Nitrogen - Its Fate as it Moves Through the Watershed
Approximately two thirds of US estuaries are being adversely impacted by eutrophication; therefore watershed nitrogen control is required in many areas of the country. To efficiently control nitrogen loadings to estuaries it helps to understand not only the sources of nitrogen inputs, but also the fate of the nitrogen as it moves through the watershed.
Nitrogen inputs to a watershed
Nitrogen inputs to a watershed vary substantially depending on the population density and land use. In the lesser developed, highly forested watersheds of New England the nitrogen input loadings are generally less than 12 pounds of N per acre per year. In the more urban watersheds of the northeast the nitrogen loadings range from 12 to 60 pounds per acre per year. The northern New England watersheds typically see nitrogen loadings on the low end of this range, while the central and southern New England watersheds typically see loadings in the mid-to upper end of this range. The various sources of nitrogen also vary significantly by watershed. The chart below shows the typical inputs for non-urban, largely undeveloped watersheds. In more urban and more developed watersheds, a larger fraction of the inputs are from food (wastewater) sources (typically 35 to 75% of the total inputs). The atmospheric loading in more developed areas is about the same as the lesser developed areas, but as a percentage of the total, it tends to be less (typically 10% to 30% of the total).
Nitrogen export from a watershed
Nitrogen movement through the watershed is a hot topic, but one that has been the subject of only a limited amount of recent research. While more research is needed in this area, we do know that only a fraction of the nitrogen inputs (i.e., nitrogen from upwind air emissions, food (wastewater) sources, fertilizer/agricultural sources, etc.) to a watershed are exported from the watershed to downstream estuaries. In the typical watershed, 20% to 60% of the nitrogen inputs are exported from the watershed via riverine and groundwater flow. The amount of nitrogen exported can vary substantially and is watershed specific. The less developed, highly forested watersheds export on the order of 20% of the nitrogen inputs, whereas the highly developed urban watersheds can export 40% to 60% of the nitrogen inputs. Smaller watersheds with granular soils and few rivers/streams, like much of Cape Cod, tend to export more of the nitrogen because natural denitrification is minimized in these environments. Studies by the Massachusetts Estuaries Project show that these watersheds typically export 80% or more of the nitrogen inputs.
Where does the nitrogen end up?
Exported nitrogen is less than the input nitrogen due to processes collectively called “attenuation.” To effectively manage nitrogen it helps to know how and where that attenuation is occurring. Some of the nitrogen is:
• Stored in the watershed in the soil, in the groundwater and in plant matter (dead and living).
• Removed from the watershed as crops (food and wood).
• Volatized to the atmosphere in the form of ammonia.
• Denitrified in soil, wetlands, ponds and rivers to return non-reactive nitrogen gas to the atmosphere.
The chart in the sidebar shows the typical nitrogen attenuation in non-urban watersheds. This chart shows that denitrification in the landscape and the rivers is the most significant nitrogen removal mechanism, responsible for transforming about 50% of the nitrogen inputs to non-reactive nitrogen gas in these watersheds. In the urban areas, there tends to be more of the nitrogen exported from the watershed to downstream estuaries, because there is less landscape and river denitrification and less nitrogen tied up in biomass.
There is growing concern that the nitrogen storage reservoirs (soil, vegetation,and groundwater) may ultimately become saturated and result in higher nitrogen export from the watershed in the future. In largely undeveloped watersheds, groundwater is generally not a significant storage sink for nitrogen. However, in recently developed areas the groundwater system can be a significant short term nitrogen storage reservoir, as it often takes years for groundwater to exit a watershed and thus it can take a long time to reach steady state conditions.
Nitrogen removal mechanisms
The various nitrogen removal mechanisms impact the sources of nitrogen in different ways. Knowledge of these mechanisms is important in selecting the most efficient means to reduce nitrogen loadings to estuaries. Some examples:
Precipitation
One pound of atmospheric nitrogen deposited via precipitations onto a forest is apt to result in less than 0.1 pounds of nitrogen exported out of the watershed. The same amount of precipitation-delivered nitrogen in an urban area could result in one pound of nitrogen exported out of the watershed. So obviously there is a lot more benefit controlling runoff from urban areas than forest runoff.
Septic systems
One pound of wastewater-generated nitrogen entering a septic tank typically results in about 0.5 to 0.6 pounds of nitrogen discharged to the groundwater. If the septic system is located in well drained soils in close proximity to an estuary, 0.5 to 0.6 pounds of nitrogen is likely to get delivered to the estuary. If the septic system is located upstream in the watershed, and the groundwater flow was intercepted by wetlands and then by a river, as little as 0.2 pounds of nitrogen might be exported. So if septic system nitrogen control is necessary, focusing on the systems that export the most nitrogen would be the most cost-effective means to reduce the nitrogen loading.
Wastewater treatment plants
One pound of wastewater-generated nitrogen entering a conventional secondary treatment plant typically results in about 0.7 pounds of nitrogen discharged to a river. For a treatment plant with its discharge point near the estuary, most of this nitrogen will be exported to the estuary. For a second plant with its discharge point significantly upstream of the estuary, significant river denitrification could occur and the exported nitrogen could be reduced to as little as 0.2 pounds. If you had to reduce the nitrogen loads, the secondary treatment plants could be upgraded to effect as much as 90% nitrogen removal, reducing the one pound of nitrogen input to 0.1 pounds. This would provide much more benefit to the estuary (in terms of nitrogen removed from the estuary and dollars spent per pound of nitrogen removed from the estuary) when applied to the downstream plant as opposed to an upstream plant.
Two obvious strategies to reduce nitrogen loadings to estuaries.
1. Reduce the nitrogen inputs to the watershed.
2. Maximize the nitrogen storage and removal mechanisms in the watershed.
Achieving efficient nitrogen reductions requires knowledge of the specific nitrogen inputs and the nitrogen attenuation factors.