The large wetland complex is in the Lower La Crosse River’s flood plain.
That assurance could give the city more options to address the problem, La Crosse Mayor Tim Kabat told Wisconsin Public Radio . He said removing the pellets would cost the city millions of dollars and could do more harm than good.
Kabat said city officials want to do long-term monitoring of insects and animals in the marsh to make sure the contamination isn’t spreading.
The Wisconsin Natural Resources Department will decide next steps.
"(The DNR) will tell us if they want further study or if they want a different kind of remediation or whatever it is they decide that they want," said Phil Ostrem, a city councilman. "There’s a new head of the DNR so we really have no inkling of what’s going to happen."
The mayor said city officials plan to send their recommendation to the department next month.

As coastal ecosystems feel the heat of climate change worldwide, new research shows the humble mussel and marsh grass form an intimate interaction known as mutualism that benefits both partner species and may be critical to helping these ecosystems bounce back from extreme climatic events such as drought.
The study, led by the University of Florida, finds that when mussels pile up in mounds around the grass stems, they provide protection by improving water storage around the grass roots and reducing soil salinity.
Mussels protect grasses from drought by improving water storage around the grass roots and reducing soil salinity.
With mussels’ help, the study found, marshes can recover from drought in less than a decade.
"It’s a story of mutual benefit between marsh grass and mussels," said Christine Angelini, an assistant professor of environmental engineering sciences in UF’s Herbert Wertheim College of Engineering and lead author on the paper.
They found that wherever there were clusters of mussels embedded in the mud around the base of the grass stems, the grass survived; in fact, grass growing in mussel clusters had a 64 percent probability of surviving versus a 1 percent probability in areas where there were no mussels.
One of the research team’s marsh study sites was in the backyard of Dale Aren of Charleston, South Carolina.
After noticing the marsh behind her home was dying, she did some online research and found a paper about that very problem written by Angelini’s colleague, Brian Silliman, an assistant professor of biology at UF and a co-author on Angelini’s paper.
"We were worried," Aren said.
"The Spartina [grass] is beautiful and the increasing area of mudflats were very unattractive and did not look healthy."

Pressures from grazers hastens ecosystem collapse from drought.
The researchers found that these tipping points can happen much sooner than current models predict because of the added pressures placed on drought-weakened plants by grazing animals and fungal pathogens.
The researchers performed a series of experiments before, during and after the drought to test how pressures from heavy grazing by crabs — the main natural enemy of plants in the protected marshes — affected resilience to and recovery from the drought.
"In test plots where crabs were excluded, we found that vegetation loss was significantly lower and many plants could survive the drought," said Qiang He, a postdoctoral researcher in Silliman’s lab.
"But in control plots, grazing by crabs decimated the drought-stressed plants, resulting in nearly complete vegetation loss," said He, who has studied the Chinese marshes for more than 10 years since he was a student at Beijing Normal University.
"Plants in forests and grasslands can also be decimated by natural enemies, such as insects and fungal pathogens, during droughts."
"The problem is that most of the plant-tolerance models we currently use to predict these outcomes are based on laboratory studies or plant physiological studies, so they often don’t factor in the compounding effects of simultaneous stressors like drought and pressure from natural enemies.
The result has been that ecosystems are dying well before our models predict they will.
"Pressures from grazers hastens ecosystem collapse from drought: Experiments show grazing pressures compound drought stress, delay recovery."
"Pressures from grazers hastens ecosystem collapse from drought: Experiments show grazing pressures compound drought stress, delay recovery."