After more than eight years of study amid ongoing discussions, the Washington Department of Ecology has made public a far-reaching plan for reducing human sources of nitrogen that contribute to the destructive low-oxygen conditions in Puget Sound. The plan, called the Puget Sound Nutrient Reduction Plan, calls for reductions in nitrogen from sewage-treatment plants, agricultural operations and a variety of other upstream sources.
The plan is based on decades of monitoring to identify the locations of low-oxygen problems, investigations to quantify sources of nitrogen (both natural and human-derived), and computer modeling to reveal how specific source reductions could affect water quality.
Nitrogen, an essential ingredient for growth, has been shown to be a problem at times when released into Puget Sound. Excess nitrogen triggers excessive plankton blooms. The plankton eventually die and sink to the bottom, spurring the growth of bacteria, which consume oxygen supplies needed for healthy marine populations.
Besides low-oxygen effects on sea life, some plankton species disrupt the food web, because they cannot be eaten by herring and other small fish that serve as prey for larger fish and marine mammals. Other plankton species excrete dangerous toxins, resulting in “harmful algal blooms” that can kill fish, birds and marine mammals and disrupt commercial shellfish operations. Even beneficial plankton in overabundance can block sunlight and impair eelgrass beds and other essential habitats.
The need for a plan to address the oxygen problems throughout Puget Sound grew out of studies beginning in the 1980s. By the early 2000s, local problems were better understood. It became clear, for example, that low-oxygen levels in Budd Inlet and other South Puget Sound bays were affected by waters coming from the rest of Puget Sound to the north. Since parts of Puget Sound are naturally low in oxygen, an ongoing debate surrounds the extent to which human sources of nitrogen are to blame.
To help pull the information together and develop a plan of action, Ecology launched the Puget Sound Nutrient Source Reduction Project In the spring of 2017. A discussion group, called the Puget Sound Nutrient Forum, started up a year later to hear reports about the latest findings. Participants included researchers, government officials, sewage-treatment plant operators, tribal representatives, environmentalists and others.
Homing in on nutrient sources
By 2019, Ecology staffers were convinced that human sources of nitrogen, including sewage treatment plants, were playing a key role in pushing oxygen concentrations below state water quality standards in some areas. The Salish Sea Model, designed to simulate actual conditions in Puget Sound, accounts for physical processes — currents, tides and nutrient transport — along with chemical and biological processes that transform chemical compounds, including nitrogen. To the surprise of early researchers studying these dynamic changes, models revealed that nitrogen released at one point could move many miles away and trigger the growth of plankton in remote areas.
Model runs reported in 2019 were based on data from the years 2006, 2008 and 2014 and published in the Ecology report “Model Updates and Bounding Scenarios.”
“Modeling results show that portions of Puget Sound, primarily South Sound and Whidbey Basin, experience a large number of days when the marine DO (dissolved oxygen) water quality standard is not met,” the report states. “In multiple locations within these two regions, the total number of noncompliant days is over three months.”
Model refinements in 2021 (PDF) began to focus on regions of Puget Sound and to combine scenarios that involved greater or lesser nitrogen discharges from treatment plants versus the upstream watersheds. Further refinements this year provided the foundation for the newly released nutrient reduction plan.
Outcomes of recent model runs have shown that most of the impaired areas are close to meeting the state’s natural conditions criteria, which allow dissolved oxygen levels to be as much as 0.2 milligrams per liter above prehistoric levels, as calculated by the model. More than half are within 0.1 mg/l of this standard by some calculations. Nevertheless, significant improvements will be needed to achieve full compliance.
“When it comes to nutrient pollution, there are proven ways to solve the problem, and we will need to use all our tools to get to clean water,” said David Giglio, manager of Ecology’s Water Quality Program in a news release. “This includes significant investments for wastewater infrastructure. As a region, we need to be as efficient as possible with our resources while we work toward a healthy Puget Sound and restoring salmon runs.”
So-called watershed sources include upstream fertilizers used on farms and in urban areas, sewage-treatment plants, septic systems, animal wastes and atmospheric deposition, all washing off the land, eventually entering Puget Sound through 193 rivers and streams in the current model. While nitrogen is a key factor in reducing oxygen levels, the Salish Sea Model also calculates the effects of inflowing organic materials that can reduce oxygen levels through decomposition.
