Chicken poop and arsenic
New ES&T research shows how the arsenic in chicken feed supplements may lead to surface-water and groundwater contamination.
Farmers have used animal manure as fertilizer since the beginning of recorded history, but during the past decade a growing body of research has revealed that the practice can have some polluting side effects. For example, when chicken poop is used to “improve” soils, excess nutrients like nitrogen and phosphorus can make their way into nearby waterways. Scientists have suspected that chicken waste could also be a source of arsenic—a known carcinogen. A research article published today on ES&T’s Research ASAP website (DOI: 10.1021/es051981o) supports those suspicions with some persuasive evidence linking arsenic and chicken poop.
Now that the U.S. EPA’s new drinking-water standard of 10 parts per billion for this toxic element has gone into effect, scientists are beginning to look more carefully at roxarsone, a common feed additive used in poultry operations. They are zeroing in on how it can be transformed from a relatively harmless organic arsenic species into more toxic inorganic forms after being released into the environment when chicken manure is used as fertilizer.
The U.S. Department of Agriculture estimates that in 2006, 9.1 billion broiler chickens [52KB PDF] will be produced in the U.S. About 70% of these broiler chickens are fed roxarsone as a supplement to control infections and increase weight. They excrete it mostly unaltered, and their arsenic-laden waste is typically disposed of as a fertilizer spread onto nearby farmlands.
In earlier reconnaissance studies, U.S. Geological Survey (USGS) researchers didn’t find any roxarsone in soils and stream sediments in and near fields where chicken waste had been applied. However, they did see higher concentrations of water-extractable arsenate [As(V)] and arsenite [As(III)] compared with similar soils and sediments with no history of poultry fertilizer use. What they didn’t know was the mechanism for how the roxarsone became As(V) and As(III). The new research fills in some of the blanks.
Scientists from the University of Arizona and USGS found in laboratory experiments that in the anaerobic conditions typical in underwater sediments and subsurface soils, roxarsone was rapidly transformed into its corresponding aromatic amine, 4-hydroxy-3-aminophenylarsonic acid, in a matter of days. Over a longer of period of time (230 days), this compound biologically degraded into As(III), one of the more toxic and very mobile species of arsenic, as well as smaller amounts of As(V).
“The roxarsone itself doesn’t have a high toxicity, [but] if it ends up in an anaerobic environment, then we see that it’s eventually going to be converted to arsenite, which has a very high acute toxicity,” says Jim Field, an environmental engineer at the University of Arizona and the paper’s corresponding author. “That means if people are land-applying this material in areas with a high water table, the stuff would go immediately into an anaerobic environment where you’d get accumulation of arsenite.” Likewise, the poultry manure itself is very wet, so if it’s stockpiled, the roxarsone could transform into As(III) within the compost, and rainwater runoff could send it into waterways, where the material could accumulate in sediments.
Other studies since USGS’s initial work have shown how under aerobic conditions roxarsone is degraded over time into As(V), which is toxic to plants, notes Gregory Cutter, a chemical oceanographer with Old Dominion University. But sediments in freshwater lakes and coastal environments are typically anoxic, particularly in the Mid-Atlantic region of Delaware, Maryland, and Virginia, where many industrial poultry farms operate. “That’s the importance of this paper,” Cutter says. These facilities “are very close to sensitive bodies of water such as the Chesapeake Bay.”
Ellen Silbergeld, an environmental health scientist at Johns Hopkins University, agrees. “My concern is the scenario of [the arsenic] leaching into groundwater and [Maryland’s] Eastern Shore, where everyone drinks groundwater, and drinking water is drawn from a fairly shallow aquifer.” She recommends that arsenicals be eliminated from animal feeds altogether because land disposal of the resulting waste is unregulated. “This is a huge volume of waste being placed in a relatively restricted area where the absorptive properties of the soils are being exceeded,” she notes.
The National Chicken Council disagrees. “Our industry just doesn’t share the view that there are significant issues with this,” says Richard Lobb, a spokesperson for the trade group. “Attempts in the past to tie poultry farming to arsenic in drinking water haven’t been successful, so while this may be a theoretical concern, we don’t have any reason to believe it’s a real-world concern.”
Indeed, USGS hasn’t found a real problem, either. “We haven’t identified any particular areas that were impacted by this practice,” says John Garbarino, a USGS research chemist and one of the paper’s coauthors. However, Tracy Connell Hancock, a USGS hydrologist, points out that the potential exists for arsenic to leach into groundwater that may be used for drinking water, particularly in karst regions. “We just haven’t seen that in our studies, but we have very limited sampling,” she notes.
