Did an experimental project leak herbicides into Lake Tahoe? We asked scientists

Amy Alonzo
Reno Gazette Journal

Did herbicides work their way into Lake Tahoe in the first few months of a pilot program to kill aquatic invasive species in the Tahoe Keys?

No, according to scientists involved with the project.  

“Nothing has reached close to the West Channel or the lake,” according to Lars Anderson, a retired United States Department of Agriculture aquatic plant and invasive species biologist monitoring the project, “so that looks good.”  

In May, herbicides were applied to the manmade Tahoe Keys, an area home to the largest infestation of aquatic invasive species in the Tahoe Basin.

The Tahoe Keys is a roughly 372-acre development with more than 1,500 houses and townhouses on Tahoe’s southwest shore. It was built in the 1960s by dredging the Truckee Marsh and reshaping the area into buildable, waterfront land. The development’s extensive waterways are connected to Lake Tahoe by two channels, the East Channel and the West Channel.

In January, two regulatory agencies — the Tahoe Regional Planning Agency Governing Board and the Lahontan Regional Water Quality Control Board — approved the one-time use of aquatic herbicides in the Keys to treat an infestation of aquatic invasive species. Before the vote, the use of aquatic herbicides was prohibited.

Herbicides were applied to roughly 17 acres of the 172-acre waterway.

Why were herbicides needed? 

The goal, according to proponents, is to reduce rapidly-spreading curlyleaf pondweed, Eurasian watermilfoil and coontail in the waterway by 75 percent over three years.

The infestation is so bad in the Keys that agencies aren't attempting to completely eradicate the plants, according to Jesse Patterson, a former marine biologist who now works as the chief strategy officer for the League to Save Lake Tahoe. Once a substantial reduction is reached, treatment methods including UV light and aeration can be used to keep the plant populations from spiraling out of control again.

The invasive aquatic plants, also found in parts of Lake Tahoe, can devastate aquatic ecosystems and negatively impact recreation opportunities on the lake, according to the Tahoe Regional Planning Agency, the agency tasked with monitoring the project. 

Far from the West Channel, two test areas – A and B – were selected for the pilot program. Turbidity curtains — impermeable barriers used to trap sediment in water — were installed to prevent the herbicides from spreading beyond the test areas.

What amount of herbicides remain in the water? 

The Tahoe Keys are infested with aquatic invasive species.

More than three months after the herbicides were applied, they are no longer detectable in test area B, according to Anderson. The herbicides degraded by more than 99 percent within the first four to six weeks, reaching levels of just five to six parts per billion. Now, those levels have dropped down to reach one part per billion.

Just how small is that?

“To grasp the scale of one part per billion, it is equivalent to 1 inch in 16,000 miles; or three seconds in a century; or a penny in $10 million; or about an inch and a half compared to the circumference of the earth,” Anderson said.

In test area A, herbicides are expected to reach non-detectable levels by late September.

Although the project spans three years, it involved just a one-time application of herbicides. In the spring, scientists will be able to see the long-term effects of the treatment, Anderson said.

“I think it looks promising, but it’s too soon to tell,” he said. “We’re really interested to see what happens next spring.”

What's next? 

Opponents to the project including the Tahoe Area Sierra Club, Friends of the West Shore and the California Sportfishing Protection Alliance previously argued that herbicides aren’t a viable long-term solution and will foul the lake.

The Sierra Club did not return an email to the Reno Gazette Journal for this story.

A preliminary report on the project will be compiled by the Tahoe Regional Planning Agency and the Tahoe Keys Property Owners Association later this fall.

An official report is due to the Lahontan Water Quality Control Board on March 1

Amy Alonzo covers the outdoors, recreation and environment for Nevada and Lake Tahoe. Reach her at Here's how you can support ongoing coverage and local journalism