compounds biodegrade, reducing the potential for human and ecological exposure over the long term. Metals, on the other hand, are infinitely persistent. Radionuclides undergo natural radioactive decay that, for some compounds (such as tritium), may significantly reduce risks over relatively short time periods. However, for other radionuclides (including various isotopes of Tc, U, Pu, and Th), half-lives are very long, meaning that risks posed by the presence of these compounds will persist for a very long time. As shown in Table 3-2, half-lives for radionuclides vary quite significantly depending on the isotopes present.

The potential for humans or sensitive ecosystems to be exposed to metals and long-lived radioactive materials is strongly affected by a number of factors that must be considered in assessing these contaminants. Some metals and radioactive contaminants have more than one oxidation state, which differ in mobility and toxicity (see Box 3-1). Like organic compounds, metals and radioactive contaminants can partition into organic matter present in soils. They also can be sorbed by other soil components, including cation exchange sites and metal oxides, and they can precipitate. Because of the mul-