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Sludge study relieves environmental fears

By Cat Holmes
University of Georgia

Toxic levels of heavy metals don’t accumulate in soil or hay when properly treated municipal sewage sludge is used as fertilizer over long periods, according to a new University of Georgia study.

The UGA research group, in collaboration with researchers from the U.S. Environmental Protection Agency, examined soil and hay from fields to which treated sewage sludge, also known as biosolids, had been applied regularly for periods ranging from one to 12 years. They then compared the data to fields that had never received biosolids applications.

Biosolids land application programs are regulated under the Clean Water Act, 43 CFR Part 503, known as the 503 regulations, instituted in 1993.

503 regulations

"Some individuals have questioned whether the 503 regulations are protective of the public and the environment," said UGA scientist Julia Gaskin, who headed the research team. "This study puts some of those fears to rest."

The study found that in the soil of treated fields, concentrations of metals like arsenic, cadmium, lead and mercury found were statistically the same as those in fields that never received biosolids applications. The same was true of hay grown in most fields treated with biosolids.

Three of the fields studied did produce hay with higher concentrations of cadmium than the National Research Council recommends, Gaskin said.

Cadmium levels

These were fields where sludge was applied before 1993, she said. They’re likely to have received sludge with high cadmium levels. Current regulations would not allow land application of that type of sludge.

"The recommended cadmium level is set to prevent cadmium from accumulating in the food chain from long-term exposure in the hay, and assumes the animals have no other source of food," Gaskin said. "It would be a problem for food chain accumulation if animals didn’t eat anything but the high cadmium hay grown in those fields."

The researchers also noted that copper and molybdenum levels were higher in soil from fields treated with biosolids for the longest times. Increased copper could be beneficial for farmers, because copper is naturally low in Southeastern soils. There wasn’t a similar increase of copper levels, however, in the hay grown in treated fields.

Copper in hay

"While other studies have reported an increase in the copper in hay grown with biosolids fertilizers," Gaskin said, "the current study did not see a statistically significant increase."

Molybdenum levels, however, were higher in hay from the fields receiving biosolids for the longest times, she said. High concentrations of molybdenum can lead to copper deficiency in cattle and other ruminants.

"The increased molybdenum levels found in the hay in this study were below levels thought to induce copper deficiencies in most animals," Gaskin said. "Since copper is a necessary nutrient for cattle, we encourage farmers to use copper supplements as a good management practice."

"The study is important because it evaluates the risk of metal contamination in the soil and hay from farms participating in a biosolids land application program," Gaskin said. "This study indicates that the 503 regulations are protective and land application programs following the 503 regulations should not pose a risk of metal contamination."

The concerns raised by the study "can be addressed by good management," Gaskin said.

(Cat Holmes was a science writer with the University of Georgia College of Agricultural and Environmental Sciences.)

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