Herbicide use will not be part of Marin Municipal Water District’s plan to prevent wildfires and curb invasive species on its lands, the district’s board of directors decided last Tuesday. 

Herbicides had been included as a potentially major component of the district’s draft Wildfire Protection and Habitat Improvement Plan, which outlines a 15-year strategy for fortifying firebreaks and controlling weeds on 22,000 acres on Mount Tamalpais and around Nicasio and Soulajule Reservoirs.

“There has been a lot of public concern expressed, in particular because we provide drinking water,” said district spokeswoman Libby Pischel. “It’s something that people have a lot of concerns about.”

Though the district’s plan found wildfire prevention to be more effective and cost-saving with the use of herbicides, board members relented to public pressure and a series of studies on herbicides’ environmental and health risks.

The district will manage invasives exclusively by mechanical and hand removal, prescribed burning and mowing. The approach will cost the district an estimated $5.6 million per year, much higher than the $1.6 million approach that combined mechanical removal and herbicide use. 

(The plan addresses approximately 18,900 acres on Mount Tam, 1,600 acres around Nicasio Reservoir and 1,100 at Soulajule.)

The water district predicts the new approach will allow the growth of French, Scotch and Spanish broom to increase by about 300 acres over the 15 years; the herbicide-included approach would have decreased broom growth by about 165 acres, the district said. 

“I think what the water district is going to be stuck with right now is containing, not combating,” said Nona Dennis, a vice president of the Marin Conservation League. “In the absence of herbicides, what they’ll have to do now is contain what they can. But they’ll have no chance now…to go after those large concentrations.”

Invasive broom has infested over 1,200 acres on Mount Tam and spread at a rate of 30 acres per year, the draft plan states. Broom, which makes up the vast majority of the watershed’s invasive weeds, has infested almost half the district’s firebreak areas, often in large stands that can reach as high as 10 feet, weakening the ability of a network of breaks to halt wildfire spread.

The district’s decision—which maintains a ban on herbicides that has been in place since 2005—follows the Point Reyes National Seashore’s decision to use herbicides to combat invasive species based on a finding of no significant environmental impact. Despite sharp criticism, last week the seashore reiterated its intention to fight European beachgrass and iceplant on 600 acres of dunes with a combination of mechanical removal and the application of glyphosate and another herbicide.

Glyphosate is the key component in one of six herbicides—a product called AquaMaster—for which the water district conducted intensive risk-assessment studies. The district had been considering using a mixture of AquaMaster and Competitor, a surfactant designed to increase the effects of the herbicide, as a means to curtail broom.

A 2011 district report noted that glyphosate had the potential to wash away from application areas, especially during winter storms, and enter water bodies that provide drinking water. Glyphosate also risked contaminating groundwater, the report stated. An experiment conducted by the district, described in that report, showed no evidence of contaminated runoff but did show localized soil contamination, with the highest concentrations in the top soil. (A new study by the World Health Organization’s International Agency for Research on Cancer also concluded that glyphosate is “probably carcinogenic to humans.”)

So far, the district has not decided whether it will move forward with plans to use certain organic herbicides for weed treatment, an option that was included in the mechanical-only approach. Though less controversial, organic herbicides are also less effective than their conventional counterparts and are highly caustic, posing immediate burn and inhalation problems for workers applying them, the district said.

“Organic herbicides….were not sizing up to offer much utility,” said Janet Klein, the district’s natural resources program manager. “They’re not being 100 percent excluded from the plan, but right now I can’t see any specific product that would be used.”

With conventional herbicides off the table, the district will reinitiate the process to draft an environmental impact report. That report will require the issuance of a new project description and scoping phase, as well as a public comment period.

Three-fourths of the drinking water Marin Municipal provides to its 186,000 customers in the county is stored in West Marin’s Nicasio and Soulajule Reservoirs and in reservoirs on Mount Tamalpais. 

 

This article was amended on July 17 to clarify findings in the district’s 2011 report on herbicides.