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Minnesota native Leeann Rock often travels to the Twin Cities to visit family. While waiting for aircraft to be de-iced at the Minneapolis-St. Paul International Airport, she's occasionally wondered where all that fluid ends up and if it leaches into the nearby Mississippi and Minnesota rivers.

Rock, now of Mount Airy, Md., sought answers from Curious Minnesota, the Star Tribune's community-driven reporting project fueled by questions from readers.

The short answer: Excess de-icing fluid is collected in a holding tank after being applied to aircraft, and ultimately treated or recycled.

De-icing is an important safety tool for airplanes during the winter season — a build-up of ice can prevent airplanes from performing properly.

Given Minnesota's harsh climate, MSP is one of the busiest airports in the country for de-icing operations.

When travel was considered "normal" before the COVID-19 outbreak, 19,811 aircraft were de-iced at MSP between September 2019 and April 2020, according to the Metropolitan Airports Commission (MAC).

The amount of de-icing fluid used depends on the level of ice, snow or frost built up on the airplane. The chemicals not only de-ice the plane, but stay on its surface to delay ice forming again.

Delta Air Lines spokeswoman Martha Whitt says orange-colored Type I de-icing fluid, a heated mixture of a propylene glycol, other chemicals, inhibitors and water, are used at MSP. (The Atlanta-based airline, the dominant carrier at MSP, oversees local de-icing operations.)

If an additional step is needed to prevent ice from re-forming on the plane before takeoff, green Type IV anti-icing fluid is applied, usually on the wings and tail after the de-icing process is complete, she added.

On average, Delta uses more than 900,000 gallons of Type I fluid a season, and over 200,000 gallons of Type IV fluid. It's up to an airplane's pilot to determine whether de-icing is needed.

Leftover fluid is collected from containment sites at MSP's five de-icing pads and transferred to the airport's Glycol Management Facility. There, it is either treated or recycled, MAC spokesman Jeff Lea said. The recycled fluid is not reused on aircraft.

So has any of this fluid made its way to local rivers and lakes?

Minnesota Pollution Control Agency Spokesman Stephen Mikkelson said an agreement was reached in 2004 between the agency and the MAC resolving violations related to de-icing chemicals and petroleum-contaminated water draining off the airport's property into the Minnesota River, Snelling Lake and Mother Lake.

The pact reached called for the MAC to make improvements to decrease the amount of de-icing chemicals and other pollutants leaving the airport through storm sewers. Since then, there have been no major violations of the agreement, Mikkelson said.

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