Green Lake Beach Bacteria Monitoring

Studies Show Water Quality Impacts of Homeless Waste Disposal at Green Lake

Statistical analysis of weekly fecal bacteria data shows a significant increase in levels at West Beach but not East Beach in the past 2 years. This finding suggests that the increase may have been due to human waste disposal in the lake since West Beach is down current of homeless encampments and RVs, while East Beach is not near encampments and was closed to swimming and more populated by geese in this period. Human waste pathogens and floating hypodermic needles pose a risk to swimmers on the west shore of the lake.
Or read the presentation in this post below:

See a presentation of the analysis from FOGL’s July meeting here. 

Green Lake Beach Bacteria Monitoring

FOGL Meeting July 27, 2021

Rob Zisette


King County Beach Bacteria Monitoring Program

Weekly grab samples collected in triplicate on Tuesdays May–Sept. at east and west beach

Closures and results posted at:

Spotty data records:

  • Fecal coliform at east beach for 1996-2002 and 2014-2019
  • Fecal coliform at west beach for 2003-2019
  • E. coli at east beach for 1998-2002 and 2019-2020
  • E. coli at west beach for 2003-2008 and 2019-2020


King County Beach Bacteria Closure Protocol

Low concern daily average <200 CFU/100 mL

Moderate concern and safe for swimming daily average > 200 and < 1000 CFU/100 mL

Beach usually closed if one or both of these criteria are met:

  • Daily average > 1000 CFU/100 mL
  • 3-Day geomean > 200 CFU/100 mL

Beach usually reopened if both of these criteria are met:

  • 3-Day geomean < 200 CFU/100 mL
  • Daily average for previous 2 days < 200 CFU/100 mL


King County Beach Fecal Bacteria Time Series/Trend Analysis

(Click chart to enlarge)

Kendall’s Tau Temporal Trend Analysis:

  • West Beach (Impact) significant increasing trend (p = 0.005)
  • East Beach (Control) no significant trend (p = 0.18)


King County Beach Fecal Bacteria Box Plot

(Click chart to enlarge)

Wilcoxon Rank Sum Test Between 2014-2019 and 2020-2021:

  • West Beach (Impact) significantly higher in COVID Camp period (p = 0.0003)
  • East Beach (Control) not significantly different between periods (p = 0.36)


Seattle Parks Test Plan

Plan by Jean Lee on 7/23/21:

  • Collect triplicate samples at two SW shore locations ~15 feet from shore on July 27, 2021.
  • Decide actions or further testing based on these results.

My recommendation:

  • Sample two distances at 3+ locations (e.g., 10 and 20 feet)
  • Time collection with beach sampling
  • Sample on 3+ occasions
  • Also analyze for human biomarker HU2 at SW shore and both beaches.


King County Beach Fecal Bacteria Conclusions

  • Fecal bacteria have significantly increased at West Beach but not East Beach during 20202021 COVID Camp period.
  • Fecal bacteria reached a moderate level of concern twice in 2020 and once so far in 2021, and not at all before COVID camps.
  • West Beach increase is not due to waterfowl because waterfowl have increased at East Beach due to closure and had no apparent effect.
  • RV waste dump takes 12 hours to flow north to West Beach in slight southerly wind.
  • Posting weekly beach sample results 2 days after collection is not sufficient to insure safe conditions during lake use.


Green Lake Swimming Risk Factors

  • Human fecal contamination and hypodermic needle sources on west shore are a significant concern to all contact recreation due to RVs and camps.
  • Waterfowl fecal contamination also pose risks for gastrointestinal disease (Salmonella) from water ingestion, while human diseases can be more serious.
  • Avoid swimming along west shore and water ingestion anywhere during RV and camps are present.
  • Beach bacteria results on website are not current.