The Eurasian watermilfoil is an invasive aquatic plant that can pose a major threat to other species if left to grow unchecked, so this thick, matty plant is often controlled with herbicides such as 2,4-D, triclopyr, fluridone and others.
However, solving one problem may introduce another, and University of Wisconsin-Madison PhD student Amber White is studying how herbicides used to combat the plant in Wisconsin might also linger in the environment. White is a student in the Environmental Chemistry and Technology program within the College of Engineering’s Department of Civil and Environmental Engineering.
Eurasian watermilfoil can be a big problem when it spreads in lakes. According to the U.S. Department of Agriculture’s Invasive Species Center, when the plant spreads in shallow water, it can create thick blankets just under the surface that block sunlight and kill off other species within the water.
White has worked on a pair of projects, both funded by grants from the Wisconsin Department of Natural Resources, to study two different herbicides used against Eurasian watermilfoil. The first focused on 2,4-Dichlorophenoxyacetic acid, or 2,4-D; currently, she’s studying florpyrauxifen-benzyl.
To do so, White is combining lab work with extensive field sampling in several Wisconsin lakes to monitor how the chemicals break down once applied in the environment, and to determine what factors drive that process. She’s taken samples from Random Lake, Eagle Lake, McCarry Lake, Pleasant Lake and Round Lake for the 2,4-D project, and from South Twin Lake, Muskellunge Lake and Silver Lake for the florpyrauxifen-benzyl project.
To study the herbicides’ breakdown process, White took water and sediment samples from the lakes before they were treated with the chemicals. Once in the lab, her team set up microcosms to test the water and soil, then added the herbicide to observe certain factors.
“In some of our lab tests, we try to isolate the bacterial community to see how quickly it breaks down the chemicals,” White says. “That removes the effects of things like sunlight or runoff. In other tests, we’re looking at photochemical degradation, to see how quickly sunlight alone breaks down the herbicide. We can take that and compare it to additional samples we take after the actual lakes have been treated to see what’s driving those processes.”
Once the lakes were treated, White and her team aggressively sampled the sediment and water in the first 72 hours after application, then slowed to weekly sampling. After a few weeks, she shifted to monthly samplings to continue to observe how the chemical degrades in the water.
Though the florpyrauxifen-benzyl project has not yet reached a stage for comparative analysis between field samples and lab results, White says the team found significant evidence that bacteria drive 2,4-D’s degradation after it’s introduced to the environment.
“In the lab, we saw the photolytic degradation of 2,4-D is very slow,” she says. “It was taking hundreds of days to see the half-life, which is how long it takes for the chemical to decrease by half. In the lake, we were observing half-lives of six to 24 days, which is much too fast to be driven by the sun. That was more in line with what we observed in our bacterial microcosms, where saw half-lives that varied from five days up to 37 days.”
White and her team also monitored for chemical loss due to runoff from the lakes, but she says that ultimately accounted for little relative change, compared to observations on bacteria breaking down the material.
“We really believe microbes in the sediment are driving the degradation,” she says. “There’s a lot of previous literature that suggests bacteria can use 2,4-D as a food source. Really, they’re just eating it, like they would eat anything else.”
While taking field samples, White is also monitoring how the chemicals’ presence in the environment stacks up against various U.S. Environmental Protection Agency regulatory concentration guidelines. She says the observed 2,4-D concentrations of 0.5 to 0.6 parts per million fell well below the EPA’s regulatory concentrations of 3 to 4 parts per million for human contact, while still above the recommended 0.1 parts per million the EPA recommends for the herbicide to be effective.
While none of the observed lakes are used for drinking water, White says they are used recreationally. “We were seeing that 2,4-D was gone from the lakes usually within a month of treatment,” White says. “It usually fell below 0.1 parts per millions threshold within 28 days, or sooner in some places. There were a few places where I could measure 2,4-D out to day 65 or 70, but that’s because we have really great, sensitive equipment. We could measure down to 10 or 20 parts per billion, which is not effective against the milfoil and is well below human contact or even drinking water regulations.”
Though there’s still sampling to do on florpyrauxifen-benzyl, White says the substance already seems to be “stickier” than 2,4-D, meaning it may be more likely to cling to the soil or suspended carbon-based solids in the water.
White studies under Civil and Environmental Engineering Associate Professor Christy Remucal and Vilas Distinguished Achievement Professor Trina McMahon. For the project, she’s also worked with three undergraduate civil and environmental engineering students and a master’s student who’s studying bacteriology. It’s a great example of the types of collaboration common in UW-Madison’s research.
“Not only is this research cool, but I’m especially proud that we’ve been able to have so many undergraduates involved and getting hands-on experience, and to bring in another student from a different area of study,” she says.
Author: Alex Holloway