First turbidity curtains being removed after successful Tahoe Keys test results | South Lake Tahoe -

2022-09-03 00:06:25 By : Ms. LEO LI

Turbidity curtain at site 15 before being removed. Photo provided.

SOUTH LAKE TAHOE, Calif. - Herbicide levels are now at non-detect levels in Area B of the Tahoe Keys, allowing the turbidity curtains to be lifted and those waterways to be used for the first time since the Control Methods Test (CMT) started May 25.

The three-year-long Tahoe Keys Property Owners Association (TKPOA) project is to test innovative methods to control the largest infestation of aquatic invasive weeds in the Tahoe Basin. One area was treated with ultra-violet light, another with EPA-approved herbicides, and the third a combination of the two. During the test period, herbicides are only applied once and the effect on invasive species will be monitored for the next two years.

“This is the first step in the effective implementation of the Control Methods Test and a major milestone in the battle against aquatic invasive weeds which threaten Lake Tahoe’s clarity,” said Dr. Lars Anderson, an aquatic ecology/invasive species specialist working on the project. “The results certainly portend major progress as the two applied herbicides acted specifically and effectively on the target invasive weeds: Eurasian watermilfoil, curlyleaf pondweed and coontail, while ensuring safety guidelines for people, pets and wildlife.”

The turbidity curtains have been in place since May to restrict herbicide movement. Originally planned to be removed in mid-July, the levels now meet the stringent guidelines of the permit. Forecasts in Area A of the Keys suggest levels will reach non-detect status by mid-to-late September, with curtain removal there soon after.

The three-year field test project was preceded by a multi-year collaborative public planning process and extensive environmental review by the Tahoe Regional Planning Agency and Lahontan Regional Water Quality Control Board.

As TKPOA began its CMT the League to Save Lake Tahoe initiated the laminar flow aeration program, a process to aerate the lagoon bottom sediments with small bubbles to reduce plant growth in designated test areas. Tahoe Regional Planning Agency simultaneously launched independent rigorous monitoring for the project to collect data on the efficacy of treatments, water quality, and overall data on how the natural environment responds to the various treatments.

“TKPOA and the Control Methods Test team are committed to following the permit conditions and stringent guidelines of the thorough and intensely monitored program,” said Anderson.

The herbicides levels degraded by over 99 percent within 4-6 weeks to about 5-6 parts per billion (ppb), with the last half-percent to reach 1 ppb now realized.

“It’s also important to note that the CMT permit-driven herbicide threshold to allow curtain removal is 400 times lower than EPA-stated safe ‘receiving water limits’ established for other lake systems throughout the U.S.," Anderson added. “To grasp the ‘scale’ of 1 part per billion, it is equivalent to 1 inch in 16,000 miles; or 3 seconds in a century; or a penny in $10 million; or about an inch and half compared to the circumference of the earth. The data that is being collected will allow us to determine long-term solutions for not only the Tahoe Keys lagoons but Lake Tahoe. We certainly appreciate the understanding and patience of the homeowners and public.”

“Preliminary hydroacoustic scans and imagery of the weeds are encouraging,” said Pete Wolcott, chair of the TKPOA Water Quality Committee. “We have tens of thousands of data points this year alone, and we will be looking closely at this data to assess the first-year results of the test.”

A status report on initial testing efforts will be released this fall with the preliminary results at No additional herbicide application or boating restrictions will occur with the project, but extensive monitoring will continue alongside non-chemical methods for the next two years. The results of the three-year test will inform long-term management plans for the Tahoe Keys and for addressing aquatic invasive weeds in Lake Tahoe.

There has been unusually extensive algae growth inside test area A that includes an unpleasant odor and widespread discoloration. These conditions are due to decomposing invasive weds, as well as prolonged isolation and lack of circulation. Conditions have improved somewhat over the last week and should continue a return to normal as air and water temperatures begin to drop.

In an unrelated circumstance, algae and harmful algal blooms (HABs) have been noted this summer throughout California’s aquatic ecosystems, including beaches around Lake Tahoe and in the Tahoe Keys due in part to the low lake levels. While no area at the Lake is currently considered in a danger zone, caution is suggested. Inside the Keys during the test period, algae growth continues to be extensively monitored, and to date, “warning level” HABs have only been observed outside the Tahoe Keys CMT Project test areas.

Learn more about the CMT project at

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