The thought of deliberately applying herbicide to drinking water may sound repugnant, but according to officials at the Mobile Area Water and Sewer Service (MAWSS) and elsewhere, it can be a safe and effective means to battle a problem that, left unchecked, can pose a much greater threat to water quality.
Earlier this summer, MAWSS announced it was closing Big Creek Lake to boaters after the utility discovered the 3,600-acre lake — the primary drinking water reservoir for MAWSS customers in Mobile, Prichard and Spanish Fort — was contaminated with giant salvinia, an invasive species of floating plant native to South America. MAWSS is unique among local water providers for having a surface reservoir as its primary supply rather than wells, but it comes with unique challenges.
During a virtual meeting Sept. 13, the MAWSS Board of Water and Sewer Commissioners voted to approve an emergency purchase of containment booms and aquatic herbicide treatments to combat the problem, but it also raised questions about possible contamination of the water supply.
After initially denying a request for details about its plan to battle the giant salvinia in Big Creek Lake last week, MAWSS reversed course Monday and told Lagniappe it is in fact applying three chemical products to affected portions of the lake in roughly 100-acre increments for an undetermined amount of time. But MAWSS stressed the treatment has “all been approved by [the Alabama Department of Environmental Management] for use in water sources and we would not use [them] if there was any thought that it was not completely safe.”
Specifically, MAWSS is applying two herbicides — penoxsulam and flumioxazin — and a surfactant, methylated seed oil (MSO), to the giant salvinia. MAWSS said it is not currently applying any other herbicide or pesticide to the reservoir for any other reason.
P.J. Waters, associate extension professor at Auburn University’s School of Fisheries, Aquaculture and Aquatic Sciences, said giant salvinia (salvinia molesta) spreads quickly and easily, and has become a problematic invasive species in all Gulf states.
“It’s pretty easy to recognize. It can be pretty aggressive at spreading and we’re seeing more of it all the time,” he said. “In terms of management, there are herbicides that have varying degrees of success.”
Waters said some herbicides are specifically designed for aquatic use, even in potable water, and labeling provides such guidance as efficacy, toxicity and usage recommendations.
“There are definitely active ingredients formulated for use in water,” he said. “[Herbicides] typically get labeled as excellent, good, fair or poor, depending on how effective the specific active ingredient is against a particular plant … and all herbicides are labeled with withdrawal periods. So whether you are using the water for irrigation, fishing, swimming, etc., it’ll tell you how long you have to wait after applying to resume that type of activity.”
Penoxsulam, marketed as Galleon, carries labeling that warns the chemical may pose a health hazard if inhaled. But the accompanying Environmental Protection Agency (EPA) pesticide fact sheet indicates “there are no risks of concern from the use of penoxsulam” and “the risk due to exposure to residues in food and water was calculated below the agency‘s level of concern for all population subgroups, including infants and children.”
The label does advise that a cocktail of Galleon and another herbicide, endothall, requires a seven-day use restriction on potable water and a buffer of 600 feet from functioning potable water intakes. But it makes no mention of any adverse interaction with flumioxazin, which the EPA has separately determined “is practically nontoxic to vertebrates and birds on an acute basis.”
MSO is an agent that helps break down the waxy layers of vegetation, Waters said, and although the label warns that direct exposure is “hazardous to humans and domestic animals,” such surfactants “may be formulated for aquatic use.”
Galleon’s label doesn’t quantify its efficacy against giant salvinia specifically (it’s referred to there as “water fern”), but anecdotal evidence suggests it has helped battle outbreaks under certain conditions. At a 2011 hearing of the U.S. House of Representatives’ Subcommittee on Fisheries, Wildlife, Oceans and Insular Affairs, Robert Barham, secretary of the Louisiana Department of Wildlife and Fisheries, called giant salvinia “a horrific problem” in his state, and testified that Galleon is an effective herbicide, but it “must remain in the water column from 60 to 90 days.”
“If you get a rain event, it dilutes it and it doesn’t work, and Galleon cost over $1,850 a gallon, so you can see with my budget, I don’t have the money to use Galleon everywhere, and it’s not the silver bullet,” Barham said.
Although MAWSS didn’t provide itemized expenses, online listings indicate the price of Galleon has increased since 2011, to an average of around $2,300 per gallon today. The MAWSS Board approved a total expenditure of $122,739 on the herbicides and boom.
Last week, MAWSS spokeswoman Monica Allen said the herbicide had already been applied since the purchase was approved, meaning it was applied during a period when the Mobile area experienced daily rainfall from the remnants of Hurricane Nicholas. The label includes retreatment instructions if treated areas are subject to heavy rainfall.
On Monday, Allen emailed to express that the future of the project remains uncertain.
“We are working on a timeline for this problem. At this time, we don’t know. The goal is to control the giant salvinia and keep it from expanding. We are making headway using the chemicals and mechanical methods (booms). We also have to look at some practices to educate those who use the lake and control what enters the lake in the future.”
The only recreational boat access to the lake is at Fox Landing on Howells Ferry Road, and maps released by MAWSS show the salvinia outbreak is mostly confined to the easternmost portions of the lake around the boat launch. Allen said MAWSS “hopes to reopen the lake when able, but nothing is off the table when it comes to ensuring water quality.”
Without knowing the details of MAWSS’s approach, Waters said the best way to battle giant salvinia is early and often.
“It’s a floating plant, so if it took over completely, it would block the sunlight, shut down photosynthesis and drop the oxygen levels. You’d have a massive fish kill,” he said. “You wouldn’t want to let it go, particularly with it being a nonnative, because it could spread to other areas and downstream. If you get on it early, you could minimize the cost and effort to eradicate it.”
But Waters also warned the species is particularly tenacious, so the effort isn’t guaranteed to succeed.
“You can treat or mechanically remove it from an entire area but it’s only going to take that one plant you miss to start spreading again,” he said.
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