2003 Lake Washington (Hunts Point) Monitoring

Description of the Site

Eurasian watermilfoil (Myriophyllum spicatum) has been infesting Lake Washington since the 1970s. The shallower embayments of this very large lake, such as Cozy Cove and Fairweather Bay, have been severely impacted by dense beds of milfoil. The primary goal of this project was to use the selective aquatic herbicide 2,4-D to kill milfoil while allowing native aquatic communities to recover and recolonize milfoil-dominated sites in the lake waters around Hunts Point. The town of Yarrow Point (adjacent community) discourages herbicide use, so the applicator was careful to apply herbicide to Lake Washington only within the jurisdiction of the town of Hunts Point.

2,4-D Treatment

The littoral area of Lake Washington around Hunts Point (Fairweather Bay and Cozy Cove) was treated with two formulations of 2,4-D on July 16, 2003. The liquid formulation (DMA*4IVM) of 2,4-D was used to treat approximately 50 acres along the inshore areas of Cozy Cove and Fairweather Bay where it was unlikely to be dispersed by currents and drift (illustrated by the blue circles on the map below). The granular formulation of 2,4-D (AquaKleen) was used to treat the deeper, more exposed areas of these bays where the herbicide could potentially drift off target (27 acres total). The AquaKleen treatment area is denoted by the red x's below. The applicator indicated that the actual treatment area for AquaKleen is much narrower than is shown on the map.

Map of the treatment areas and sampling sitesApproximately 100 pounds per acre of AquaKleen and about 1.5 gallons per acre foot of DMA*IVM was applied. The estimated water concentration of 2,4-D for the AquaKleen application at these rates was 1 mg/l and the estimated 2,4-D water concentration of DMA*IVM was 1.2 mg/l.

Water samples for 2,4-D analysis were taken by an environmental consultant. Lake water samples were collected pre-, and 1, 3, and 5 days post-treatment. Water was taken from mid-depth at each of five sampling sites per embayment. Water from the 5 sites was composited to create a single 2,4-D water sample from Cozy Cove (CC-01-CC05) and two duplicate composited samples from Fairweather Bay (FB-01-FB-05). The consultant sampled twice at each site in Fairweather Bay to create two composited duplicate samples. The blue squares on the map represent the sampling stations for the composite water samples, plant biomass data, and the water quality data. In Fairweather Bay two of the sampling locations were outside of the treated area (FB-04 and FB-05). In Cozy Cove three of the sampling locations were outside of the treated areas (CC-01, CC-02, and CC-03).

Discrete samples (not composited) were also taken outside of the AquaKleen treated area (FB-OTL) and immediately adjacent to the AquaKleen treated zone (FB-OTG) 1, 3, and 5 days after treatment. These sampling locations are represented on the map by yellow squares. All 2,4-D samples were analyzed using an Ecology-accredited laboratory for 2,4-D using EPA method 8151A. The map is from Jones and Stokes. Application information from AquaTechnex.

Summary of 2,4-D  Residues

Location

Pre-treatment

1-day

3-days

5-days

Cozy Cove

Not Analyzed

88.5 ppb

83.9 ppb

38.6 ppb

Fairweather Bay

Not Detected

1,570 ppb

693 ppb

81.2 ppb

Fairweather Bay

Not Detected

1,060 ppb

960 ppb

716 ppb*

FB-OTG (outside of treatment area)

Not Analyzed

384 ppb

266 ppb

Not analyzed

FB-OTL (outside, but  adjacent to the treatment area)

Not Analyzed

58.6 ppb

49.9 ppb

94 ppb

* Not sure why there is such variation between these duplicate samples. The laboratory reported no analysis problems.

Judging from the treatment map supplied by the applicator, a larger percentage of Fairweather Bay was treated with herbicides than was Cozy Cove. Also three of the five composited samples were taken outside of the treatment area in Cozy Cove. This likely accounts for the lower 2,4-D concentrations in Cozy Cove.

This treatment was a combination of liquid and granular formulations, with nearly double the acreage being treated with the liquid formulation (50 acres versus 27 acres for the granular). Generally the granular formulations have given much lower water concentrations in Washington lakes after treatment. In Spring Lake where the liquid formulation was used, 2,4-D concentrations remained higher and were more persistent than in lakes where the granular formulation was used. It is possible that the higher concentrations seen at Hunts Point are due to the use of the liquid formulation. Also the liquid formulation was used in the more sheltered areas where it would be less likely to be diluted with untreated waters.

