typical value (at low water concentrations) on the order of 1,000; and that, in sediments, estimated Kd values range from 1 to 160,000, with a typical value (at low water concentrations) on the order of 6,000.

Plants are contaminated with cadmium via two routes—one is uptake of cadmium in soil through the roots and the other is deposition of cadmium from air onto leaf surfaces with translocation to other plant parts. Cadmium residues in plants are typically less than 1 mg/kg (IARC 1993).

The plant-soil partition coefficient, Kps, expresses the ratio of contaminant concentration in plant parts in mg/kg (plant fresh mass) to concentration in wet root-zone soil, in units of mg/kg. Root uptake of cadmium as Cd+2 in plants is passive and occurs though uptake by roots of cadmium dissolved in water; cadmium is highly mobile in plants and readily translocated to other plant parts (Bodek et al. 1988). Plant-soil partition coefficients have been reported in the range 0.015 to 2.1 mg/kg with a likely value on the order of 0.1 mg/kg (Bowen 1979; Friberg et al. 1979; Nriagu 1980; Baes et al. 1984a).

According to Bodek et al. (1988), airborne deposition is believed to contribute to concentrations of cadmium found in plant leaves. At low concentrations, the ratio of plant-leaf concentration to air concentration, when air and plant environments are in contact, can be estimated based on the balance of gains from wet and dry deposition versus losses by wash-off and plant decay.

Atmospheric emissions of cadmium from human sources are estimated to exceed those from natural sources by about an order of magnitude (IARC 1993). ATSDR (1997a) summarizes references indicating that the mean levels of cadmium in ambient air range from less than 0.001 µg/m3 in remote areas to 0.003 to 0.04 µg/m3 in the United States. Cadmium metal and cadmium salts exist in ambient air primarily in fine suspended particulate matter. When inhaled, some fraction of this particulate matter is deposited in the lung airways and the rest is exhaled. In urban areas, an individual who breaths 20 m3 of air will inhale about 0.2 µg/day cadmium.

Cadmium enters drinking water directly from pollution sources, deposition from air to surface water, soil runoff to surface water, or leaching from rocks and soils into ground water. The concentration of cadmium dissolved in the open ocean is less than 0.005 µg/L (Nriagu 1980; IARC 1993). The concentration of cadmium in drinking water is generally reported to be less than 1 µg/L but it may increase up to 10 µg/L as a result of industrial discharge and leaching from metal and plastic pipes (Friberg et al. 1974; ATSDR 1997a). An individual who consumes 2 L of water daily with a cadmium concentration of 1 µg/L will have an intake of 2 µg/d.

For aquatic organisms, the bioconcentration factor (BCF) provides a measure of chemical partitioning between tissue and water and has units of mol/kg (fish) per mol/L (water). Bodek et al. (1988) report both ocean- and freshwater-fish bioconcentration factors in the range 200-50,000 L/kg, with 2,000 L/kg being a typical value in this range of reported values.

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