## Example

A final link in the food chain transport of selenium is cow's milk. What is the selenium concentration in cow's milk?

Solution The concentration in cow's milk is calculated from Eq. 8.17:

### Cmilk UCvFmilk

From Table 8.2 the vegetative uptake rate for dairy cows is 36 kg(w)/d, from Table 8.1 the milk transfer factor for selenium is 1.0 x 10-2 d/L(milk), and from Example 8.6 the selenium concentration in vegetation is 0.038 mg(c)/kg(v,wet). Thus,

Cmilk = t/CFmilk = [36 kg(v,wet)/d][0.038mg(c)/kg(v,wet)][1.0 x 10-2 d/L(milk)] = 0.014 mg(c)/ L(milk)

A final consideration is the time required for food products to reach consumers. This time, known as a holdup time, can range from 20 days for beef to 14 days for fruits and vegetables to 1 day for leafy vegetables and dairy products (NRC 1992). For crops exposed to long-term releases of contaminants that degrade quickly, such as short-lived radionuclides, holdup times may significantly reduce the amount of contaminant taken up by humans. In such cases, the appropriate degradation correction should be applied.