Efficacy of Treatment

Two types of diver surveys were performed by an environmental consulting firm to determine the efficacy of milfoil removal. Biomass sampling at pre-selected sites within the two coves was conducted pre- and 30 days post treatment. Biomass was collected by deploying 0.25 m2 quadrats and cutting the aquatic vegetation from within the quadrats. Plants were then separated to species, water was shaken off, the plants were air dried, and weighed. The sample locations for the biomass samples were the same locations where the composite water samples were taken. The plant species observed included: Myriophyllum spicatum, Elodea canadensis, Egeria densa (also a state-listed noxious weed), Chara sp., Nitella sp., Vallisneria americana, two species of thin leaved Potamogeton (not identified to species), Potomogeton crispus (non-native), Potomogeton praelongus, Najas sp., and Ceratophyllum demersum.

Some of the biomass data is summarized in the table below. The red lettering indicates sample locations within the treated area. The black lettering indicates sample locations outside of the treated area.

Sample Number

Depth (feet)

Total Plant Pre-biomass gm2

Total Plant Post-Biomass gm2

Pre-Milfoil  Biomass gm2

Post-Milfoil biomass gm2

Pre- Percent milfoil of total

Post-Percent Milfoil of Total

% Milfoil control 30 days post

FB01

9.7

4648

420

1144

0

24.6

0

100

FB02

10.6

204

1484

68

0

33.3

0

100

FB03

10.6

92

1236

36

24

37.5

1.8

66.7

FB04

8.8

1032

1620

36

56

3.3

3.5

No

FB05

13.8

544

656

520

532

95.8

81.0

No

CC01

6.0

1816

1928

544

0

30

0

100

CC02

7.1

704

1076

692

680

98.4

63.2

1.7

CC03

12.1

1620

2776

1620

2540

100

91

No

CC04

11.2

1112

656

1088

12

98

1.7

99

CC05

8.5

1260

1068

1244

0

99.1

0

100


Divers also conducted pre and 30 days post treatment visual dive transects to observe percent plant cover and distribution of milfoil in key areas. The red lines on the treatment map indicate the areas where the diver transects occurred.

Diver transect observations made both before and 30 days after herbicide application generally corroborated the biomass data where significant quantities of actively growing milfoil were only observed in areas that had not been treated with 2,4-D. Limited areas of growing milfoil were observed in areas of application in particular on the western portion of Cozy Cove.

The 2,4-D treatment appeared to effectively remove milfoil biomass from the sampling sites within the treatment areas. Areas outside of the herbicide application generally did not show effective removal of the milfoil. Unfortunately no biomass samples were taken in the more open treated waters along the Point. Even though 2,4-D concentrations were substantially lower in Cozy Cove than in Fairweather Bay, both areas seemed to be effectively treated.

2,4-D was selective in removing milfoil biomass. The total plant biomass was reduced in some treated sites, but increased at other treated sites. At all sampling sites at 30 days post treatment, healthy non-milfoil plant biomass remained as habitat and cover for fish and wildlife.

Water Quality Data

Water column profiles measuring dissolved oxygen and temperature were conducted before (July 11) and 30 days after herbicide treatment (August 15) using a hydrolab. Measurements were taken by an environmental consultant at each of three depths (just below the surface, mid-depth, and 1 foot off the bottom) per sampling site, although the measurements for temperature were only taken at the bottom in July. The sampling stations were the same as used for 2,4-D water collection and biomass. In addition, two “control” stations were selected in Yarrow Bay (no herbicide treatment) consisting of a dense milfoil site (YB1) and an open water site (YB2). These sampling locations are not shown on the map, but were located in Yarrow Bay to the east of Cozy Cove and well outside of the 2,4-D treatment area.

Measurements were made 2-3 times throughout the day beginning before sunrise and extending to the afternoon. Data is shown below for dissolved oxygen and temperature from stations FB-01, CC-05 and the control sites in Yarrow Bay. FB-01 is located within the treatment area in Fairweather Bay. CC-05 is located within the treatment area in Cozy Cove. At both sites, milfoil biomass was substantial prior to treatment and milfoil was 100 percent controlled after treatment.

