The number of plastic and paper bags Chicagoans used to haul home their groceries dropped 42 percent in the first month after city officials imposed a 7 cents-per-bag tax in February. View Full Caption Flickr/life_chasing

CHICAGO — The good news is that Chicago's effort to keep plastic and paper bags out of area landfills by imposing a 7 cents-per-bag tax is succeeding beyond officials' wildest dreams.

The bad news is that the success of the fee in dissuading shoppers from taking single-use bags means the city's coffers are taking a steep hit.

Chicago officials balanced the city's 2017 spending plan based on an assumption that the city would earn $9.2 million this year from the tax.

But that assumption appears to have missed the mark by at least $1.5 million based on data complied by the city through June 18.

The city has earned just $2.4 million in the five months the tax has been in effect, said Molly Poppe, a spokeswoman for the city’s Finance Department. If bag use continues at the current pace, that means the city would net just $7.7 million from the tax for the year.

A study paid for by the city — and conducted by ideas42, a behavior design lab, as well as researchers from New York University and the University of Chicago Energy and Environment Lab — found that the number of plastic and paper bags Chicagoans used to haul home their groceries dropped 42 percent in the first month after the tax was imposed.

The study measured plastic and paper bag use at large grocery stores in Chicago one month before and one month after the tax was imposed.

Mayor Rahm Emanuel said he was thrilled by the study's findings.

"By decreasing our paper and plastic bag use, Chicago is making important progress in reducing our carbon footprint as well as reducing street litter and improving recycling operations," Emanuel said in a statement in April.

Before the tax went into effect Feb. 1, shoppers took home an average of 2.3 disposable bags every time they shopped at a big grocery store. After the tax went into effect, shoppers took home one fewer bag, according to the study.

The tax was prompted after the city's ban on single-use, thin plastic bags approved by the City Council in 2014 was largely considered a failure after many retailers switched to thicker bags that were considered worse for the environment.

Under the current tax, the city gets a nickel from the sale of each bag, with the store owner getting the other two cents.

The city expects to release additional data about the impact of the bag tax in the coming months.