to adequately evaluate the potential impact of the metal. In both sediment and soil, a large fraction of the total metal is associated with minerals. This portion of the metal is bound so firmly that it is effectively non-bioavailable. Other natural processes also reduce metal availability. Sediments contain large concentrations of sulphide which sequester metals such as copper, lead, nickel and zinc. There are few instances that have been reported where metals in sediment are toxic to aquatic organisms. In soils, metals associate with organic matter and with iron and manganese oxide coatings, which reduces their bioavailability.

The effectiveness of these natural processes in rendering metals non-bioavailable is well-understood in the agricultural community. For example, large portions of the agricultural soils of the USA, Australia and other countries have trace element deficiencies, particularly of zinc. The effectiveness of fertilizer additions depends on the form of the added metal. With time, the added zinc becomes less available as it is incorporated by the soil.

Most soils, natural waters and sediments contain a large fraction of metals in non-bioavailable forms. All chemical analyses tend to overestimate the concentration of metal that is bioavailable. To obtain better estimates of the bioavailable metal in environmental samples, scientists are relying on analytical procedures other than total metal for the purposes of risk assessment and regulatory compliance. Because only a fraction of the total metal in any given environmental medium is bioavailable, it is important for appropriate chemistry and biology to be considered in regulatory programmes, such as risk assessments.