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Monitoring update: April 14, 2017

The Wastewater Treatment Division is conducting water quality and environmental monitoring to assess any impacts to Puget Sound. We are coordinating closely with the State Department of Ecology. Download and view the environmental monitoring work plan .

Summary: Marine and Environmental Monitoring Results as of April 14, 2017 Summary: Marine and Environmental Monitoring Results as of April 14, 2017

King County Wastewater Treatment Division (WTD) takes our commitment to environmental stewardship seriously.

WTD is conducting permit-required and supplemental monitoring following a major equipment failure at the West Point Treatment Plant on Feb. 9. Flooding of the treatment plant resulted in bypasses of untreated stormwater and wastewater into Puget Sound and reduced level of wastewater treatment during restoration. Currently, wastewater entering West Point is receiving partial treatment, disinfection (killing of bacteria and pathogens), and dechlorination before discharge through a deepwater outfall.

The focus and primary objectives of this work plan are:

  • Provide comprehensive, representative, and accurate monitoring and analyses of West Point discharge conditions and effects to the marine environment of Puget Sound.
  • Document West Point treatment performance and effluent quality improvements over time as repairs to West Point are made.
  • Evaluate the potential for short-term water quality effects of wastewater discharges to Puget Sound while repairs to West Point are underway.
  • Evaluate West Point effluent quality for correlations with any observed significant changes in marine water quality from bi-weekly offshore marine monitoring program data.
  • Evaluate the potential long-term water quality and sediment quality effects in Puget Sound (if any).

Below you will find information about water quality monitoring data being collected at West Point while the restoration of the plant is underway, status of compliance with NPDES permit requirements, potential water quality effects in Puget Sound, and how effluent disperses in central Puget Sound.

King County is monitoring the West Point wastewater coming into the plant (influent) and the discharge to Puget Sound (effluent) for multiple parameters that can affect water quality: flow, total suspended solids, biochemical oxygen demand, pH, fecal coliform bacteria, residual chlorine, and nutrients. These parameters are routinely sampled either daily, weekly, or monthly, as required in the NPDES permit for West Point that is overseen by the Department of Ecology. This data is used with the marine water quality monitoring data, and compared to permit limits.

Information can be found here regarding:

  • Bi-weekly updates of laboratory data for routine West Point effluent monitoring and comparisons to our NPDES permit limits.
  • Results of effluent tests that evaluate the potential for acute and chronic toxicity on marine life.

West Point Treatment Plant Restoration Routine Effluent Water Quality Monitoring Data

The effluent monitoring data collected in February (April 7, 2017 report) and March (April 14, 2017 update) shows that with the reduced level of treatment at West Point while repairs are underway, effluent is not meeting the NPDES permit limits for total suspended solids and biochemical oxygen demand. Additionally, the reduced level of treatment also was making it challenging to control the residual chlorine levels in the disinfection process at times during March.

The first quarter 2017 test for acute toxicity in West Point effluent was conducted for samples collected on March 21, 2017, while the treatment plant was at a reduced level of treatment due to the flooding incident on February 9th. The results showed that the effluent quality is normal with no substantial toxicity. View the detailed laboratory report of results, March 31, 2017.

King County conducted its required 1st quarter monitoring for EPA priority pollutants (i.e., primarily metals and organic compounds found in trace levels) in the influent and effluent. While repairs to West Point are underway, we are collecting additional samples for the priority pollutants on a periodic basis, and will continue with generally bi-weekly sampling until the plant is back to normal operations. We compare the data to water quality criteria and historical conditions to evaluate effects on Puget Sound.

Information can be found here regarding:

  • Bi-weekly updates of routine and additional West Point monitoring data for the priority pollutants.
  • Comparisons to water quality criteria and historical conditions to evaluate potential effects on Puget Sound.

West Point Treatment Plant Restoration Effluent Priority Pollutant Monitoring Data

West Point Treatment Plant Restoration Near-Field Effluent Mixing Water Quality Analysis

This information addresses questions about how effluent is discharged from the West Point deepwater outfall, and how it disperses with tidal flow in Puget Sound.

The West Point effluent is discharged approximately 3,600 feet offshore from West Point at a depth of about 240 feet below mean lower low water. The last 600 feet of the outfall pipeline contains multiple ports to increase the dispersion of effluent.

The discharge pattern and mixing (or dispersion) of the West Point effluent in the tidal flows within the central basin of Puget Sound are generally well understood through numerous studies conducted over the past 40 years.

  • The effluent is buoyant and rises through the water column to the water surface during the winter and spring periods of the year.
  • The diffuser creates rapid mixing of effluent with seawater such that by the time the effluent reaches the surface, the mixture is about 1 part effluent to 99 parts seawater (or 100:1 dilution).
  • Within this initial zone of mixing, the mixture of effluent must not exceed Washington’s water quality criteria.
  • During the winter and spring cold and wet months, typical of conditions while the West Point restoration is underway, the tidal currents result in effluent being dispersed along a dominant north-south axis near the outfall.
  • During each tidal cycle, the initial buoyant and rapidly mixed plume then disperses as a generally well-defined plume and moves about 5 miles north on ebb tides, and 2.5 miles south on flood tides.
  • As the plume moves with each tidal cycle, the plume with initial 100:1 diluted plume is further mixed by an additional factor of 5 to 10 fold (i.e., diluted to ratios of 500:1 to 1000:1).
  • During slack tide conditions, and through the reversal of tidal flow direction, additional and greater levels of dispersion and dilution occur.
  • Dye tracer study by King County and University of Washington researchers in the 1970’s found that the area of detectable dye levels, indicating effluent dispersion, was limited to the east side of the sound (grey outline shown in the map).

Map of Typical Winter/Spring Dispersion of the West Point Treatment Plant Effluent , April 2017

Historical technical reports on effluent plume studies of West Point:


For questions or inquiries related to the West Point restoration, please do not call the West Point Treatment Plant.

Contact us:

206-477-5371 (after hours, leave message)

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