Monitoring Water-Quality Changes in the Chesapeake Bay Watershed

Science Center Objects

The U.S. Geological Survey (USGS), in partnership with the Chesapeake Bay Program (CBP) watershed water-quality monitoring network, measures changes in nutrients and sediment in the Chesapeake Bay watershed. The data from the network are used to help scientists and managers assess water-quality conditions and long-term trends as management practices are implemented to reduce the amount of nutrients (primarily nitrogen and phosphorus) and sediment reaching the streams in the watershed and the Bay. The data will also be used to help measure progress towards meeting the Chesapeake Bay Total Maximum Daily Load (TMDL). The TMDL is a "pollutant diet" designed to reduce nutrients and sediment to improve water-quality conditions for fish and underwater grasses in the Bay.

Major conclusions from the CBP for nutrient and sediment concentration trends through 2009

  • Trends over the past 25 years have improved. Nitrogen and phosphorus concentrations have decreased at almost 70 percent of the 32 monitoring sites within the Bay watershed. Sediment has decreased at about 40 percent of the sites.
  • Several of 32 sites had increasing trends, which indicates degrading conditions. Two sites had increasing trends for nitrogen, four for phosphorus, and four for sediment.
  • Multiple factors are affecting water-quality changes in the watershed. The primary factors include (a) changes in nutrient sources and land use, (b) population increases, (c) implementation of management actions, and (d) the influence of watershed characteristics, including the lag time between implementing a management action and detecting a water-quality improvement.
  • These long-term trends suggest that pollution-reduction efforts, such as improved controls at wastewater treatment plants and practices to reduce nutrients and sediment from farms and suburban lands, are improving water-quality conditions in many areas of the watershed. However, nutrients, sediment, and contaminants will need to be further reduced to achieve a healthier Bay and streams.
  • The USGS is developing additional techniques to assess changes in nutrient and sediment loads; these techniques will provide an improved approach in which to compare monitoring information and progress toward the allocations in the Bay TMDL.

 

Over the past 25 years, the trends for nutrient and sediment concentrations are improving at the majority of sampling locations in the Chesapeake Bay watershed

The concentrations of nutrients and sediment in rivers vary greatly each year because they are influenced by changes in the amount of annual rainfall. The USGS has developed techniques to adjust for the effects of rainfall variability in order to better evaluate if long-term changes in waterway conditions are related to human activities and management efforts. These techniques, called "flow-adjusted trends," provide information on how nutrient and sediment concentrations would change over time if they were not influenced by changes in river flow.

Each year the USGS updates information on trends (using techniques described in Langland and others, 2007, see WWW link below) based on new data collected from the CBP nontidal water-quality network. Between 1985 and 2009, specific results at 32 sampling locations include:

  • Twenty-two sites showed downward flow-adjusted trends for nitrogen concentrations, two showed upward trends, and eight showed no trend or small trends that are not statistically significant.
  • Twenty-two sites showed downward flow-adjusted trends for phosphorus concentrations, four showed upward trends, and six showed no trend or small trends that are not statistically significant.
  • Twelve sites showed downward flow-adjusted trends for sediment concentrations, four showed upward trends, and sixteen showed no trend or small trends that are not statistically significant.

Maps showing these results are at the end of this document.

Additionally, the USGS has begun to compute trends for the past 10 years (2000 to 2009) to provide information on more recent changes in water-quality conditions. These results will be available in the upcoming CBP Bay Barometer, an annual assessment report that is planned for release in the summer of 2011. For more detailed information on the techniques to compute trends see: Langland M.J., Moyer, D.L., and Blomquist J. D., 2007, Changes in Streamflow, Concentrations, and Loads in Selected Nontidal Basins in the Chesapeake Bay Watershed, 1985-2006, U.S. Geological Survey Open-File Report 2007-1372.

 

Additional techniques are being developed to assess changes in water quality

The USGS has developed a new method that allows for the determination of trends in river loads for nitrogen, phosphorus, and suspended sediment. This new method, known as Weighted Regressions on Time, Discharge, and Season (WRTDS), complements our historical statistical approach for quantifying loads and flow-adjusted concentration trends such that we can now determine changes in nutrient and sediment loads during any time interval. Additionally, this new method provides:

  • An innovative approach for removing the confounding influence of river flow for better trend determination called “flow normalization.”
  • Information on the processes and pathways by which water-quality conditions are changing.
  • A direct link, through flow-normalized trends in load, for assessing progress towards meeting the TMDL allocations.

The figure below provides an example of the flow-normalized load results obtained from WRTDS. The nine panels show annual phosphorus loads (shown as a yield that is the load divided by the watershed area) (black dots) and the flow-normalized trend in load (black line) for each of the nine river-input monitoring stations. The plot shows that the total amount of phosphorus delivered to the estuaries of the Bay is increasing in the Choptank River basin and decreasing in the Patuxent River basin. Additionally, the Susquehanna and the Potomac Rivers had increasing loads during the 1980s and 1990s, while more recently the loads are beginning to decrease. The remaining five sites have marginal to no change.

Flow-normalized load results obtained from WRTDS

Flow-normalized load results obtained from WRTDS

More information can be found in the article: Hirsch, Robert M., Moyer, Douglas L., and Archfield, Stacey A., 2010, Weighted Regressions on Time, Discharge, and Season (WRTDS), with an Application to Chesapeake Bay River Inputs, Journal of the American Water Resources Association.

The USGS also has a PODCAST explaining this information:

 

Background information on monitoring sites used to measure water-quality changes in the Bay watershed

The USGS, in cooperation with CBP partners, monitors streamflow, nutrients, and sediment as part of the CBP Watershed Water-Quality Monitoring Network. The network includes 85 monitoring sites in each of the major river basins draining to the Bay (see map below). These sites include the river-input monitoring (RIM) stations (located at the head of tide of each major river basin) and upstream sites. The RIM stations collectively represent 78 percent of the area of the watershed and range in size from the Susquehanna River basin (27,000 square miles) to the Choptank River basin (100 square miles). At each site in the network, the amount of river flow is measured and 20 samples per year are collected and analyzed for the concentrations of nutrients and sediment. The USGS computes the amount (loads) at sites with at least 5 years of data and conducts analysis of changes over time (trends) at sites with at least 10 years of data. The results are used by the CBP partners to determine annual loads into the estuary and the impact to the estuary and to help assess the effectiveness of management actions.

Additional information on the river flow and loads to the Bay can be found in the USGS feature on this topic.

 

Maps Showing Results of Flow-Adjusted Concentration Trends

Monitoring sites in the nontidal network. A subset of these sites is included in an annual water-quality update.

Monitoring sites in the nontidal network. A subset of these sites is included in an annual water-quality update.

Long-Term Flow-Adjusted Concentration Trends for Total Nitrogen for 32 Sites in the Chesapeake Bay Watershed, 1985-2009

Long-Term Flow-Adjusted Concentration Trends for Total Nitrogen for 32 Sites in the Chesapeake Bay Watershed, 1985-2009

Long-Term Flow-Adjusted Concentration Trends for Total Phosphorus for 32 Sites in the Chesapeake Bay Watershed, 1985-2009

Long-Term Flow-Adjusted Concentration Trends for Total Phosphorus for 32 Sites in the Chesapeake Bay Watershed, 1985-2009

Long-Term Flow-Adjusted Concentration Trends for Suspended Sediments for 32 Sites in the Chesapeake Bay Watershed, 1985-2009

Long-Term Flow-Adjusted Concentration Trends for Suspended Sediments for 32 Sites in the Chesapeake Bay Watershed, 1985-2009