Geoscientists follow arsenic from chicken feed to streambeds

10/11/05

Blacksburg, Va. What happened to the chicken when she crossed the road is less important that what happens to what she eats when it is used as fertilizer.

Organic arsenic is fed to poultry to prevent bacterial infections and improve weight gain. A little bit of arsenic is taken up by the tissue and the majority of it is excreted in urine. Poultry litter -- the wood chips, feathers, droppings, and urine from under poultry houses -- is rich in nitrogen and phosphorous, so is a logical fertilizer. But what happens to that arsenic?

Virginia Tech geoscientists are determining what happens to such feed additives when they are part of the manure applied to agricultural fields. They will present their research at the Geological Society of America national meeting in Salt Lake City Oct. 16-19.

In research funded by the U.S. Department of Agriculture, Madeline Schreiber, associate professor of geosciences at Virginia Tech, carried out field and laboratory studies to discover the fate of arsenic fed to poultry. She and her graduate students found that bacteria in the litter and in shallow subsurface soil transform organic arsenic to inorganic arsenic. Organic arsenic is not highly toxic to humans, but inorganic arsenic, with its organic component removed, is toxic.

"We found that organic arsenic is highly soluble in water and is rapidly biotransformed to inorganic arsenic," Schreiber said. Despite laboratory findings that show a strong adsorption of inorganic arsenic to minerals in the soils and aquifer sediments, a surprising finding from water samples from streams receiving runoff is that low concentrations of arsenic are transported to streambeds instead of being retained by the aquifers, Schreiber said. "We think that the arsenic is adsorbed onto nanoscale particles that pass though our filters and through the soil column," said Schreiber. "This suggests that particle transport is an important mechanism in arsenic cycling in these watersheds."

Graduate student Mary Harvey is currently studying the adsorption potential for organic arsenic to iron oxides and clay minerals. Although much of the organic arsenic is biotransformed to inorganic arsenic before it reaches the aquifer, rapid flushing of organic arsenic in to the subsurface during storm events introduces organic arsenic to the subsurface; thus, understanding its adsorption is important.

Schreiber emphasized, "All of the arsenic concentrations we are finding at our field site are below the drinking water standards, even below the new standards of 10 parts per billion, which will come into effect in February 2006."

Schreiber is now collaborating with microbiologist John Stolz of Duquesne University, and expert on bacteria that transform arsenic. Schreiber and Stolz are looking at bacteria present in the litter and soil and the conditions under which biotransformation occurs.

Schreiber will deliver her paper, "Arsenic cycling in agricultural watersheds: The role of particles," at 3:10 p.m. at the Salt Palace Convention Center, room 250 DE. Harvey will present "Adsorption characteristics of roxarsone, an organoarsenic poultry feed additive," following Schreiber's talk. Co-authors with Harvey are Schreiber and Professor Christopher Tadanier, Research Assistant Professor of Geological Sciences at Virginia Tech.

Source: Eurekalert & others

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