T.C. Hancock1, J.M. Denver, G. F. Riedel, and C.V. Miller
U.S. Geological Survey, 1730 E. Parham Rd., Richmond, Virginia
(tel) 804-261-2618; (fax) 804-261-2659; (email) 

The source, transport, and fate of several arsenic compounds are being investigated in water, soil, and sediment of the Pocomoke River Basin in Maryland and Delaware. This basin in the Chesapeake Bay Watershed has a high concentration of poultry-feeding operations where arsenic feed amendments are used extensively. Concentrations of total arsenic in fresh poultry manure collected from the Pocomoke River Basin were as high as 27 mg/kg, whereas older, composted manure contained less than 2 mg/kg total arsenic. Arsenic in composting manure may be volatilized and during rain events, it is likely leached into water. Total arsenic in agricultural and forest soil of the Pocomoke River Basin was 1-2 mg/kg; these soils had similar concentrations of arsenic as the composted manure sample. Base-flow concentrations of total arsenic in suspended particles and in bed sediment in the Pocomoke River and its tributaries were 0.8-21 mg/kg in both 1997 and 1999. Concentrations of total dissolved (0.45 m filtered) arsenic in water samples from the Pocomoke River did not exceed 1.6 g/L during base-flow. Concentrations of total dissolved arsenic in agricultural ditches and in the Pocomoke River increased during high flow, presumably due to runoff. 

Although the initial input of arsenic to the basin from poultry waste is in the form of organic arsenic compounds, we found mostly inorganic arsenic (As III and As V), and relatively low concentrations of methylated arsenic in pore water in cored sediments collected beside an agricultural field. Total arsenic in pore water in surface sediments and at depths to 8 feet ranged from below detection limits to as high as 29 g/L. The As (V) concentrations were elevated in pore water from the near-surface sandy soil (29 g/L) and from an iron-rich clay silt layer at depth (24 g/L). As (III) was not detected in the near-surface sediments, but increased with depth to 11 g/L.

Shallow ground water from piezometers near agricultural fields, which had total dissolved arsenic concentrations as high as 23 g/L, appears to be an important reservoir for arsenic cycling in the Pocomoke Basin. Water from a deeper, semi-confined part of the surficial aquifer system, which is a drinking water source, had total dissolved arsenic concentrations as high as 8 g/L. This deeper ground water also had relatively high concentrations of dissolved iron (25-29 mg/L) and did not contain any obvious signs of agricultural influence. These findings suggest two sources of arsenic in the basin: poultry waste spread on land, and a natural source associated with iron-rich sediments, particularly at depth. Elevated arsenic concentrations were observed in ground water with low dissolved oxygen content, and under these reducing conditions, ground water has the potential to mobilize arsenic from the reduction of metal oxides. 

To understand all the potential sources and sinks of arsenic in the basin, total arsenic concentrations in soil and bed sediment samples representing varying intensities of agricultural land use will be analyzed. Core sediment collected from various locations and representing various land uses in the basin will be analyzed for total arsenic, carbon, and sulfur. In addition, grain-size analyses and sequential partial extractions will be performed to characterize the sediments. Storm-runoff samples will be measured for total arsenic and arsenic speciation in both whole water and dissolved (filtered) samples. Sampling also will be expanded to other tributaries in the Chesapeake Bay Watershed affected by high-density poultry operations, such as the Shenandoah Valley of Virginia.

Hancock, T.C., Denver, J.M., Riedel, G.F., Miller, C.V., 2001, Reconnaissance for Arsenic in a Poultry Dominated Chesapeake Bay Watershed -- Examination of Source, Transport, and Fate, [abs.], US EPA Workshop Managing Arsenic Risks to the Environment: Characterization of Waste, Chemistry, and Treatment and Disposal, May 1-3, 2001, Denver, Colorado.

U.S. Department of Interior, U.S. Geological Survey
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