Trends and status of flow, nutrients, and sediments for selected nontidal sites in the Chesapeake Bay Watershed, 1985-98

Abstract

Data from 30 stream sites in nontidal portions of the Chesapeake Bay Watershed were analyzed to document annual nutrient and sediment loads and trends for the period 1985 through 1998 as part of an annual water-quality update for the Chesapeake Bay Program. Annual loads were estimated by use of the U.S. Geological Survey ESTIMATOR model and are available upon request. Trends were estimated by use of either linear regression, Seasonal Kendall, or Kendall-Theil tests. Trends were reported for monthly-mean flow, monthly load, flow-adjusted concentration, and flow-weighted concentration. Mean yields and median concentrations were calculated to help assess current waterquality status and to facilitate comparisons between basins. The trend in flow was upward at 12 of the 30 sites. Trends of nutrient and sediment load were upward at these same 12 sites. Trends in flow–adjusted and flow-weighted concentrations varied by species. Trends in flow-weighted concentrations were generally upward or not significant for the nitrogen species and parallel or not significant for phosphorus species. Trends in flowadjusted concentrations were downward at nearly all sites for sediment and dissolved solids in the Susquehanna and Potomac River Basins and generally were not significant in the Virginia river basins. INTRODUCTION The ecosystem of the Chesapeake Bay has been adversely affected by nutrient enrichment. The Chesapeake Bay Program (CBP), a multi-jurisdictional restoration effort, established a goal to reduce controllable nutrient loads into the estuary by 40 percent by the year 2000. Results from the CBP watershed and water-quality models indicated that water quality would be sufficiently improved to support living resources if the 40-percent nutrient-reduction goal was met. Individual nutrient-reduction goals and associated strategies were established for the major rivers delivering nutrients to the Bay. Progress toward these reduction goals have been reevaluated by use of the watershed and water-quality models. Additionally, water-quality and living-resource data are compiled and analyzed to assess the response of the watershed to nutrient-reduction efforts. The 1998 data-analysis effort is part of an annual effort to document changes in water quality and living resources in response to Nutrient Reduction Strategies (NRS) in the Chesapeake Bay Watershed. Two programs, the River Input Monitoring and Multi-Agency Nontidal Programs, provide information about water-quality conditions in the nontidal areas of the Bay. As part of the River Input Monitoring Program, water-quality and streamflow data are collected at nine stations near the Fall Line (fig. 1). Through the Multi-Agency Nontidal Program, water-quality and streamflow data are collected by several agencies at 21 sites upstream from the River Input Program sites in the nontidal watershed (fig. 1). The water-quality and streamflow data are used to determine trends in streamflow and in concentration and load of nutrients. At 15 of the 21 sites, trend calculations were completed by the U.S. Geological Survey (USGS) in cooperation with Maryland Department of Natural Resources (MdDNR), Virginia Department of Environmental Quality (VaDEQ), and Washington, D.C., Council of Governments (WashCOG); and at 6 sites by the Susquehanna River Basin Commission (SRBC) as part of the Susquehanna Nutrient Assessment Program. Trends and Status of Flow, Nutrients, and Sediments for Selected Nontidal Sites in the Chesapeake Bay Watershed, 1985-98