The problem of establishing ecologically based soil quality criteria: The case of lead

Soil quality guidelines are being developed to address the problem of lead in soils. They define an acceptable total soil concentration of lead. Recent studies indicate that only a small portion (found in the soil solution) of the total is immediately available to biota and therefore a direct ecological hazard. This paper focuses on the difficulty of assessing the ecological risk of lead in soils and the factors controlling the fraction of the total that is available to soil organisms. Toxicity data on plants and soil animals used to create guidelines indicate that the lowest observed adverse effects occur at concentrations ranging from 50 to over 40 000 mg Pb kg(-1) soil; however, the tests, using clean soil spiked with soluble lead salts, do nor adequately reflect the conditions likely to be found in the field. In most cases the lead in contaminated soils can be expected to be transformed into relatively stable, insoluble forms over time; although the final form of the metal will depend on the form in which the lead was originally introduced and the environmental conditions that have persisted. The selective chemical extraction procedures used to define different ''pools'' of lead in soils, and to define the amounts of bioavailable lead, have not been properly validated. The philosophy behind the derivation of soil quality guidelines is discussed and recommendations are made for the development of scientifically defensible guidelines for lead in soils.