The main cause of the Bay's poor water quality and aquatic habitat loss is elevated levels of two nutrients: nitrogen and phosphorous. Nitrogen occurs naturally in soil, animal waste, plant material and even the atmosphere (78 percent of the Earth's atmosphere is inert nitrogen gas). However, most of the nitrogen delivered to the Bay comes from:
- Nitrogen oxide that is deposited on the watershed and the Bay (primarily due to emissions to the atmosphere from vehicles, industry and electric utilities).
- Ammonia that is deposited on the watershed and the Bay (primarily due to emissions to the atmosphere from agricultural sources).
- Excess chemical fertilizer and manure applied to agricultural and developed (urban/suburban) lands.
- Human waste treated and discharged from municipal wastewater treatment plants and wastewater discharged from industrial facilities.
- Septic systems that treat household wastewater and discharge effluent to groundwater in the Bay watershed.
Based on long-term average hydrology simulations, it is estimated that 281 million pounds of nitrogen were delivered to tidal waters of the Bay in 2007, including 19 million pounds deposited directly on tidal waters. This is compared to approximately 359 million pounds in 1985, including 25 million pounds deposited directly on tidal waters. (Loads due to atmospheric deposition are simulated from the CBP Airshed Model.)
Point source loads are monitored and non-point source loads are simulated using the Phase 4.3 Watershed Model (based on reported implementation of best management practices, or BMPs, that reduce nutrient pollution). The simulation removes annual hydrological variations to measure the effectiveness of BMP implementation and converts the numerous BMPs, which have various pollution reduction efficiencies depending on type and location in the watershed, to a common currency of nitrogen reduction.
Atmospheric
Nitrogen compounds are emitted to the atmosphere on the order of billions of pounds of emissions. The compounds include nitrogen oxides, ammonia and organic nitrogen. A portion of the nitrogen from these emissions is deposited on the Bay and its watershed when it rains and snows (wet deposition) or via settlement (dry deposition).
Approximately 400 million pounds of nitrogen is deposited on the Bay’s watershed each year. However, most of the nitrogen is used by forests or other vegetation and in other biological processes before it reaches the Bay. Of all the nitrogen that is deposited on the watershed annually, approximately 75 million pounds actually reaches the Bay’s tidal waters. Another 19 million pounds is deposited directly on the tidal waters of the Bay, for a total of approximately 94 million pounds or 33 percent of the total nitrogen load to the Bay.
Nitrogen Oxides
Nitrogen oxides are responsible for approximately two-thirds of the total nitrogen emissions that eventually end up as pollution loads to the Bay. Nitrogen oxides are primarily released into the air as a by-product of combustion (the burning of fossil fuels such as coal, oil or gas).
The Chesapeake Bay airshed – the estimated geographic area where emission sources have the greatest potential to deposit nitrogen to a watershed – includes Delaware, Indiana, Kentucky, Maryland, Michigan, New Jersey, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia and West Virginia. The following shows the relative percentage contributions of nitrogen oxide emissions from sources in the 14 states in the Chesapeake Bay airshed:
- Vehicles (on-road cars and trucks): 39 percent
- Electric utilities: 27 percent
- Off-road mobile sources (boats, lawn mowers, construction equipment, etc.): 16 percent
- Industry: 12 percent
- Area Sources (homes, small businesses, agriculture, etc.): 6 percent
Ammonia
Ammonia is responsible for approximately one-third of the total nitrogen emissions that eventually end up as loads to the Bay. Ammonia is primarily released into the air by volatilization (releasing into the atmosphere as a vapor) and emissions from industrial processes.
The following shows the relative percentage contributions of ammonia emissions from sources in the 14 states in the Chesapeake Bay airshed:
- Agriculture (livestock and fertilized soils): 80 percent
- Vehicles: 10 percent
- Other (off-road mobile sources, industries, utilities, area sources): 10 percent
These excessive nitrogen loads to the Bay could be reduced by:
- Further controlling emissions from the above-mentioned sources beyond that which may be required for air quality purposes.
- Reducing/preventing the use of nitrogen containing-products.
- Using state-of-the-art technology to generate products and power that lead to reduced or no nitrogen emissions.
- Maintaining healthy forested watersheds, which help keep the air clean and take up much of the nitrogen that deposits on them.
Manure Applied to Agricultural Land
Approximately 49 million pounds, or 17 percent, of the total nitrogen load to the Bay is due to excess animal waste from agricultural areas in the Bay watershed.
These detrimental loads to the Bay could be reduced by using best management practices (BMPs) that:
- Ensure only the amount of manure needed for crop growth is applied to the land.
- Prevent any excess manure from seeping into groundwater or reaching local waterways that lead to the Bay.
Chemical Fertilizer Applied to Agricultural Land
Approximately 43 million pounds, or 15 percent, of the total nitrogen load to the Bay is due to excess fertilizer from agricultural areas in the Bay watershed.
These detrimental loads to the Bay could be reduced by using BMPs that:
- Ensure only the amount of chemical fertilizer needed for crop growth is applied to the land.
- Prevent any excess nitrogen in chemical fertilizer from seeping into groundwater or reaching local waterways that lead to the Bay.
Chemical Fertilizer Applied to Urban/Suburban Land
Approximately 29 million pounds, or 10 percent, of the total nitrogen load to the Bay is due to excess fertilizer from urban/suburban areas in the Bay watershed.
These detrimental loads to the Bay could be reduced by using BMPs that:
- Ensure only the amount of chemical fertilizer needed for lawn growth is applied to the land.
- Prevent any excess nitrogen in chemical fertilizer from seeping into groundwater or reaching local waterways that lead to the Bay.
Municipal and Industrial Wastewater
Approximately 53 million pounds, or 19 percent, of the total nitrogen load to the Bay is due to municipal sewage treatment plants and industrial facilities that discharge treated wastewater to waterways in the Bay watershed.
These detrimental loads to the Bay could be reduced by:
- Using state-of-the-art technology that reduces nitrogen concentrations in sewage treatment effluent.
- Changing industrial processes that result in wastewater containing less or no nitrogen.
Septic Systems
Approximately 12.5 million pounds, or 4 percent, of the total nitrogen load to the Bay is due to septic systems that treat household wastewater and discharge effluent to groundwater in the Bay watershed.
These detrimental loads to the Bay could be reduced by using state-of-the-art technology that reduces nitrogen in septic system effluent.
Sources Not Accounted For in the Chart
The ocean is also a significant source of nutrients to the Bay, but is not accounted for in this chart. Contributions from tidal shoreline erosion are not included either.
Chesapeake Bay Program Phase 4.3 Watershed Model 2007 Simulation and the Airshed Model.