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Chesapeake Bay Water Quality Improving

Data show drop in nutrient and sediment pollution, the leading causes of the Bay’s poor health

Annapolis, MD (September 21, 2016)

The amount of nutrient and sediment pollution entering the Chesapeake Bay fell significantly between 2014 and 2015, helping improve water quality in the nation’s largest estuary. Experts attribute this drop in pollution loads to dry weather and below-normal river flow, but note local efforts to reduce pollution also played a role. Indeed, related research shows “best management practices”—including upgrading wastewater treatment plants, lowering vehicle and power plant emissions, and reducing runoff from farmland—have lowered nutrients and sediment in local waterways.

The Chesapeake Bay Program (CBP) tracks pollution loads and trends as it marks progress toward improving the health of the Bay. According to data from the CBP and the U.S. Geological Survey (USGS), nitrogen, phosphorus and sediment loads to the Bay were below the long-term average in 2015. Between 2014 and 2015, nitrogen loads fell 25 percent, from 290 million pounds to 217 million pounds. Phosphorus loads fell 44 percent, from 17.7 million pounds to 9.9 million pounds. Sediment loads fell 59 percent, from 7.2 billion pounds to 2.9 billion pounds. Below-average loads are considered positive because reductions in nitrogen, phosphorus and sediment pollution can improve water quality.

The most recent assessment of water quality—which examines dissolved oxygen, water clarity and chlorophyll a (a measure of algae growth) in the Bay and its tidal waters—makes these improvements clear: an estimated 37 percent of the tidal Chesapeake met water quality standards between 2013 and 2015. While this is far below the 100 percent attainment needed for clean water and a stable aquatic habitat, it marks an almost 10 percent improvement from the previous assessment period.

A large portion of pollution loads enters the Bay from the rivers within its watershed. Accordingly, the USGS tracks annual pollution loads and trends in these loads at monitoring stations along nine of the biggest rivers that feed the Bay. In some cases, long-term pollution trends at these stations—which span from 1985 to 2015—reflect efforts to improve water quality. Long-term trends in nitrogen, for example, are improving at six of the nine monitoring stations, including those on the Susquehanna, Potomac, James and Rappahannock (the four largest rivers in the watershed). Long-term trends in phosphorus and sediment, however, are more variable: phosphorus is improving at three monitoring stations and degrading at five, while sediment is improving at three stations and degrading at four. Short-term pollution trends—which span the last decade—show less improvement.

In June, the U.S Environmental Protection Agency released its two-year milestone evaluations of federal agencies’ and watershed jurisdictions’ work toward the Chesapeake Bay Total Maximum Daily Load (Bay TMDL). This “pollution diet” calls for all needed pollution control measures to be in place by 2025, with measures that would achieve 60 percent of pollution load reductions in place by 2017. Computer simulations show these measures are in place to achieve 31 percent of the nitrogen reductions, 81 percent of the phosphorus reductions and 48 percent of the sediment reductions necessary to reach our clean water goals. Evaluations from EPA indicate it is unlikely jurisdictions will meet the 2017 target for reducing nitrogen.

While continued improvements in water quality will take time—due in large part to the lag between the implementation of a conservation practice and the visible effect of that practice on a particular waterway—the ecosystem is beginning to respond to protection and restoration efforts. Last year, researchers observed more than 91,000 acres of underwater grasses in the Bay, which surpassed the Chesapeake Bay Program’s 2017 restoration target two years ahead of schedule and marked the highest amount ever recorded by the Virginia Institute of Marine Science aerial survey.


The U.S. Geological Survey (USGS) monitors nitrogen, phosphorus and sediment loads entering the Chesapeake Bay from the nine largest rivers in the watershed. Together, pollution loads computed at all nine River Input Monitoring (RIM) stations reflect pollution loads delivered to the Bay from 78 percent of its watershed. Additional monitoring and modeling information is used to estimate the total nitrogen, phosphorus and sediment loads delivered to the Bay in a given water year.

