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9611 SE 36th Street
9601 SE 36th Street
Mercer Island, WA 98040
Phone: 206.275.7608
Fax: 206.275.7814
Contact: Maintenance Department
Email: mi_maint@mercergov. . .
Hours: 7:00 AM - 3:30 PM (Mon-Fri)
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City of Mercer Island / Maintenance / Stormwater Utility / Mercer Island and Lake Washington Water Quality

Mercer Island & Lake Washington Water Quality

Mercer Island and Lake Washington Water Quality
Mercer Island and Lake Washington Water Quality

The City of Mercer Island, in partnership with King County, has been studying how our community's storm water runoff affects water quality in Lake Washington since 1999.
 
In that time, we have learned a lot. Most importantly, Mercer Island is a relatively good steward of the Lake's water quality when measured against water quality standards used by the state and other agencies:

  • We are complying 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).
  • Over the course of this eleven year project, only one herbicide or pesticide has been detected at concentrations above available guidelines, and that was diazinon from a sample collected from Basin 32 in 2001.

But this doesn't mean we can't do better:

  • The insecticide diazinon and compounds found in weed-n-feed products have been detected in water samples from both residential and commercial areas. Diazinon concentrations exceeded the Environmental Protection Agency¡¯s water quality criteria in 10 of 58 samples from 1999 to 2002, but only once in 2003.
  • Nitrate and phosphate concentrations often have been higher than the federally recommended concentrations for western Washington waters. These nutrients are found in fertilizers and animal wastes 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 started the monitoring program in 1999 because new state and federal regulations require Mercer Island to examine how it impacts Lake Washington's water quality and work to minimize those impacts. The City's Storm and Surface Utility has been trying to control erosion in the Island's most unstable ravines, and we want to know how effective our projects are, and how to prioritize future projects. Also, we want to target educational efforts to get the best possible results.

Here are some simple things you can do to protect water quality in our 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 King County Hazardous Waste Management Program and read about disposal options at King County facilities.
  • Use detergents with low or no phosphates.
  • If you live 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 armoring concentrates storm flows and leads to increased velocities, which can promote erosion of adjacent unarmored stream segments and contribute to excessive discharge of sediments.

In the last few years, our efforts have shifted toward monitoring aimed at finding significant sources of dissolved metals in Basin 10 and 32.  The first step in this work occurred in 2011-2012.  To read the report, please click here.  Contact City Engineer Patrick Yamashita, at 206-275-7722.



 

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