City Hall: Stormwater Pollution Report 'Mostly Positive' Despite Exceeding Water Quality Standards

The City of Mercer Island exceeded federal EPA guidelines on water quality 13 times from 2008 to 2010.

announced the latest findings Wednesday in an ongoing water-quality study of Lake Washington, giving the city "many positive" marks, according to a city press release.

Conducted in partnership with King County, the study focusing on local stormwater runoff finds that Mercer Island complies with basic state and federal standards for ammonia-nitrogen concentrations, temperature (except during July and August), and pH (water samples are neither too acidic nor too alkaline). The only herbicide or pesticide detected at concentrations above available guidelines was diazinon. 

However, stormwater levels of fecal colliform bacteria in the city's outfalls into Lake Washington exceeded state standards seven times during storm events from 2008 to 2010. Levels of fertilizer and heavy metals that exceed EPA guidelines have also been detected. Specifically:

  • The insecticide diazinon and compounds found in weed-n-feed products have been detected in 10 of 58 water samples from both residential and commercial areas from 1999 to 2002 exceeding the Environmental Protection Agency's water quality criteria.
  • Nitrate and phosphate concentrations found in fertilizers and animal wastes often have been higher than the federally recommended levels and contribute to an overabundance of algae and other plants in lakes.
  • Erosion of steep ravines continues to send an excessive amount of sediment to Lake Washington, reflected in turbidity levels that exceed state standards during major storms.
  • Basins 10 and 32 continue to have high copper and zinc concentrations.

To read the final version of the 2008-2010 report, please click here.
The City's Storm and Surface Utility has been working to control erosion in the most unstable Island ravines. The City recommends Island property owners take these simple steps to help protect water quality in Island streams and Lake Washington:

  • Apply pesticides, herbicides, and fertilizers sparingly, only on windless days, and never near streams or Lake Washington or when rain threatens. Click to find out more about Natural Yard Care.
  • Dispose of lawn and garden chemicals carefully. For information about recycling chemical yard wastes, call the Household Hazards Line at 206.296.4692 or visit the  King County Hazardous Waste Management Program.
  • Use detergents with low or no phosphates.
  • If your property is adjacent to a stream or Lake Washington, keep the banks naturally vegetated because trees and shrubs shade the water, keeping it cool for fish, preventing erosion, and filtering pollutants from storm water.
  • Do not build up stream banks with rocks because it concentrates storm flows and leads to increased velocities, which can promote erosion and contribute to excessive discharge of sediments.

Surrounded by Lake Washington, Mercer Island's management of natural resources is an issue of regional and statewide importance for recreation, tourism and the protection of migratory fish and birds, in particular several native species of salmon. Earlier this year, the by strengthening regulations on the composition, widths and transparency of new or substantially repaired docks and piers.

In the next few years, the City will be monitoring and investigating potential sources of dissolved metals in Basin 10 and 32.  Full copies of the monitoring program annual reports, including laboratory data, are available for review by contacting City Engineer Patrick Yamashita, at 206.275.7722.

— Information presented is an excerpt from a City of Mercer Island press release


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