Chicken farmers on the Delaware?Maryland?Virginia peninsula are introducing between 20 and 50 tons of arsenic to the environment annually, and researchers aren?t sure where it?s ending up. At some point, this arsenic could contaminate surface and groundwater.
The poultry industry in the region raises 600 million chickens annually, according to the Department of Agriculture. In the process, these chickens are fed organic arsenic compounds, like roxarsone, to control infections and increase weight gain. Keeping within U.S. Food and Drug Administration limits, little of the roxarsone is retained in the meat. Most of it ends up in the 1.5 million tons of manure produced annually by the chickens.
Transporting this litter for disposal elsewhere costs money, so farmers stockpile it in long rows on fields called windrows, or apply it directly to corn and soybean fields as fertilizer.
U.S. Geological Survey (USGS) researchers have been analyzing arsenic concentrations in soils from agricultural fields where poultry waste has been applied, soils from more pristine environments such as nearby forests, bed sediments from the Pocomoke River, water from the river itself, surface water from ditches throughout the river?s basin, and groundwater throughout the basin, trying to trace the arsenic.
Dissolved arsenic concentrations in the Pocomoke were not much higher than normal, but during storms, levels increased 4 times, says Tracy Connell Hancock, a USGS hydrologist. Arsenic concentrations in ditch water samples also rose during storms, indicating manure particles in the soil flowed into the water from storm runoff.
Hancock and his colleagues also looked at levels of arsenic in pore water from cored sediments and shallow groundwater from wells beside an agricultural field. Arsenic concentrations were highest at the surface, fluctuating with depth, also indicating a surface source. However, "the levels we?re seeing aren?t incredibly high," Hancock says.
The only place they?ve found pure arsenic is directly in the poultry litter, says John Garbarino, a USGS research chemist. USGS scientists have experimented with gradually diluting the litter with water at room temperature and steadily monitoring the amount of arsenic. "After 24 to 48 hours, all of the [arsenic] is gone, and it seems to change into a compound we haven?t identified yet," Garbarino says. The rate of transformation seems to be bacteria-related because when the litter extract was sterilized, the arsenic remained stable for at least 10 days.
Indirect evidence indicates that the arsenic undergoes more extensive degradation after it is introduced into the soil. But so far, Garbarino says, "even though a lot of arsenic is being added to the environment, we can?t account for it all."
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