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Nitrogen Loads and River Flow to the Chesapeake Bay

During the 2015 water year (October 2014 to September 2015), approximately 217 million pounds of nitrogen reached the Chesapeake Bay. This is below the long-term average of 335 million pounds per year. River flow averaged 41 billion gallons per day during this time, which is below the long-term average of 51 billion gallons per day.





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Importance

Each day, billions of gallons of fresh water flow from rivers and streams into the Chesapeake Bay. The amount of water that flows out of these tributaries has a direct impact on pollution: as snowmelt or rainfall increase river flow, more pollution is pushed into the estuary. In other words, precipitation and river flow are two factors that affect pollution loads and water quality.

Nutrient pollution is one of the leading causes of the Bay’s poor health. When nitrogen and phosphorus enter rivers, streams and the Bay, they can fuel the growth of algae blooms that lead to low-oxygen “dead zones” harmful to fish, shellfish and other aquatic life.

Goal

In 2014, the Chesapeake Bay Program adopted a goal to reduce pollution loads and achieve the water quality necessary to protect human health and support the region’s underwater resources.

The amount of nitrogen delivered to the Chesapeake Bay can change dramatically from year to year and is influenced by changes in land use, land management and river flow. This complicates our efforts to determine trends in nitrogen loads over time.

During the 2015 water year (October 2014 to September 2015), approximately 217 million pounds of nitrogen reached the Bay. Sixty-four percent of the total nitrogen load—or 137 million pounds—entered the Bay from non-tidal rivers in the watershed. Of the 80 million pounds of nitrogen that entered the Bay downstream of these non-tidal rivers, 37.8 million can be attributed to nonpoint pollution sources, 25.6 million to wastewater treatment facilities and 16.5 million to the atmospheric deposition of nitrogen.

The total nitrogen load during the 2015 water year is below the long-term average of 335 million pounds per year and 68 million pounds less than the 2014 load. River flow averaged 41 billion gallons per day during this time, which is below the long-term average of 51 billion gallons per day.

Additional Information

Scientists use the following information to estimate nitrogen loads to the Chesapeake Bay:

  • Water samples collected at River Input Monitoring (RIM) sites
  • Water samples collected at wastewater treatment plants downstream of RIM sites
  • Model estimates of nitrogen loads from nonpoint pollution sources downstream of RIM sites
  • Atmospheric deposition of nitrogen to tidal waters

This indicator helps us understand and explain trends in nitrogen loads and water quality. Another indicator—Reducing Nitrogen Pollution—reports computer-simulated estimates of the amount of nitrogen delivered to the Bay following the implementation of efforts to reduce pollution. These estimates are generated with the Chesapeake Bay Program’s Watershed Model (Phase 5.3.2). They help us understand the effects of our management actions on pollution loads and develop “what-if” scenarios that project the future impacts of our actions on water quality. Because of the differences in the data behind these indicators, it is possible for them to report different pollution load amounts for the same period of time.

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