Herbicide testing possible in Lake Tahoe by 2018 | SierraSun.com

Herbicide testing possible in Lake Tahoe by 2018

Claire Cudahy
ccudahy@tahoedailytribune.com

SOUTH LAKE TAHOE, Calif. — The Tahoe Keys Property Owners Association recently announced it is seeking a permit to test herbicides in 2018 as a way to combat aquatic invasive plants — an ongoing (and longtime) issue in the 172-acre lagoon system.

Aquatic invasive plants, primarily curly leaf pondweed and Eurasian watermilfoil, have taken over more than 90 percent of the Tahoe Keys and present an immediate threat to Lake Tahoe.

"These plants threaten the lake's ecosystems, the water's clarity, and our recreation and economy," said Dr. Lars Anderson, UC Davis aquatic plant expert. "In spite of ongoing efforts, they continue to grow in the Keys, and with the Tahoe Environmental Research Center's State of the Lake Report showing record-breaking increases in lake temperatures, the threat to Lake Tahoe is greater than ever."

Between 13,400 and 18,600 cubic yards of weeds have been removed annually since 2011. The plants have also become home to non-native warm-water aquatic species including goldfish, catfish and bullfrogs.

Stakeholders have been combating the issue for more than 25 years, using a variety of plant-fighting methods, including harvesting and fragment collection, dredging, bottom barrier mats, rotovating, dewatering, nutrient reduction and other biological controls.

Results from these efforts have been mixed — and costly.

Recommended Stories For You

TKPOA's harvesting attempts amount to roughly $400,000 a year, while the harvesting process itself also generates 4,000 plant fragments per acre harvested, which could go on to regrow elsewhere if not properly collected.

A decision by the U.S. Environmental Protection Agency last September determined that some herbicides may be approved for use on a case-by-case basis in Lake Tahoe, paving the way for a new method of invasive species control in the Keys.

"This is one evaluation of one method the association is considering in our comprehensive plan to gain control over the invasive plants," said John Larson, chair of the association's Water Quality Committee.

If approved, the association would apply low levels of three herbicides — Endothall, Triclopyr and Penoxsulam — at nine test sites in 2018. The test sites would cover about 13 acres, or 8 percent, of the Keys and would be in dead-end lagoons far from the lake.

According to TKPOA, the test sites would have multiple surface-to-bottom barriers to ensure the herbicides, which are considered nontoxic to humans, fish and wildlife, would not reach the lake.

Additionally, they would be diluted to between 0.02 and 2 parts per million, or about half the maximum concentrations allowed by the EPA.

"We understand that people have strong feelings about the potential use of herbicides in the Keys," Larson said. "We also understand, however, that to really address the aquatic invasive plant issue not only in the Keys lagoons but for all of Lake Tahoe, we must be willing, as a community, to try a variety of state-of-the art tools to see what combination of options are best for moving forward."

TKPOA pointed to dye studies conducted in 2011 and 2016, which demonstrated that water movement would not carry the substances into Lake Tahoe during the test period.

The herbicides also break down by light, microbial action and other processes within a few days to two weeks.

Dr. Sudeep Chandra, a professor at University of Nevada, Reno, who specializes in aquatic ecosystems and has studied the Keys extensively, agrees it is necessary to use all available tools to combat the issue of invasive species.

Chandra, who coauthored an implementation plan for control of aquatic species in Lake Tahoe in 2015 with Dr. Marion Wittmann, said it's a complicated decision with emotional ties.

"It's a complex subject, including complex feelings on my emotional side, my non-science side. We've watched the Tahoe Keys build a population of invasive species and plants. We know there is a problem and the Keys is at the heart of it," said Chandra. "On the science side, we need to have as many tools to combat this as possible."

TRPA spokesman Tom Lotshaw said that as a region, the best options for managing invasive species in Lake Tahoe are still being investigated, and may vary by species and from site to site around the lake.

The organization, he said, will thoroughly assess the environmental impacts of testing these herbicides in the Tahoe Keys.

"Even a limited pilot project to test aquatic herbicides as a way to kill invasive weeds in waters adjoining Lake Tahoe would be a first for our region," said Lotshaw.

Other lakes that have used these herbicides include Discovery Bay south of the California Delta, Big Bear Lake in Southern California, Clear Lake in Northern California, Loomis Lake in Washington, and areas within the Sacramento-San Joaquin Delta.

The Tahoe Keys was created in the late 1950s by dredging an estimated 5 million cubic yards of material from the marsh at the mouth of the Upper Truckee River. The effort destroyed much of the river's marsh, a major filtration system from Lake Tahoe's largest tributary.

Save the date

On Nov. 1 a public workshop will be held from 6 to 8 p.m. at the South Tahoe Public Utility Building (1275 Meadow Crest Drive) to educate the public on the efforts made to combat aquatic invasive species in the Keys.