The amount of nitrogen, phosphorus and sediment entering the 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 pollution loads over time. The USGS analyzes trends in flow-normalized pollution loads—which account for changes in weather and river flow—to better understand the changes in pollution that can result from changes in land use and management practices.

Last year’s decline in pollution loads can, in large part, be attributed to favorable weather. While high precipitation can increase river flow and push pollution into the Bay, river flow was below normal in 2015. However, 2015 was not a drought year. A related analysis from the University of Maryland Center for Environmental Science highlights the significance of this fact: previously observed improvements in water quality have been linked to lower rates of river flow than those seen in 2015. The long-term decline in pollution loads can also be attributed to on-the-ground pollution-reducing practices, which jurisdictions put in place to meet first the 1983 Chesapeake Bay Agreement, then similar agreements signed in 1987 and 2000, and later the requirements of the Chesapeake Bay Total Maximum Daily Load (Bay TMDL).

The Chesapeake Bay Program uses the following data to determine the total nitrogen, phosphorus and sediment loads entering the Bay:

  • Nitrogen, phosphorus and sediment loads computed at nine RIM stations;
  • Nitrogen and phosphorus levels in water samples collected at wastewater treatment plants downstream of RIM stations;
  • Computer-simulated estimates of nitrogen and phosphorus loads from nonpoint pollution sources downstream of RIM stations; and
  • Computer-simulated estimates of the atmospheric deposition of nitrogen to tidal waters.

Water quality in the Bay and its tidal tributaries is evaluated using three parameters: dissolved oxygen, water clarity or underwater grass abundance, and chlorophyll a (a measure of algae growth). These parameters are monitored by the Maryland Department of Natural Resources and the Virginia Department of Environmental Quality. The resulting data is used to develop an indicator of the estimated attainment of a set of water quality standards that can be monitored at this time. During the 2013 to 2015 assessment period, an estimated 37 percent of the Bay and its tidal waters met water quality standards. This marks an increase of almost 10 percent from the previous assessment period, during which an estimated 34 percent of the Bay and its tidal waters met water quality standards. While this indicator does not represent a complete accounting of all of the water quality standards for the Bay and its tidal tributaries, it does reflect trends in water quality over time. If the Bay and its tidal tributaries are to function as a healthy ecosystem, all water quality parameters for all aquatic habitats must be met.


Excess nutrients and sediment are among the leading causes of the Bay’s poor health. Nitrogen and phosphorus can fuel the growth of algae blooms that lead to long-duration, low-oxygen “dead zones” in deep water and short-duration “mortality moments” in shallow water. Sediment can block sunlight from reaching underwater grasses and suffocate shellfish. By tracking pollution loads into rivers and streams, the Chesapeake Bay Program can ensure our partners are on track to meet our clean water goals. By measuring the achievement of water quality standards, we can observe changes in Bay health over time. By reporting on these environmental indicators together, we gain a better picture of how pollution from the watershed can affect the health of the Chesapeake Bay.


“It is critical that we track our pollution control efforts and assess the ecosystem response that result from those efforts. The ecosystem of the Chesapeake Bay watershed is large and complex and can be affected by a variety of different factors. We are witnessing improvement in a number of our indicators—bay grasses, water clarity and water quality standards attainment, as well as a number of our fisheries such as blue crab population. But we must stay focused and ramp up our pollution reduction efforts if we are to be successful over the long term.”

-- Nick DiPasquale, Director, Chesapeake Bay Program

“While our job is not done, our determined efforts to date give us great hope for further improvements in water quality in the Bay and its tributaries and the living resources that depend on healthy aquatic habitats.”

-- Molly Joseph Ward, Virginia Secretary of Natural Resources, and Chair, Chesapeake Bay Program Principals’ Staff Committee

“While the lowered amount of pollution entering the Chesapeake Bay in 2015 is encouraging, the trends of nutrients and sediment over the last decade in the major rivers flowing into the Bay show mixed results. There will need to be improving trends in all of these rivers to support improvement in the Bay’s health.”

-- Scott Phillips, Chesapeake Bay Coordinator, U.S. Geological Survey

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