Of all the human sources of nitrogen under consideration, about two-thirds arrives from direct discharges from sewage-treatment plants, with about one-third from upstream sources flowing into rivers and then into the Sound. Natural sources are estimated to contribute about 90 percent of all nitrogen in Puget Sound, but where and when each of these sources arrive in the Sound are key factors in determining oxygen levels at various locations, as accounted for in the model.
Puget Sound currents, driven by tides and incoming freshwater flows, play a major role in oxygen levels at every location in Puget Sound. Most vulnerable to low oxygen levels are small bays as well as the terminal ends of longer inlets, where low circulation and slow flushing rates allow an accumulation of low-oxygen waters on the bottom. Hood Canal, for example, has no major inputs of nitrogen from industry or sewage treatment, yet the southern dead-end portion of the waterway (Lynch Cove) shows persistent low-oxygen conditions, in large part from natural sources of nitrogen.
In some areas, such Central Puget Sound, nitrogen released from treatment plants in Seattle and Tacoma can have far-reaching effects, because the large inputs of nitrogen are carried by strong currents.
Latest model results
In March, the latest refinements in the Salish Sea Model and fresh findings from model runs were revealed in an online meeting of the Puget Sound Nutrient Forum (PDF). More than a dozen model scenarios looked at combinations of various nitrogen levels coming from treatment plants, paired with various nitrogen reductions from watersheds. Improvements were measured by calculating areas of Puget Sound that could meet water quality standards for each combination. Over a full year, for each combination, the model was able to determine minimum oxygen levels as well as the number of days that water-quality violations would occur anywhere in Puget Sound, as described in the report Model Updates and Optimization Scenarios, Phase 2.
After studying the options based on nutrient reductions at specific locations, Ecology officials selected on option announced today in the draft plan, which establishes regional targets for nitrogen levels in both wastewater and watersheds. Point-source targets will be used to set effluent limits for sewage-treatment plants and industrial facilities.
Watershed targets are aggregated to each of Puget Sound’s eight defined basins, including the Strait of Juan de Fuca, Main Basin, South Sound and so on. Watershed targets will be a starting point for a 25-year cleanup effort, including two cleanup plans by 2027.
“The watershed targets are represented as a single annual TN (total nitrogen) load for each of the eight Puget Sound basins,” the plan says. “Note that these loads represent all upstream nonpoint and point sources of TN in the 163 distinct watersheds draining to Puget Sound.”
To meet water-quality standards, the plan calls for a 68 percent reduction in nitrogen coming from human sources in the large watersheds on the east side of Puget Sound, including South Sound. For the small watersheds in that area, the proposed reductions are 61 percent. The reduction goal is 53 percent for Hood Canal and Admiralty Inlet. Watersheds in the Strait of Juan de Fuca and the Strait of Georgia would be capped at existing levels with no further reductions needed.
An important exception is proposed in the “recalcitrant areas” that present major challenges to meeting water quality standards. Specifically, those areas are Lynch Cove in Hood Canal ; Sinclair Inlet and Liberty Bay on the Kitsap Peninsula; and Carr and Henderson inlets in South Sound. To meet water-quality standards, local streams might need to reduce their nutrient loads by 90 percent, according to the plan.
The focus on direct nitrogen inputs to Puget Sound involved 99 “point sources,” including 78 sewage treatment plants in Washington, nine sewage treatment plants in British Columbia and 10 industrial facilities.
To gain the best results in water quality with less extensive changes in treatment systems, discharges of nitrogen would be reduced the most during July, August and September — the so-called “hot months” — when sunlight, nutrient buildup and slow mixing triggers plankton blooms, leading to the lowest oxygen levels of the year. The second-highest reductions would be the surrounding months of April, May, June and October, the “warm months” that typically have mixed light levels and water movement. “Cool months,” from November through March, are known to have more dynamic changes in weather and mixing and fewer low-oxygen problems.
Using this strategy, the plan would propose hot, warm and cool periods for treatment plants discharging more than 22 pounds of total nitrogen and more than 13 pounds of dissolved nitrogen per day in these basins: Northern, with one treatment plant; Whidbey, 11 plants; Main, 14 plants; and South Sound, three plants. Specifically, average nitrogen concentrations were set to 3 milligrams per liter in hot months, 5 mg/l in warm months and 8 mg/l in cool months. For plants on Sinclair Inlet, the limit would be 3 mg/l year-round.
Most large treatment plants discharging into the main basin of Puget Sound, such as Seattle and Tacoma, would be limited to 3 mg/l in both hot and warm months, with 8 mg/l in cool months. West Point in Seattle would use the three limits for hot/warm/cool periods mentioned above. Limits would be set to 2014 nitrogen loads for smaller treatment plants and for those discharging into Hood Canal, Admiralty Inlet, Strait of Juan de Fuca and Straight of Georgia.
