by Miller, C.V., T.C., Hancock, and J.M. Denver
Managing waste from animal agriculture is not a new issue, but recent trends to condense livestock into intensive animal feeding operations (AFOs), combined with new chemicals and pharmaceuticals that enhance weight gain in the animals, have exacerbated the problems. Approximately 450,000 AFOs are now active in the United States, and because of the large volume of manure that is produced, AFOs can pose serious risks to local and regional water quality, ecology, and human health. The U.S. Geological Survey (USGS) is conducting research on the environmental fate and transport of arsenical feed amendments from poultry houses in several small watersheds including the Pocomoke River Basin on the southern Delmarva Peninsula in Maryland. The Pocomoke River watershed is underlain by Coastal Plain sediments, which are iron-rich and heavily ditched. Ditching enhances the potential for high infiltration and contaminant transport, but the iron deposits provide a high adsorption potential for arsenic and may be trapping the arsenic in the watershed.
Roxarsone, or 3-nitro-4-hydroxyphenylarsonic acid, is currently the most commonly used arsenical compound in poultry feed in the United States, with a usage of 23 to 45 grams of chemical per ton of feed for broiler chickens for increased weight gain, feed efficiency, improved pigmentation, and prevention of parasites. By design, most of the chemical is excreted in the manure. Concentrations of total arsenic in poultry manure normally range from 15 to 35 parts per million. For the Delmarva Peninsula, where an estimated 600 million chickens produce more than 1.5 billion kilograms of raw manure annually, this translates to an annual load of 20 to 50 thousand kilograms of arsenic. Base-flow concentrations of arsenic in suspended particles and bed sediment in the Pocomoke River and its tributaries ranged from 0.8 to 21 milligrams per kilogram in 1997 and 1999, and were moderate to high when compared to national averages for river samples. Concentrations of arsenic in filtered surface water were all less than 1 microgram per liter during base flow. Values for dissolved and particulate arsenic were within the ranges found in other tributaries of the Delmarva Peninsula that also are dominated by agriculture, but have a lower density of poultry houses. Shallow ground-water concentrations of dissolved arsenic ranged from less than 1.0 to 7.6 micrograms per liter, and this appears to be an important reservoir for arsenic cycling in the Pocomoke Basin. Future sampling will focus on speciation methods for inorganic forms of arsenic, for roxarsone and potential organic degradation pathways, and sampling will also be expanded to other tributaries in the Chesapeake Bay watershed where there are high densities of AFOs.
Miller, C.V., T.C., Hancock, and J.M. Denver, 2000, Environmental Fate and Transport of Arsenical Feed Amendments for Animal Agriculture, American Geophysical Union, 2000 Spring Meeting: Integrative Geoscience Solutions -- A Start for the New Millennium, May 30 - June 3, 2000, Washington, DC.
U.S. Department of Interior, U.S. Geological Survey