Dissolved Oxygen

July - Pre-Treatment

August - Post-Treatment

 

FB01

CC05

YB1

YB2

FB01

CC05

YB1

YB2

Morning

mg/l

mg/l mg/l mg/l mg/l mg/l mg/l mg/l

Surface

9.6

9.0

10.35

9.4

7.39

6.82

7.44

8.23

Mid

9.9

9.6

10.05

9.2

7.32

6.8

7.29

7.83

Bottom

9.6

10.3

9.5

9.5

6.42

6.74

7.16

8.2

Noon

 

 

 

 

 

 

 

 

Surface

9.3

8.9

10.35

9.4

-

-

-

-

Mid

10.3

9.2

10.05

9.05

-

-

-

-

Bottom

10.2

9.8

9.5

9.1

-

-

-

-

Afternoon

 

 

 

 

 

 

 

 

Surface

10.7

8.8

9.8

8.9

8.48

7.58

8.26

8.58

Mid

11.3

9.9

10.5

9.1

8.19

7.8

8.32

8.59

Bottom

13

9.6

10.7

10.2

6.48

7.87

7.97

7.67

 

Temperature

July - Pre-Treatment

August - Post-Treatment

 

FB01

CC05

YB1

YB2

FB01

CC05

YB1

YB2

Morning

0 C

0 C

0 C

0 C

0 C

0 C

0 C

0 C

Surface

-

-

-

-

23.57

23.5

23.28

23.04

Mid

-

-

-

-

23.57

23.51

23.24

22.91

Bottom

22

22.8

22

22

23.56

23.51

23.23

22.83

Noon

 

 

 

 

 

 

 

 

Surface

-

-

-

-

-

-

-

-

Mid

-

-

-

-

-

-

-

-

Bottom

22

21.6

22

22

-

-

-

-

Afternoon

 

 

 

 

 

 

 

 

Surface

-

-

-

-

24.3

24.03

23.55

23.26

Mid

-

-

--

 

23.87

23.8

23.52

22.44

Bottom

23

22

22

22

23.55

23.56

23.47

22.32

In general, DO concentrations were lower in Cozy Cove than in Fairweather Bay. Cozy Cove is more sheltered from the open expanse of waters of Lake Washington than Fairweather Bay and this may account for the overall lower oxygen levels in the Cove. Dissolved oxygen levels decreased at all sites between July and August, but this decrease did not appear to be related to 2,4-D treatment since the non-treated control sites DO concentrations also declined. The decrease is likely related to the higher water temperatures seen in August. These data did not show the DO sag that has been reported to occur under dense milfoil beds in other lakes and in Lake Washington. They also do not show an increase in temperature at the water’s surface that is sometimes seen in dense plant beds.

Washington has set a Water Quality Standard for a minimum DO concentration for surface water of 9.5 mg/L for salmon core areas including Lake Washington. Dissolved oxygen in August frequently dropped below this standard. However, salmon can survive in 5 mg/L DO. Temperatures varied little in the shallow sampling areas, but always exceeded the Water Quality Standard for salmon core rearing areas (16 degrees C).

Conclusions

The 2,4-D treatment appeared to effectively remove milfoil biomass from the sampling sites within the treatment areas. Areas outside of the herbicide application generally did not show effective removal of milfoil. Unfortunately no biomass samples were taken in the more open treated waters along the Point. Even though 2,4-D concentrations were substantially lower in Cozy Cove than in Fairweather Bay, both areas seemed to be effectively treated. Unfortunately compositing treated and untreated waters for 2,4-D analysis complicates data interpretation for persistence and off site movement.

2,4-D was selective in removing milfoil biomass. The total plant biomass was reduced in some treated sites, but increased at other treated sites. At all sampling sites at 30 days post treatment, healthy non-milfoil plant biomass remained as habitat and cover for fish and wildlife.

Temperature and dissolved oxygen did not appear to be affected by the herbicide treatment or milfoil presence or absence. Temperature exceeded the Water Quality Standard for core salmon rearing areas pre- and post-treatment.

Click Here to Link to the Consultant Report (pdf file)