Welcome to this week’s installment of the BayBlog Question of the Week! Each week we'll take a question submitted through the Chesapeake Bay Program website and answer it here for all to read.

This week, Dave is trying to get a sense of “who is causing what” in relation to the Chesapeake Bay’s pollution issues. He wants to know: what are the main sources of nitrogen, phosphorous and sediment to the Bay?

It’s important to know where Chesapeake Bay pollution comes from because we can use that knowledge to do our part to reduce the amount of pollutants each of us contributes to the Bay and its local waterways.


Nitrogen occurs naturally in soil, animal waste, plant material and the atmosphere. However, most of the nitrogen delivered to the Bay comes from:

  • Manure, emissions and chemical fertilizers from farmland and animal operations (36 percent)
  • Nitrogen oxide emissions from sources including vehicles, industries and electric utilities (27 percent)
  • Human waste treated and discharged from municipal wastewater treatment plants and wastewater discharged from industrial facilities (20 percent)
  • Chemical fertilizers applied to lawns, golf courses and other developed lands (11 percent)
  • Septic systems that treat household wastewater and discharge effluent to groundwater in the Bay watershed (5 percent)


Phosphorous, like nitrogen, occurs naturally in soil, animal waste and plant material. But these natural sources account for just 3 percent of the phosphorous loads to the Chesapeake Bay. Here are the major sources of the Bay’s phosphorus pollution:

  • The largest source is agriculture: manure and chemical fertilizers from farms contribute 45 percent of the total phosphorus load to the Bay.
  • Runoff from developed cities, towns and suburbs, as well as legacy sediments from streams, account for 31 percent of the Bay’s phosphorus pollution.
  • Municipal and industrial wastewater is the source of the remaining 21 percent of phosphorous loads.


Sediments are loose particles of clay, silt and sand. When suspended in the water, sediment can block sunlight from reaching underwater bay grasses. As sediment settles to the bottom of the Bay and its rivers, it smothers bottom-dwelling animals (such as oysters). Sediment can also carry high concentrations of phosphorus and toxic chemicals.

Most of the sediment to the Bay comes from agriculture. Natural sources, stormwater runoff and erosion from streams make up the rest of the sources of sediment to the Bay and its local waterways.

While some sources of pollution may be larger than others, one source is not more important to prevent than any other. We must take any and all steps to reduce nitrogen, phosphorous and sediment loads to the Bay. Think about how your daily actions contribute pollution to the Bay and its rivers. Be sure to check out our Help the Bay tips to learn how you can do your part.



Rachel Felver

Hi Jasmine,

About half of the water in the Chesapeake Bay is salt water, coming from the Atlantic Ocean. The other half is fresh water -- half of that fresh water comes from the Susquehanna River, the Bay's largest tributary. For more facts and figures about the Bay, please visit our website: https://www.chesapeakebay.net/discover/facts



From which source does most of our water in the watershed come from?
(chesapeake bay watershed)

Rachel Felver

Hi Margaret,

The three biggest pollutants entering the Chesapeake Bay are nitrogen, phosphorus and sediment. There are a variety of reasons why these pollutants enter the Bay, including agricultural runoff, wastewater treatment plants, air pollution and urban and suburban runoff (e.g. stormwater). Agricultural runoff is the number one source of pollution into the Bay. For more information, please visit our Learn the Issues page on Nutrients (https://www.chesapeakebay.net/issues/nutrients) or you can view specific charts with this information on ChesapeakeProgress (http://www.chesapeakeprogress.com/clean-water/watershed-implementation-plans).



What are the 2 major pollutants causing issues in the Bay?, What is the major source of these sources?

Sami Alamiri

Great, thanks. One more question how do these pollutants harm the dwellers of the bay (plants and animals)? Thanks.

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