2003 Spring Lake Glyphosate Monitoring - 2004 Efficacy Report

Description of the Site

Spring Lake, a 68-acre lake located 3.5 miles northwest of Maple Valley in unincorporated King County, has infestations of four noxious aquatic weeds: Eurasian watermilfoil (Myriophyllum spicatum), purple loosestrife (Lythrum salicaria), yellow iris (Iris pseudacorus), and fragrant water lily (Nymphaea odorata). This project aims to eradicate these four species initially, and establish a community-based program of long-term monitoring and control of these aquatic noxious weeds. Eradication of Eurasian watermilfoil is of primary importance, as it threatens to deteriorate lake conditions if left untreated.

Control methods included aquatic herbicide applications, manual removal methods, and installation of bottom barrier material near the boat launch. These activities will be performed according to the approved Spring Lake Integrated Aquatic Vegetation Management Plan (IAVMP).

Glyphosate Treatment

Spring Lake was treated with 67-ounces of AquaPRO® (1.5% solution) mixed with 44.8-ounces of LI 700® surfactant (1% solution) on August 13, 2003 by a state-licensed contractor. Noxious weeds were spot-sprayed on and along the margins of Spring Lake for a total treatment area of 3-acres. Targeted species included Lythrum salicaria, Iris pseudacorus, and Nymphaea odorata. The applicator used a combination of backpack and low pressure sprayer mounted on an airboat with a long hose.

Noxious weeds missed during the first treatment on August 13 were treated on August 26, 2003. The contractor spot-sprayed noxious weeds on the margins of Spring Lake totaling approximately 1-acre, using 34.4-ounces of AquaPRO® (1.5% solution) mixed with 23-ounces of LI 700® surfactant (1% solution). (See figure for treatment areas)

Glyphosate Monitoring

King County staff collected baseline, 1-hour post-treatment, and 24-hour post-treatment samples for each glyphosate application. On August 13, surface water samples were taken adjacent to a dense stand of Iris as well as adjacent to the largest patch of fragrant water lily treated. On August 26, samples were taken at Station 2 adjacent to the largest untreated patch of yellow iris. (See figure below for the treatment sites)

 

Results

All glyphosate samples were well below the EPA drinking water standard of 700 ppb.

Samples taken adjacent to a yellow flag iris treatment on August 13th

Time Laboratory Results
Pre-Treatment (August 13) No Detection
1 hour after treatment (August 13) 30 ppb
24 hours after treatment (August 14) No Detection

Samples taken adjacent to a yellow flag iris treatment on August 26 (largest patch untreated on August 13th)

Time Laboratory Results
Pre-Treatment No Detection
1 hour after treatment 120 ppb
24 hours after treatment No Detection

Samples taken adjacent to a fragrant water lily treatment on August 13th

Time Laboratory Results
Pre-Treatment No Detection
1 hour after treatment 310 ppb
24 hours after treatment No Detection

Efficacy

2003

In summary, the Spring Lake glyphosate treatment went according to plan and was effective in controlling the targeted weeds, with the expected quick degradation of the herbicide. Higher one hour concentrations after the fragrant lily treatment were expected since the herbicide had more opportunity to enter the water than for the yellow flag iris treatments. Additional glyphosate treatments will likely occur next year to eliminate returning weeds.

2004

Good efficacy was reported on fragrant water lilies and purple loosestrife using glyphosate. However, some follow-up treatments took place in 2004 to control "skips". Unfortunately the yellow iris was able to grow out of the glyphosate treatment.

Click here to see the sampling forms (word document)