Chesapeake Bay Water Quality Shows Mixed Results

Annapolis, Maryland—Analysis of recent tidal and nontidal data show lower amounts of nitrogen, phosphorus and sediment entering the Chesapeake from its 64,000-square-mile watershed, although Bay water quality remains relatively unchanged from the previous assessment period.

The Chesapeake Bay Program estimates the water quality of the Bay through four indicators: 

• Water quality standards attainment for dissolved oxygen, water clarity (measured by underwater grass abundance) and chlorophyll a (a measure of algae growth).
• Monitored and modeled amounts of nitrogen, phosphorus and sediment entering the Chesapeake Bay.
• Monitored nitrogen, phosphorus and sediment trends in the watershed.
• Progress toward the Chesapeake Bay Total Maximum Daily Load (Bay TMDL), measured by combining monitoring and modeled data to estimate the progress of nitrogen and phosphorus reductions in response to implemented best management practices.

Water Quality Standards Attainment 

The Chesapeake Bay is estimated to have met 29.4% of water quality standards during the 2021-23 assessment period, a slight decline from its previous score of 29.8%. Detailed assessments for this time period show a decline in dissolved oxygen in the deep waters of the Chesapeake but a moderate improvement in the open waters.

As dissolved oxygen and water clarity are significantly impacted by the amount of nitrogen, phosphorus and sediment entering the Bay, water quality correlates with changes in rainfall patterns across the region. In 2018-19, higher than average river flows brought a larger abundance of nitrogen, phosphorus and sediment into the Chesapeake, resulting in low dissolved oxygen, poor water clarity and reduced underwater grass acreage. Weather patterns were more average in 2021-23, leading experts to anticipate a rebound in the water quality standards attainment score. Although the data does not show that rebound, there may be delays, referred to as lag times, in seeing that improvement. At this time, water quality in the Chesapeake Bay remains below the 100% attainment necessary to support a healthy Bay ecosystem.

Monitored and Modeled Nitrogen, Phosphorus and Sediment Entering the Chesapeake Bay 

In 2023, an average of 42.5 billion gallons of water per day flowed into the Bay from its tributaries, a 10% decrease from the previous year. These flows brought 199 million pounds of nitrogen, 9.7 million pounds of phosphorus and 12 billion pounds of sediment into the Chesapeake in 2023. These are decreases from the previous year (2022) of 21.7%, 26% and 15.5%, respectively. The amount of nitrogen, phosphorus and sediment entering the Chesapeake vary year-to-year depending on a number of factors, including land use, fertilizer and manure application, and river flow. Monitoring data from River Input Monitoring (RIM) stations located on the nine largest tributaries of the Bay represent the amount of nitrogen, phosphorus and sediment entering the Chesapeake from 78% of the watershed, while the remaining amount is estimated through the combination of wastewater treatment plant data and computer-simulated estimates of nitrogen and phosphorus from nonpoint sources. 

“Maintaining a robust water-quality monitoring network is critical for the Chesapeake Bay Program to understand how the watershed and estuary are changing over time, and it provides important data to evaluate the effects of management efforts across the watershed,” said Chesapeake Bay Coordinator Ken Hyer with the U.S. Geological Survey.

Monitoring Nitrogen, Phosphorus and Sediment Trends in the Watershed 

The U.S. Geological Survey assesses trends in the amount of nitrogen, phosphorus and sediment entering the Chesapeake Bay from the nine RIM stations. These trends are adjusted for annual differences in streamflow so experts can better understand how changing amounts of nitrogen, phosphorus and sediment are affected by actions taken on land. Trends from 2014-2023 show a mix of improving and degrading conditions throughout the watershed.

