Battle against invasive weeds not just in the water: Lawsuit threatens herbicide testing

SOUTH LAKE TAHOE, Calif. - Aquatic invasive plants (predominantly curly leaf pondweed and Eurasian watermilfoil) have taken over more than 90 percent of the 172-acre Tahoe Keys lagoon system, and, according to the 2015 UNR Implementation Plan, they are considered the most immediate threat to Lake Tahoe.

To combat the plants, the Tahoe Keys Property Owners Association (TKPOA) teamed up with local agencies to experiment with different ways to remove them, including herbicides. In 2022, a controversial Control Methods Test (CMT) began to poison invasive underwater weeds, something that divided people and environmental groups in Lake Tahoe.

The CMT used targeted herbicides in one area, UV-C light in another, and a combination of the two in a third area of the Tahoe Keys west lagoon and Lake Tallac. The CMT was approved by the Lahontan Water Quality Control Board (Lahontan) and the Tahoe Regional Planning Agency (TRPA) Governing Board, both certifying the Final Environmental Impact Statement, thus approving the Tahoe Keys Lagoons Aquatic Weed Control Methods Test Project.

The Sierra Club and the California Sportfishing Protection Alliance (CSPA) sued in June 2022 and have now claimed victory after a ruling in El Dorado County Superior Court on April 25, 2024.

El Dorado County Judge Gary Slossberg handed down a ruling, issuing a writ of mandate commanding Lahontan to set aside its approval of the project. The Court found Lahontan failed to approve a Final EIR (Environmental Impact Report) without an analysis of the "reasonably foreseeable consequences of repeated pesticide use."

"From the beginning of the Control Methods Test project and throughout the project’s environmental analysis and final permit processes, the Sierra Club Tahoe Area Group and CSPA commented that allowing the discharge of herbicides before non-chemical methods were fully analyzed and demonstrated not to work would violate the Board’s own regulations in their Basin Plan," said the Sierra Club after the ruling.

“We are very pleased with the Court’s ruling requiring the Board to follow its own rules,” said Tobi Tyler, a volunteer with the Sierra Club’s Tahoe Area Group. “The Board violated the public trust and circumvented Basin Plan regulations that were diligently established after a great deal of public input and scrutiny.”

Lahontan did not respond to a request for comment, though TRPA and TKPOA did comment.

"We think it is incorrect and unjust," said TKPOA board member Pete Wolcott of the judge's ruling. "There were considerable items that didn't get presented to the court by Lahontan and others."

Wolcott said they are starting the second summer of post-application testing and do not include any further herbicides or light testing, just checking results in the treated channels.

"The train is already one-half mile out of the station," said Wolcott. "We've been working together for this biological threat, and that is the heart of our disappointment."

Wolcott said TKPOA will be meeting with their environmental partners to get their next steps planned.

"This summer is the third and final year of the test that will provide data on a variety of weed control options to knock back the largest infestation of invasive weeds in Lake Tahoe. No further chemical treatments have been permitted," said Jeff Cowen of TRPA. "Aquatic invasive weeds pose a serious threat to Lake Tahoe’s water clarity and native species, and the lagoons in the Tahoe Keys have been identified as the highest priority for treatment. The infestation covers more than 150 acres and is considered a source of aquatic invasive weeds in other areas of the lake."

Cowen said the goal of the test was to see if it is possible to maintain weed control for the duration of the three-year test without herbicide after the major knockback.

"Ongoing use of herbicide was not considered and is not part of the test," said Cowen. "If a combination of methods that includes EPA-certified herbicide is recommended in the future, a new environmental analysis and public review process would be required."