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Saving Puget Sound

South Puget Sound Dissolved Oxygen Study:
Overview of the study
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What is the problem?


In the 2008 Water Quality Assessment, Ecology found that 24 locations in South Puget Sound were impaired due to a lack of dissolved oxygen. Ecology identified another 27 locations as waters of concern.

The locations of greatest concern are Carr, Case, and Budd Inlets. Fish need oxygen. In areas with low levels of dissolved oxygen, fish and other marine life become stressed and die or are forced to flee their habitat.

Map of Puget Sound showing areas designated as Impaired Waters and Waters of Concern

What is the study area?

This effort focuses on South Puget Sound, south of the Tacoma Narrows (shown in orange). However, because Central Puget Sound sources may influence South Puget Sound water quality, Ecology is including the entire South and Central Puget Sound in the initial effort (shown in green). Ecology will evaluate all human loads, including north of the boundary.

What are the sources of nitrogen?

Nitrogen enters Puget Sound from many different human and natural sources. For this study, Ecology grouped the sources of nitrogen into four categories:

  1. Wastewater Treatment Plants (WWTPs): This study includes 29 municipal wastewater treatment plants that discharge nitrogen directly into Puget Sound.
  2. Watersheds: This study includes 39 rivers and streams that flow into Puget Sound. It also includes the other upland areas that drain directly into Puget Sound. Nitrogen in rivers comes from septic systems, stormwater, WWTPs, upland atmospheric deposition, other point and nonpoint sources, and natural sources.
  3. Atmospheric deposition, directly on Puget Sound, is a smaller but measurable source of nitrogen.
  4. Exchange of marine water with northern parts of Puget Sound and the Pacific Ocean. This source will be analyzed with the water quality model developed in the next phase of the study.

>> See additional information on Nitrogen in the Puget Sound Ecosystem.

How much nitrogen enters South Puget Sound?

As part of the study, Ecology analyzed river and WWTP discharges to Puget Sound for their dissolved inorganic nitrogen content. Dissolved inorganic nitrogen is the sum of nitrate, nitrite, and ammonium. It is the form of nitrogen we are most interested in for Puget Sound.

On an annual basis, wastewater treatment plants south of the Tacoma Narrows send an average of 6,000 pounds of dissolved inorganic nitrogen into South Puget Sound per day. Another 11,000 pounds of dissolved inorganic nitrogen came from all other human and natural sources in the watershed. During the critical summer period for dissolved oxygen when river flows are lower, the wastewater treatment plants are a larger percentage of the load. The area between the Tacoma Narrows and Edmonds has many more people and it contributes about four times more dissolved inorganic nitrogen than South Puget Sound. Future work will quantify the amount of nitrogen coming into South Puget Sound from the Pacific Ocean.

The amount of dissolved inorganic nitrogen (DIN) per day from each WWTP and watershed is shown on the map below. The bigger dots represent larger amounts of DIN. Watersheds are in pink and WWTPs are in blue. See the nutrient loading report for more information about nitrogen entering Puget Sound.

map showing Annual Nitrogen Loads