How could Duluth’s city water, safe for people to drink, kill so many fish? – Duluth News Tribune

DULUTH — The municipal water system sends an average of 13 million gallons of water to more than 100,000 people in and around the city every day.

Reports filed annually show Duluth’s water, drawn from Lake Superior at the Lakewood Pumping Station and then given minimal treatment, is clean and safe, exceeding federal safe drinking water standards.

So why then is an accidental release of a half-million gallons of Duluth drinking water considered the likely culprit in

killing more than 1,000 fish and other aquatic creatures in Tischer Creek

on Aug. 1?

Biologists say it’s likely the small amounts of chemicals added to make the lake water safe for people are what made it unsafe for fish.

According to reports filed with the state, Duluth drinking water contains chlorine (as high as 1.72 parts per million) and fluoride (about 0.74 ppm) as the only chemicals intentionally added to the Lake Superior water. (The reports also show very tiny amounts of trihalomethanes and haloacetic acids, in parts per billion, both unintentional byproducts of the chlorine disinfection process.)

While that level of chlorine is enough to kill E. coli and other microorganisms that might make people sick, it is entirely safe for people and pets to drink every day. But brook trout, crayfish and other aquatic creatures are another story.

Every good angler knows not to put their minnows or leeches in tap water because that little bit of chlorine is too much for their systems to handle — the same for aquariums and backyard fish ponds.

Chlorine can kill fish in two ways, said professor Allen Mensinger, a fish physiology expert in the biology department at the University of Minnesota Duluth. It can burn their gill tissue, damaging it enough to be fatal. More profoundly, chlorine disrupts how a fish’s red blood cells carry oxygen to the body.

“The chlorine changes the hemoglobin. … The fish essentially suffocates,’’ Mensinger said. Chloramine, when present, does the same, he noted. Neither fluorides nor the other trace chemicals reported in Duluth water would have such fatal results.

Don Schreiner, a fisheries biologist with Minnesota Sea Grant and formerly the Minnesota Department of Natural Resources, gave a similar explanation.

“Fish are very susceptible because even a very small amount of chlorine, like that found in municipal drinking water to kill microbes, will affect their sensitive gills … they can suffocate by not being able to extract oxygen from the water,’’ Schreiner said. “So it is basically the very sensitive gill filaments/cells that the chlorine can damage that affects the ability of fish to breath.”

Dale Hoff, scientist at the Duluth-based Great Lakes Toxicology and Ecology Division of the Environmental Protection Agency, said any living thing that gets oxygen from water is susceptible to even low levels of chemicals in the water.

“Any organism that needs to obtain oxygen from water can be impacted by chlorine in the treated water,” Hoff said. “Chlorine readily passes with the water through the gills and can impact the integrity of hemoglobin and therefore the ability of the hemoglobin to bind to, and then distribute, oxygen to the rest of the body.

“A lot of water has to pass through gills in order to obtain enough oxygen,” Hoff said, “therefore, exposure can amount to a lot even though there may be low concentrations of chlorine in the water. None of these concerns occur when people and animals drink the water.”

Chlorine would eventually evaporate out of the water, making the water safe for fish at that point. It also would dilute to harmless levels as it reached downstream and Lake Superior.

Other issues could have been the temperature of the water, too warm for trout, or reduced oxygen in the city water. But because Duluth water is stored underground and is usually very cold, and because the incident killed many different organisms, those scenarios are less likely. It’s also possible the rush of water could have picked up something toxic between the reservoir and the creek.

An MPCA official on Monday said the results from toxicology tests on fish tissue and water samples drawn for the creek immediately after the incident were not yet available.

Neighbors began noticing dead fish in Tischer Creek on Aug. 1. The situation was first reported in the News Tribune on Aug. 2.

On Aug. 5, the Minnesota Pollution Control Agency said the city reported a release Aug. 1 of 400,000-500,000 gallons of city water from a city storage reservoir that flowed into the stormwater system and then into Tischer Creek.

The MPCA reported more than 1,000 fish died, including many brook trout. DNR crews noted that many other creatures also died in the creek, a designated trout stream, mostly downstream from Hartley Park.

City staff immediately owned up to their problem in reporting to the MPCA.

“In the course of a maintenance operation on a nearby drinking water reservoir, the city of Duluth discharged clean, potable drinking water into the stormwater sewer system, which then flowed into Tischer Creek,” the city’s report said.

“The city of Duluth would like to reaffirm that the water discharged into the stormwater system on Aug. 1 was safe, clean, and potable for humans and house pets as it left the reservoir, ” the statement said.

City and MPCA officials said there is no evidence the fish kill is related to the extensive road construction in the area.