Scientific Investigations Report 2006-5080
Bassett Creek near Ironshire, Maryland, drains a 1.22-square-mile watershed that enters Newport Bay at the northern end of the larger Chincoteague Bay. Twenty-nine water samples collected from Bassett Creek through a range of low and high flows were analyzed for their content of dissolved forms of nitrogen. The concentrations of nitrite + nitrate in the samples ranged from 0.397 to 3.02 milligrams per liter as nitrogen, and frequently exceeded published thresholds for determining the effects of human activities on watersheds in the area.
Concentrations of nitrite + nitrate in the stream under base-flow conditions varied inversely with flow magnitude, with a statistical correlation between base flow and nitrite + nitrate concentration with R2 = 0.90. Concentrations of nitrite + nitrate in the stream under stormflow conditions had a statistical correlation with water temperature with R2 = 0.25. Both correlations are statistically significant at or above the 95-percent confidence interval. The latter correlation may reflect the seasonal availability of nitrite + nitrate as determined by agricultural practices in the watershed.
The estimated nitrite + nitrate loading rates for water years 2003 and 2004 were 9.3 kilograms per day (total load of 3,400 kilograms per year or 7,500 pounds per year) and 13.9 kilograms per day (total load of 5,100 kilograms per year or 11,200 pounds per year), respectively. The base-flow component of the estimated load of nitrite + nitrate comprised approximately 61 percent and 24 percent of the total estimated nitrite + nitrate load in water years 2003 and 2004, respectively.
A seasonal pattern of estimated monthly nitrite + nitrate loading was observed over the study period, with both inferred stormflow and total estimated loads of nitrite + nitrate increasing over the winter and early spring months. Most of the estimated nitrite + nitrate load during the late summer, fall, and early winter is transported in base flow, although fall storms may result in significant stormflow loads, such as occurred in September 2003 and the late summer and fall of 2004.
The flow-weighted concentrations of nitrite + nitrate for Bassett Creek during the study period (1.35 milligrams per liter as nitrogen in water year 2003 and 1.74 milligrams per liter as nitrogen in water year 2004) are similar to the maximum value for the period spanning calendar years 1985 through 2004 for the Choptank River near Greensboro, MD (1.30 milligrams per liter as nitrogen), and lower than the range of values for calendar years 1997 through 2001 for Chesterville Branch near Crumpton, MD (4.21 milligrams per liter as nitrogen to 6.71 milligrams per liter as nitrogen). These comparative values may reflect the effect of the areal extent of agricultural land use in the respective watersheds.
First posted March 5, 2006
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Dillow, J.J.A., and Raffensperger, J.P., 2006, Estimates of the loads of nitrite + nitrate in the flow of Bassett Creek to the Maryland Coastal Bays adjacent to Assateague Island National Seashore, Water Years 2003–2004:U.S. Geological Survey Scientific Investigations Report 2006–5080, 10 p.
Streamflow and Water-Quality Measurements
Estimated Load of Nitrite + Nitrate
Summary and Conclusions