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Land application of biosolids is limited primarily by metal concentrations in biosolids and metal loading rates established in the United States Environmental Protection Agency's Part 503 Rule. Most of the data used in the risk assessment to develop this rule were generated from short-term studies (less than 5 years duration). A long-term study was conducted on small plots to evaluate the effects of NuEarth biosolids applications on metal uptake in edible parts of garden vegetables. NuEarth biosolids were applied at rates of 0, 20, 40, 80, and 100 Mg/ha annually for 5 years. Six garden vegetables representing three groups of edible tissue, leaf (Swiss chard and spinach), root (beets and carrot), and fruit (green beans and tomatoes) were grown annually during the period of NuEarth application and for 15 years after application was terminated. Concentrations of Cd (200 to 300 mg/kg) and Zn (3,300 to 4,800 mg/kg) in NuEarth and annual loading rates of these metals were higher than the concentration limits for exceptional quality biosolids and the limits on loading rates established in the Part 503 Rule. The cumulative loading of the applied metals caused significant increases in Cd and Zn concentrations in the soil.

After, termination of NuEarth application, Cd and Zn concentrations in most of the vegetables tended to decrease with time. Except for Cd in tomatoes at the 40 Mg/ha/yr NuEarth rate, the concentration of Cd and Zn in the edible tissue of fruit vegetables was lower than the concentrations predicted by the risk assessment model used to develop the Part 503 Rule. In most years following termination of NuEarth applications, Cd and Zn concentration in edible tissue of the other vegetables were lower than predicted levels. The decrease in the concentration of Cd and Zn in the edible tissue of vegetables following termination of NuEarth application is attributed to immobilization of these metals by soil minerals. This study demonstrates that the bioavailability of Cd and Zn associated with the land application of biosolids decreases with time after the termination of biosolids application. Since the Part 503 risk assessment does not consider this reduction in bioavailability of metals applied via biosolids to land, it is conservative and provides more than adequate protection to animal and public health.

Document Type: Research Article


Publication date: 2002-01-01

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