The District's collections from its so-called bag tax have fallen for the second year in a row as more residents have chosen to use reusable bags for their shopping, an evolution that the fee's supporters hailed as a validation of their efforts.

"Our city is cleaner, our rivers are cleaner and our city is better off," said Ward 6 D.C. Councilman Tommy Wells, an architect of the 5-cent tax charged for every paper or plastic bag a customer uses when shopping. "It's just been incredible to see how our citizens adapted. It was almost seamless."

According to statistics from the District's chief financial officer, the city collected $1.6 million from the fee in the 2012 fiscal year, which ended in September. That's down 11 percent from the District's $1.8 million haul in 2011.

The 2011 fiscal year was the first full budget cycle for the fee, which went into force in January 2010.

Mayor Vincent Gray said the fee "is accomplishing what it set out to do."

Before the fee went into effect, officials estimated the District went through more than 22 million bags a month, a figure that has since fallen to perhaps a few million.

The fall in bag tax revenue, though, won't dent the city's $6 billion local operating budget because the fees go toward cleaning up the Anacostia and Potomac rivers, waterways long plagued by pollution caused by litter and runoff.

Jill Lawson of the Anacostia Watershed Society said environmental groups had noticed clear progress in reducing litter in the city.

"Trash traps that we keep on the stream have many fewer plastic bags in them," Lawson said. "Driving around, walking around through the parks and such, we definitely see a lot less plastic bag litter than we used to."

The fee, however, still has its detractors.

"This bag tax has nothing to do with the environment, and it does nothing for the environment. All it does is create a slush fund," said Patrick Gleason, Americans for Tax Reform's director of state affairs. "Instead of D.C. Council members pestering residents with how they transport their groceries, they should focus on getting their own house in order."

Gleason also said his group expects bag use to climb in the future.

"We're expecting to see a rebound in usage," he said. "Once people get used to paying it, use will go back up."