Geochemistry and Characteristics of Nitrogen Transport at a Confined Animal Feeding Operation in a Coastal Plain Agricultural Watershed, and Implications for Nutrient Loading in the Neuse River Basin, North Carolina, 1999–2002
Scientific Investigations Report 2004–5283
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Chemical, geologic, hydrologic, and age-dating information collected between 1999 and 2002 were used to examine the transport of contaminants, primarily nitrogen, in ground water and the pathways to surface water in a coastal plain setting in North Carolina. Data were collected from more than 35 wells and 4 surface-water sampling sites located in a 0.59 square-mile basin to examine detailed hydrogeology and geochemical processes affecting nutrient fate and transport. Two additional surface-water sampling sites were located downstream from the primary study site to evaluate basin-scale effects. Chemical and flow data also were collected at an additional 10 sites in the Coastal Plain portion of the Neuse River basin located between Kinston and New Bern, North Carolina, to evaluate loads transported in the Neuse River and primary tributary basins.
At the Lizzie Research Station study site in North Carolina, horizontal flow is induced by the presence of a confining unit at shallow depth. Age-dating, chemical, and piezometric data indicate that horizontal flow from the surficial aquifer is the dominant source of ground water to streamflow. Nitrogen applied on cultivated fields at the Lizzie Research Station is substantially reduced as it moves from recharge to discharge areas. Denitrification in deeper parts of the aquifer and in riparian zones is indicated by a characterization of redox conditions in the aquifer and by the presence of excess nitrogen gas. Direct ground-water discharge of nitrate to surface water during base-flow conditions is unlikely to be significant because of strongly reducing conditions that occur in the riparian zones of these streams. Nitrate loads from a drainage tile at the study site may account for much of the nitrate load in the receiving stream, indicating that a major source of nutrients from ground water to this stream is artificial drainage. During base-flow conditions when the streams are not flowing, it is hypothesized that the mineralization of organic matter on the streambed is the source of nitrate and(or) ammonium in the stream. Base flow is a small contributor to nitrogen loads, because both flows and inorganic nitrogen concentrations are low during summer months.
Effects of a confined hog operation on ground-water quality also were evaluated. The use of sprayed swine wastes to fertilize crops at the Lizzie Research Station study site since 1995 resulted in increased concentrations of nitrate and other chemical constituents in ground water beneath spray fields when compared to ground water beneath fields treated with commercial fertilizer. The nitrate concentration in ground water from the spray field well increased by a factor of 3.5 after 4 years of spray applications. Nitrate concentrations ranged from 10 to 35 milligrams per liter, and one concentration as high as 56 milligrams per liter was observed in water from this well in spring 2002. This finding is in agreement with findings of other studies conducted in the Coastal Plain of North Carolina that nitrate concentrations were significantly higher in ground water from cultivated fields sprayed with swine wastes than from fields treated with commercial fertilizer.
Loads and yields of nitrogen and phosphorus in 14 streams in the Neuse River basin were evaluated for calendar years 2000 and 2001. Data indicate that anthropogenic effects on nitrogen yields were greatest in the first-order stream studied (yields were greater than 2 tons per square mile [ton/mi2] and 1 ton/mi2 or less in second- and higher-order streams) in the Little Contentnea Creek subbasin. Nitrogen yields in streams in the Contentnea Creek subbasin ranged from 0.59 to 2 ton/mi2 with typical yields of approximately 1 ton/mi2. Contentnea Creek near Evansdale had the highest yield (2 ton/mi2), indicating that a major source of nitrogen is upstream from this station. Nitrogen yields were lower at Contentnea Creek at Hookerton in 2000 and 2001 compared to previous yield estimates based on 1990 data. Along the main stem of the Neuse River, nitrogen yields during 2000 and 2001 ranged from 3.4 ton/mi2 in Bear Creek, a tributary west of Kinston, North Carolina, to 0.55 ton/mi2 in the Trent River. The total nitrogen load delivered to the Neuse estuary in 2000 was 4,807 tons or 9.61 million pounds, and the total phosphorus load was 425 tons or 850,000 pounds. It is estimated that about 17 percent of the delivered total nitrogen load is from background sources—35 percent from point sources and 48 percent from nonpoint sources.
Annual phosphorus yields in the Little Contentnea Creek subbasin for 2000 and 2001 ranged from 0.02 to about 0.15 ton/mi2. In contrast with total nitrogen yields, the larger total phosphorus yields were in higher-order streams (about three times greater), indicating either that the major source of phosphorus in the Little Contentnea Creek subbasin originates in the larger streams or the phosphorus is deposited by sediment eroded from low-order headwater streams and deposited on the bed. Phosphorus yields in the Contentnea Creek subbasin ranged from 0.02 to 0.15 ton/mi2. Phosphorus yields for Contentnea Creek at Hookerton were about 5 to 6 times the expected background yield, although yields for phosphorus reported for 2000 and 2001 for Contentnea Creek at Hookerton were considerably lower than those reported for 1990. Phosphorus yields in the Neuse River basin ranged from 0.02 to 0.29 ton/mi2, with the highest yields occurring near New Bern in the vicinity of the upper Neuse estuary.
The total phosphorus load transported to the Neuse estuary in 2000 was 425 tons. No recently published information was found on total point-source contributions of phosphorus to the Neuse River near Fort Barnwell for this study; therefore, general estimates from past studies were used. Based on this analysis, about 7 percent of the total phosphorus load in 2000 was from background sources, about 41 percent was from anthropogenic point sources, and 52 percent was from anthropogenic nonpoint sources.
Effects of cultural eutrophication with respect to phosphorus enrichment in coastal plain streams of the Neuse River basin are much greater than for nitrogen. The eutrophication loading index of phosphorus ranged between 2 and 12 times, and values typically were 5 to 6 times the estimated background yield for second- and higher-order streams. The eutrophication loading index of total nitrogen ranged between 1 and 5 times, and values typically were about 2 times the estimated background yield.
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