2004 Liberty Lake Monitoring

Description of Site

Liberty Lake is a 708-acre (1.1 square mile) soft-water mesotrophic lake situated 2.5 miles east of the City of Spokane Valley and 3 miles west of the Idaho border. Liberty Lake has a mean depth of 23 feet (7 meters) and a maximum depth of 30 feet (9 meters). The lake and incorporated area of Liberty Lake are known for their real estate and recreational value; they are heavily utilized by 80,000 to 100,000 visits per season (Funk et. al., 1982). Recreational activities on the lake include fishing, swimming, boating, water-skiing, and jet skiing. A large percentage of the lake is privately owned with two public access points for recreation. These include a County Park beach (situated in the southeast) and a Washington State Department of Fish and Wildlife boat launch (situated in the north).

A small infestation of Eurasian watermilfoil (Myriophyllum spicatum) was discovered in Liberty Lake in the fall of 1995 and was effectively controlled by hand harvesting until 1997. A private diving company utilized divers to pull the plants by hand, collect, and dispose of them. They were successful in the spring and early summer of 1997, however by late summer, many clear days and hot weather, combined with a large increase in boat traffic, caused fragments of milfoil to be scattered around the lake reinfesting many areas. After 1997, application of granular 2,4-D then became the primary milfoil management method in Liberty Lake, although hand harvesting also continued. Since that time, 2,4-D treatments have occurred in sections of the lake in 1998, 1999, 2000, 2001, 2003, and 2004. Each spring, aquatic weed diving surveys are conducted by the Liberty Lake Sewer and Water District and a private diving company to locate milfoil populations and determine treatment areas. The areas with numerous milfoil plant are treated with 2,4-D and the areas with smaller number of milfoil plants are hand harvested (Liberty Lake Aquatic Weed Management Plan, 2003).

 

 

 

 

 

 

 

2004 Treatment

Liberty Lake was treated with granular 2,4-D (Navigateā„¢) on July 30, 2004. The herbicide was applied using a granular blower at a rate of 100 pounds per acre by a licensed applicator. A total of 5.1 acres were treated around the littoral zone of the lake in the areas with growing milfoil plants. The milfoil locations and treatment areas are seen in yellow on the map below.

Surface water samples for 2,4-D residue analysis were collected from inside and approximately 100 feet outside of the southeast treatment area (2.935-acres) by the Liberty Lake manager. The samples were surface grabs taken at pre-treatment, 24 hours, 3 days and 5 days after treatment.

All 2,4-D samples were stored on ice until delivery to the laboratory. All samples were analyzed by an Ecology- accredited laboratory using method number 8151A.

Efficacy: Liberty Lake was visually surveyed after treatment by the lake manager. Although the milfoil plants within the treated areas appeared to die, numerous healthy small milfoil plants were apparent in these areas several weeks after treatment. These plants may have colonized the area from fragments after the treatment took place, but the overall results were disappointing. No adverse impacts such as algae blooms or fish kills were observed after treatment. Secchi disk readings in Liberty Lake in 2004 were the best seen for twenty years, although the clear water enhanced the growth of macrophytes in the lake.

Summary of 2,4-D Residues from Liberty Lake

All 2,4-D samples were well below the EPA drinking water standard of 70 ppb at 1 day after treatment.

 

Treatment Site: Southeast end of lake (2.935 acres treated)

Time

Inside Treatment Area

Outside Treatment Area

Pre-Treatment

No Detection

No sample taken

1 Day after Treatment

43.7 ppb

15.4 ppb

3 Days after Treatment

4.8 ppb

22.2 ppb

5 Days after Treatment

10.4 ppb

7.4 ppb