Where Does Nitrogen Come From?

We are using the Salish Sea Model to evaluate the relative effects of human (anthropogenic) and natural nutrient sources on dissolved oxygen levels in Puget Sound. This model is being used to guide the Puget Sound Nutrient Reduction Project.

Nitrogen concentrations in different sources

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This graphic illustrates the typical range of nitrogen concentrations we see in different sources. Note that the y-axis of the figure is on a log scale, so septic system effluent and wastewater treatment plant effluent concentrations are one to two magnitudes higher than most other sources.

Nitrogen load contributions

Even though concentrations of nitrogen in the incoming deep Pacific Ocean water are not the highest (relative to other sources), the large amount of water flowing into Puget Sound and the Salish Sea from the ocean means the Pacific Ocean contributes the largest load.

“Load” is an important way to quantify nitrogen contributions, and is simply calculated as:

nitrogen load = nitrogen concentration x flow

After the Pacific Ocean, there are two main terrestrial inputs of nitrogen to Puget Sound:
Wastewater treatment plant effluent discharges to Puget Sound via marine outfalls.
Rivers and streams deliver sources from the watersheds they drain.
The load entering Puget Sound from rivers and streams includes contributions from both natural sources and human watershed activities.

This time series plot shows the relative contributions from human and natural sources of dissolved inorganic nitrogen during 2006. Human sources include wastewater treatment plants discharging into Puget Sound and human activities within watersheds delivered via rivers. It is important to note that each of these loads enters the system at different points in space and time. Therefore, a higher source of loading does not necessarily mean it has the highest impact on water quality.

We use the term “reference load” to describe our estimates of nutrient loads that exclude human inputs of nutrients since this is our best estimate of the “natural load” – the true natural load cannot be estimated due to lack of water quality data before human development in this region. Our estimates of the reference nitrogen load may be refined based on ongoing analysis.

On an average annual basis, the pie chart shows that an estimated 2/3 of the nitrogen load is from human sources and 1/3 of the nitrogen load is from natural sources.

Improvements in wastewater treatment technology and watershed management actions can reduce nitrogen loading from these sources.

Comparing wastewater & river nitrogen loads

Wastewater treatment plants and river dissolved inorganic oxygen (DIN) loads to Puget Sound are similar in magnitude on an annual basis. However, summer wastewater treatment plant loads are much greater than summer river loads.

Wastewater facilities treat about the same amount of wastewater regardless of the time of year, so the magnitude does not show much variation over time. However, river loading changes with the season, since the amount of flow in rivers is based on rainfall patterns. During the summer, streamflows are at their lowest, and river DIN loads reach an annual minimum.

The seasonality in loads is important to Puget Sound water quality, because the lowest dissolved oxygen concentrations in Puget Sound have been measured in the late summer season. Low dissolved oxygen conditions in Puget Sound in the late summer may mean that the ecosystem is more sensitive to nitrogen loading in the summer months.