Puget Sound nutrient pollution studies

A healthy Puget Sound is critical to the recovery of salmon, orca, and other marine life. However, oxygen levels in many parts of Puget Sound and the rest of the Salish Sea are now below the levels needed for fish and other marine life to thrive. Nutrient pollution from human activities is worsening the region’s naturally low oxygen levels. Areas most affected are poorly flushed inlets, including Penn Cove, Quartermaster Harbor, and Case, Carr, Budd, Sinclair, and Dyes Inlets.

We recognize the need to manage human sources of nutrients in the Puget Sound region. We have evaluated the impacts of reducing nutrient pollution from municipal wastewater plants and have published a report that shares our findings. 

The effects of low levels of dissolved oxygen

In addition to depleting oxygen levels, nutrient pollution flowing into marine waters can lead to profound consequences for the ecosystem. In addition to low levels of oxygen, some effects include:

• Acidification, which can prevent shellfish and other marine organisms from forming shells
• Shifts in the number and types of bottom-dwelling invertebrates
• Increases in abundance of macroalgae, which can impair the health of eelgrass beds
• Seasonal reductions in fish habitat and intensification of fish kill events
• Potential disruption of the food web

Scientists search for solutions

Our research team evaluated changes to dissolved oxygen levels by modeling reductions in nitrogen and carbon at municipal wastewater treatment plants. We used a peer-reviewed, state-of-the-science computer modeling tool called the Salish Sea Model. It models water movement and water quality in the Salish Sea, and it extends into the coastal waters of Southwest British Columbia, Washington, and Northwest Oregon.

We recognize the need to manage human sources of nutrients in the Puget Sound region. Excessive nutrients in rivers and from point sources flowing into the Sound — such as municipal wastewater treatment plants — deplete dissolved oxygen below the water quality standards. In our report, we evaluated changes in marine dissolved oxygen due to reducing nitrogen and carbon at municipal wastewater plants.

The years 2006, 2008, and 2014 were modeled to represent a range of climate and ocean conditions affecting Puget Sound. The model scenarios tested the impacts of:

• Current levels of nutrient pollution from rivers and wastewater treatment plants discharging directly to Puget Sound.
• Reduced nitrogen and carbon at all municipal wastewater treatment plants discharging to the Sound.
• Reduced nitrogen and carbon at only midsize and large municipal wastewater treatment plants discharging to the Sound.
• Reduced nitrogen and carbon at only large municipal wastewater treatment plants discharging to the Sound.

Results of our research

The results of this first phase of modeling conducted in 2018 confirms that human sources of nutrients are having a significant impact on dissolved oxygen in Puget Sound in multiple embayments. Reducing nutrient pollution at municipal wastewater treatment plants would provide significant progress toward meeting the dissolved oxygen water quality standards in the Sound.

The tables below show estimated improvements in oxygen when nutrient pollution from wastewater treatment plants is reduced. The top table shows the percent reduction in the number of days that are not in compliance with the water quality standard, and the second table shows the percent reduction in the noncompliant area.

Improvements in dissolved oxygen

Percent reduction in noncompliant days
due to nutrient reduction  at wastewater treatment plants

Year	All plants	Middle- & large-sized plants	Large plants
2006	51 percent	43 percent	31 percent
2008	61 percent	49 percent	33 percent
2014	51 percent	42 percent	22 percent

 

Percent reduction in noncompliant area
due to nutrient reduction at wastewater treatment plants

Year	All plants	Middle- & large-sized plants	Large plants
2006	47 percent	37 percent	23 percent
2008	51 percent	41 percent	24 percent
2014	42 percent	33 percent	13 percent

Under existing conditions, approximately 20 percent of the area in the greater Puget Sound does not meet dissolved oxygen standards. If reductions are made at all wastewater treatment plants as modeled, approximately 10 percent of greater Puget Sound would not meet dissolved oxygen standards. This represents a 50 percent improvement in compliance area for the dissolved oxygen standard.

It is clear from the modeling study that it will take a combination of nutrient reductions from wastewater treatment plants and other sources of nutrient pollution in watersheds to meet marine water quality standards.

Therefore, future evaluations of nutrient reduction strategies will need to include a comprehensive suite of measures.