Skip to main content
King County logo

Monitoring update: June 23, 2017

This page provides information regarding the additional environmental monitoring of the wastewater discharges from the West Point Wastewater Treatment Plant that was conducted during the restoration of the plant following the flooding and damage that occurred on February 9, 2017. As of May 10th, repairs at West Point were completed to ensure that quality of secondary treated effluent will consistently meet all NPDES permit requirements for discharge to Puget Sound. Consequently, the additional monitoring has been discontinued and the County has resumed its routine monitoring programs required by the NPDES permit. 

The Wastewater Treatment Division is conducting water quality and environmental monitoring to assess the effects of West Point operations to Puget Sound while there was a reduced level of treatment being provided following the Feb. 9th flooding and damage to the treatment facilities. 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 June 23, 2017


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

WTD is conducting permit-required and supplemental monitoring following the 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 the level of wastewater treatment while repairs were being made.

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 occur.
  • Evaluate West Point effluent quality for correlations with any observed significant changes in marine water quality from 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, 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 monitoring update for June 23, 2017 shows the final influent and effluent data collected in May, and draft partial data collected through June 16, 2017. The data shows that with the secondary treatment process restored on April 27th, and additional restoration of processes occurring by May 10th, effluent quality met all permit requirements for discharge to Puget Sound in May, and is anticipated to continue to meet limits going forward.

The first quarter 2017 test for acute toxicity in West Point effluent was conducted for samples collected on March 21, 2017, and second quarter test for chronic toxicity was conducted for samples collected during the period of April 4-9, 2017. Both toxicity tests were conducted while the treatment plant was at a reduced level of treatment due to the flooding incident on February 9th. The acute toxicity test is conducted to observe for potential adverse effects of exposing sensitive aquatic organisms to effluent over short test periods (2 to 4 days depending on the test organism used). The chronic toxicity test is used to observe for potential adverse effects to organisms over a longer period of 7 days.

For both the acute and chronic toxicity tests, the data show that the test results have been normal with no substantial toxicity observed in the effluent. The detailed reports for download are:

Immediately following the damage to West Point on February 9th, 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 have been underway, we collected additional samples for the priority pollutants on a generally bi-weekly frequency. We are continuing to monitor priority pollutants 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

The attached report contains the final data for influent and effluent samples that were collected on May 10th and May 22nd, and the available partial data for samples collected on June 5th. Data collected over the most recent four weeks of sampling events (May 10th, May 22nd, and June 5th) indicate that measured concentrations of the priority pollutants are at normal levels and stable, and routine monitoring of priority pollutants will resume to a once-per-quarter frequency as specified in the NPDES permit.

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

  • The attached technical report discusses the effects of West Point effluent discharge to trace metals, ammonia, and residual chlorine concentrations in the initial zone of mixing created by the discharge outfall. These parameters, when elevated above water quality criteria beyond the initial zone of mixing, may cause unacceptable harmful effects to aquatic organisms.
  • The analysis compares estimated concentrations in the zone of mixing for normal West Point operating conditions to current conditions measured on May 10th and May 22nd reflecting the level of treatment since repairs to West Point were completed to a level that it is anticipated the effluent will consistently meet effluent limitations.
  • The analysis shows that under normal conditions, West Point effluent results in water quality concentrations that are well below any of the applicable water quality criteria for aquatic organisms.
  • The analysis also shows that with the current level of treatment, the effluent also is not causing any concentrations to rise above any water quality criteria.


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)

Follow WTD on social media