Bioavailability of Metals

INTRODUCTION The fate of various metals, including chromium, nickel, copper, manganese, mercury, cadmium, and lead, and metalloids, including arsenic, antimony, and selenium, in the natural environment is of great concern (Adriano, 1986; 1992), particularly near former mine sites, dumps, tailing piles, and impoundments, but also in urban areas and industrial centers. Soil, sediment, water, and organic materials in these areas may contain higher than average abundances of these elements, in some cases due to past mining and (or) industrial activity, which may cause the formation of the more bioavailable forms of these elements. In order to put elemental abundances in perspective, data from lands and watersheds adjacent to these sites must be obtained and background values, often controlled by the bedrock geology and (or) water-rock interaction, must be defined. In order to estimate effects and potential risks associated with elevated elemental concentrations that result from natural weathering of mineral deposits or from mining activities, the fraction of total elemental abundances in water, sediment, and soil that are bioavailable must be identified. Bioavailability is the proportion of total metals that are available for incorporation into biota (bioaccumulation). Total metal concentrations do not necessarily correspond with metal bioavailability. For example, sulfide minerals may be encapsulated in quartz or other chemically inert minerals, and despite high total concentrations of metals in sediment and soil containing these minerals, metals are not readily available for incorporation in the biota; associated environmental effects may be low (Davis and others, 1994). Consequently, overall environmental impact caused by mining these rocks may be much less than mining another type of mineral deposit that contains more reactive minerals in lower abundance. In aquatic environments where chemically reducing conditions may prevail, metals from mining activities may be associated with sulfide minerals. These sulfide minerals are present either in the ore deposit or formed by bacterial reduction of the sulfate in oxidized tailings. Most metal sulfide minerals are quite immobile, as long as they remain in a chemically reducing environment, and they may have little impact on biota despite anomalous metal concentrations.