Sources of nitrogen in the watersheds
In searching for upstream, land-based sources of nitrogen, scientists have discovered that every river entering Puget Sound contains a unique mix of nitrogen from fertilizers, animal wastes, sewage-treatment plants, alder trees, urban stormwater and septic systems.
The total amount of nitrogen being delivered to Puget Sound varies greatly from river to river and season to season, with the greatest amount of nitrogen arriving during the high river flows of winter, according to a report released this year by the U.S. Geological Survey.
Actual upstream sources of nitrogen in the various rivers are as different as the surrounding land uses — from farm to forest to urban development, according to the study based on a USGS watershed model called SPARROW — SPAtially Referenced Regressions On Watershed attributes. Results of the SPARROW model are under review and yet to be compared with previous models, including Visualizing Ecosystem Land Management Assessments (VELMA) developed by the Environmental Protection Agency and the Hydrological Simulation Program — Fortran (HSPF), a separate USGS model. See also Puget Sound Integrated Modeling Framework.
Of the total nitrogen reaching Puget Sound from 19 major watersheds, more than half came from four river systems: the Nooksack, Snohomish, Cedar-Sammamish and Duwamish-Green. Each of those watersheds contribute about the same amount of nitrogen — around 14 percent of the total — even though the size of the drainage areas is much different. Snohomish is the largest area with 4,836 acres, followed by Nooksack, 3,351; Cedar-Sammamish, 1,688; and Duwamish-Green, 1,361.
Within those four watersheds, the sources of nitrogen are quite different. Nitrogen in the Cedar-Sammamish and Duwamish-Green watersheds, which include urban areas, is dominated by effluent from upstream sewage treatment plants. The Nooksack, a largely rural area including farms, shows heavy inputs from crop fertilizers, livestock and alder trees. The Snohomish, which includes forested areas, gets its nitrogen largely from alder trees and sewage-treatment plants.
The watershed with the greatest drainage area, the Skagit-Samish with 8,861 acres, comes in fifth in the amount of nitrogen released into Puget Sound. Because of large, forested areas, major nitrogen sources include alder trees and the atmospheric deposition of nitrogen-containing particles.
Nitrogen from alder trees was a major source in most of the 19 watersheds, as alder trees have become more common today than in prehistoric times. Such changes were factored into a “reference scenario” to consider changes from prehistoric times.
“A reference scenario was developed to provide an estimate of the pre-industrial local and regional loads, which indicated that the largest increases in (total nitrogen) yield from historical to present were from the Cedar and Green Rivers as well as Chambers Creek,” the report says. Historic modeling could help in developing strategies for reducing nitrogen from various sources.
Septic systems were the largest source of nitrogen coming off the Kitsap Peninsula and the Deschutes and Kennedy-Goldsborough watersheds in South Puget Sound.
The model also calculates phosphorus releases for the various watersheds, because phosphorus tends to stimulate algae growth in freshwater, as nitrogen does in saltwater. Algae coming out of the streams may add to the organic load in the marine waters, contributing to low-oxygen conditions.
Watersheds contributing the greatest phosphorus load to Puget Sound are, in order, the Snohomish, Skagit-Samish, Cedar-Sammamish and Duwamish-Green.
The SPARROW model will be useful in predicting the success of various nitrogen-reduction actions in the watersheds, according to authors of the study. Actions that reduce the amount of nitrogen coming into Puget Sound at certain times of the year, particularly summer and fall, could improve oxygen levels during critical periods.
“The seasonal mass balance representation of streams from headwaters to marine-water discharge points by source is important information to support Ecology’s nutrient reduction plan,” the report states. “Certainly, there are limitations with process representation and simulation accuracy, but the modeling approach directly provided real, interpretable values… Although model uncertainty can be high when zoomed into unique reaches (mean error 50% nitrogen to 72% phosphorus), an ability to quantify uncertainty in space and time is a strength.”
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This article was funded in part by King County in conjunction with a series of online workshops exploring Puget Sound water quality. Its content does not necessarily represent the views of King County or its employees.
The Series
Part 1: The debate over oxygen in Puget Sound
Part 2: Water-cleanup plans and the search for ‘reasonable’ actions
Part 3: Computer models spell out the extent of the water-quality problem.
Part 4: Many actions may be needed to improve Puget Sound waters