2005 North Lake Monitoring

Description of Site

North Lake lies along the eastern border of Federal Way in the upper White River watershed in King County Washington. Before the project started, the 55-acre lake was moderately infested with Eurasian watermilfoil (Myriophyllum spicatum). Three other noxious weed species with expanding infestations at North Lake also threatened to degrade the ecological and recreational benefits of the system. Fragrant waterlily (Nymphaea odorata), purple loosestrife (Lythrum salicaria), and yellow flag iris (Iris pseudacorus) are well established around the shoreline and in the lake. The City along with the North Lake Community intends to eradicate these state-listed noxious weeds. Control methods include aquatic herbicide applications, manual removal methods, and possible installation of bottom barrier. These activities will be performed according to the approved North Lake Integrated Aquatic Vegetation Management Plan (IAVMP).

Herbicide Treatments

Federal Way personnel collected samples from North Lake to determine both glyphosate and 2,4-D concentrations before and after treatment in 2005. The sampling procedure was undertaken to determine lake concentrations of herbicides, and to provide an analytical measurement of the contractor’s performance. Background samples (before treatment) and post treatment samples were collected at time intervals. Samples were taken in the middle of the lake (outside the treatment areas), and inside the two individual treatment sites: one for fragrant water lily (glyphosate), and one for milfoil (2,4-D).

All samples were collected using a Wildco Alpha 2.2 liter Van Dorn style water bottle. See the figures below  for sample locations. The samples were retrieved from various depths, and combined into individual composite samples. Samples were chilled, and delivered the same day to Severn Trent Laboratories, Inc. (STL Seattle). Sampling times were adjusted to accommodate weekends and staff schedules. 2,4-D samples were analyzed by STL (DOE accredited laboratory #C1226) using USEPA Chromatography Method 8151 GC/MS Modified. Glyphosate samples were submitted to STL, and subcontracted to Coffey Labs in Portland through Edge Analytical in Burlington. Coffey Labs (DOE accredited laboratory #C1264) used USEPA Chromatography Method 547. 

2,4-D Monitoring Results

Eurasian watermilfoil was treated with Dow DMA 4, IVM for a target concentration of 2 ppm. The application was made using a pontoon boat mounted with a 50 gallon spray tank that fed a number of weighted drop hoses. Ten acres of lake infested with Eurasian watermilfoil was treated.

Date Pre/Post Inside/Outside Treatment Zone Location Concentration
7/21/05 Pre Inside In littoral zone, approximately 300’ south of public boat launch Non Detect
7/21/05 Pre Outside In middle of lake, approximately 200’ from outer edge of expected treatment plot, directly west of 3610 S. 334th St. Non Detect
8/3/05 Post - 24 hr Inside In littoral zone, approximately 300’ south of public boat launch 330 ppb
8/8/05 Post - 6 days Inside In littoral zone, approximately 300’ south of public boat launch 188 ppb

Glyphosate Monitoring Results

White Water Lily control was performed by a licensed applicator using boat mounted low pressure spray equipment. Rodeo or AquaPRO (active ingredient glyphosate) Aquatic herbicide and LI 700 surfactant were mixed in the spray tank to achieve a 1.5 percent solution. This spray solution was applied uniformly over the lily pads within the designated treatment area. This process was repeated to touch up any areas that did not uptake enough herbicide because of weather or plant wash off. Approximately one acre of the noxious aquatic weed White Water Lily were treated using glyphosate herbicide throughout the lake. On subsequent trips, smaller areas were spot sprayed if they looked like wash off had occurred during the first treatment.

Date Pre/Post Inside/Outside Treatment Zone Location Concentration
7/21/05 Pre Inside In littoral zone, approximately 700’ south of public boat launch Non Detect
7/21/05 Pre Outside In littoral zone, approximately 700’ south of public boat launch, immediately adjacent to expected treatment plot Non Detect
8/2/05 Post - 1 hr Inside In littoral zone, approximately 700’ south of public boat launch 19  ppb
8/3/95 Post - 21 hr Inside In littoral zone, approximately 700 south of public boat launch Non Detect

Efficacy of Eurasian Watermilfoil Treatment

Although colonies of Eurasian watermilfoil  were distributed throughout the littoral area in the northern and central part of North Lake in 2005, no milfoil  was detected during the 2006 initial systematic survey.