• Trends in nitrogen indicate improving conditions at four stations (Pamunkey, Patuxent, Potomac and Susquehanna), degrading conditions at three stations (Appomattox, Mattaponi, Rappahannock) and no change at two stations (Choptank, James).
• Trends in phosphorus indicate improving conditions at three stations (Patuxent, Potomac, Susquehanna), degrading conditions at five stations (Appomattox, Choptank, James, Mattaponi, Rappahannock) and no change at one station (Pamunkey).
• Trends in sediment indicate improving conditions at three stations (Pamunkey, Patuxent, Susquehanna), degrading conditions at three stations (Appomattox, Choptank, Mattaponi) and no change at three stations (James, Potomac, Rappahannock)

The U.S. Geological Survey also analyzes nitrogen, phosphorus and sediment trends at the non-tidal monitoring network stations throughout the watershed. The following are the most recent trends from 2014-2023.

• Forty-three percent of non-tidal network stations showed an improvement in the amount of nitrogen pollution, whereas 39% are degrading and 18% show no trend.
• Twenty-four percent of non-tidal network stations showed an improvement in the amount of phosphorus, whereas 47% are degrading and 29% show no trend.
• Thirty-three percent of the non-tidal network stations showed an improvement in the amount of sediment, whereas 34% are degrading and 33% show no trend.

"Pennsylvania has invested in partnerships and projects that reduce pollution and promote clean local waters,” said Secretary Jessica Shirley of Pennsylvania’s Department of Environmental Protection. “The improvements we are seeing at the Susquehanna River Input Monitoring station, which captures all of the freshwater flowing through Pennsylvania's Susquehanna River Basin, proves that what Pennsylvania's residents, farmers, municipalities and businesses are doing is making a difference—not only locally but also to the Chesapeake Bay.”

Bay TMDL Indicator

The Bay TMDL Indicator combines monitored and modeled data to estimate the progress made in reducing nitrogen, phosphorus and sediment as established under the Bay TMDL. While the Bay TMDL was established in 2009, the Bay TMDL Indicator estimates the amount of nitrogen, phosphorus and sediment that needs to be reduced on an annual basis for the Chesapeake Bay and its tidal tributaries to function as a healthy ecosystem and meet applicable water quality standards from 1995. Therefore, the Bay TMDL Indicator calls for nitrogen and phosphorus entering the Chesapeake to be reduced by 145.42 and 9.33 million pounds per year, respectively. (Note: In contrast, the Bay TMDL calls for a reduction of over 47.6 million pounds of nitrogen and over 3.9 million pounds of phosphorus annually.)

Impacts from changing environmental conditions and infill from the Conowingo Dam reservoir since the Bay TMDL was established have added approximately 10.66 million pounds per year of nitrogen and 0.82 million pounds per year of phosphorus to the original targets for pollutant reductions. Bay Program experts estimate that between 2009 and 2023, 81.83 million pounds of nitrogen and 1.63 million pounds of phosphorus were reduced through the installation of best management practices. It is believed that an additional approximately 27.21 million pounds of nitrogen and 4.05 million pounds of phosphorus will be reduced annually due to these installed best management practices but these declines have not yet been seen in the monitoring data due to natural lag times. 

Tracking water quality status and trends requires robust monitoring analysis. As such, the Chesapeake Bay Program’s Water Quality Standards Attainment and Monitoring Outcome remains on-course as it strives to continually support and improve the partnership’s efforts in monitoring and data assessment to support the annual reporting of water quality standards attainment throughout the watershed. 

“The Chesapeake Bay Program’s monitoring networks provide the foundation for understanding and protecting the health of not only the Chesapeake Bay but for local rivers and streams flowing throughout the watershed,” said Acting Director Lee McDonnell of the Environmental Protection Agency’s Chesapeake Bay Program Office. “The partnership’s continued investment in refining how we use modeling and monitoring to better understand how our actions on land impact the health of our waters is only serving to strengthen how we measure progress. As we prepare to enter the next phase of Bay restoration, sustained monitoring remains critical to ensuring our decision-making is driven by data, grounded in science and supported through strong partnerships."