Nutrient and Sediment Concentrations, Loads, and Trends for Four Nontidal Tributaries in the Chesapeake Bay Watershed, 1997-2001
By Michael P. Senus, Michael J. Langland, and Douglas L. Moyer
Abstract
Excess nutrient and sediment loads in the Chesapeake Bay can cause unbalanced water-quality conditions that reduce the amount of oxygen and sunlight available to aquatic plants and organisms. Nutrient and sediment loads and trends were analyzed at four nontidal tributaries in the Chesapeake Bay watershed to collect additional data for the ongoing (since 1985) U.S. Geological Survey Chesapeake Bay River Input Monitoring Program. These sites were Chesterville Branch in Maryland, Conodoguinet Creek in Pennsylvania, North Fork Shenandoah River in Virginia, and South Fork Shenandoah River in Virginia. Monthly base-flow and stormflow water-quality samples were collected at each site, with a total of 90-120 samples collected at each site during the study period (1997-2001). These data were used to compute annual and monthly loads for selected nutrient and suspended-sediment constituents using a mass-load estimating model, ESTIMATOR, developed by the U.S. Geological Survey. Each of the four study sites in this report was characterized on the basis of land use and water-quality data. Basins with high percentages of agricultural land use had higher nitrogen and phosphorus loads and yields than basins with higher percentages of forested land and less agricultural land. No relation between land use and suspended-sediment loads and yields was evident. Study results also indicate that suspended-sediment loads were higher in years of high mean annual streamflow at Conodoguinet Creek, North Fork Shenandoah River, and South Fork Shenandoah River. The highest total suspended-sediment loads and yields at Chesterville Branch were caused by Hurricane Floyd in 1999, which was not a year with the highest annual total streamflow. Results of this study indicate that a 5-year period of record is too short a timeframe to characterize factors affecting trends. Moreover, a combination of extreme variability in climate and lack of recent land-use data made characterizing basin nutrient and sediment relations problematic. Although the data and analysis established a baseline of water quality at the four tributaries, additional monitoring and tracking of loads would help identify long-term trends and factors affecting trends.
