Steel Lake 2,4-D Monitoring - 2004

Description of Site

Steel Lake is a 46-acre urban lake located within the city limits of Federal Way (near Seattle) in King County. It is a shallow lake (mean depth of 13 feet) with a watershed area of 243 acres which are primarily developed as residential and commercial property. The lake has a fully developed shoreline with the majority comprising single-family homes with lakefront access. Eurasian watermilfoil (Myriophyllum spicatum) was successfully removed from Steel Lake after a 1994 Sonar® treatment. The lake remained free of milfoil through 1997 when a change in city personnel and a lack of interest by lake residents resulted in lake surveys being stopped. Milfoil was detected in 2001 by King County staff. Milfoil was treated with granular 2,4-D in 2002. In 2003, milfoil numbers were so low that the City decided to control milfoil solely by hand harvesting. In 2004, milfoil populations were on the upswing and milfoil was treated using the liquid formulation of 2,4-D (DMA*4IVM™).


Steel Lake was treated with liquid 2,4-D (DMA*4IVM™) on July 9, 2004. The herbicide was applied using sub-surface injection by a licensed applicator. Six separate areas (total acreage of 5.4-acres) were treated in Steel Lake at a rate of 2 to 4 ppm (see treatment map).  A total of 50 gallons of herbicide was used for this treatment. Treatment areas were: A: 1.0 acre 9.25 gal; B: 1.0 acre 9.00 gal; C: 1.4 acre 13.0 gal; D: 0.1 acre 1.00 gal; E: 0.3 acre 2.75 gal; F: 1.6 acre 15.0 gal (see treatment map below)


Herbicide residue samples were taken by city of Federal Way staff at pre-treatment, 24 hours, and 3 days after treatment. Samples were taken in the middle of the lake (outside the treatment areas) and inside two treatment sites (one on the north shoreline and the other on the south shoreline). All samples were taken using a Wildco Alpha 2.2 liter Van Dorn style water bottle. Samples were retrieved from various depths at each sampling site, combined into individual composite samples, and chilled until delivery to the laboratory. The samples were analyzed by an Ecology- accredited laboratory using method number 8151 GC/MS modified.


Steel Lake was visually surveyed after treatment by Federal Way staff, the lake residents, and the aquatic herbicide applicator who all reported a good milfoil kill.  A post-treatment survey is scheduled by an aquatic plant management firm. No adverse impacts such as algae blooms or fish kills were observed after treatment.

Herbicide Residues

2,4-D samples generally remained above the EPA drinking water standard of 70 ppb for three (or more) days, but by day three all samples were below the EPA irrigation water restriction of 100 ppb. 2,4-D appeared to be very mobile with the concentrations in the untreated middle of the lake as high as in the treated areas.

Sampling Stations:  

Outside Treatment Areas - middle of Steel Lake

Inside Treatment Area South Shoreline - at 2200 S 308th St.

Insides Treatment Areas North Shoreline - at west of 2417 S. 304th Street 

Time Inside Treatment Area (South Shoreline) Inside Treatment Area (North Shoreline) Outside Treatment Area
Pre-Treatment No sample taken .86 ppb .84 ppb
1 Day after Treatment 118 ppb 68.4 ppb 126 ppb
3 Days after Treatment 88.3 ppb 87.3 ppb 88.6 ppb