For local governments struggling to stretch their dollars, a tax that brings in more revenue than forecast would usually be considered a blessing. Not so for Montgomery County’s five-cent levy on plastic and paper shopping bags, which went into effect a year ago this month.

The idea was not to squeeze more money from taxpayers, county officials said, but to change their behavior. The nickel fee was meant to discourage use of the bags, estimated to make up one-third of the litter found in the streams and stormwater ponds in the county and a significant portion of the more than 120 tons of trash that flows into the Anacostia River each year from Montgomery tributaries. As public awareness of the tax increased and shoppers relied on reusable bags, the thinking went, revenue would start to decline.

The cultural shift has not been as dramatic as hoped. County shoppers have been more willing to pay for their plastic than anticipated. Officials projected a 60 percent decline in bag usage for the current fiscal year — from 82.9 million to 33.1 million. But through the first five months of fiscal 2013, businesses have already sold 24.8 million traditional bags.

Money from the tax, estimated at $1 million for this fiscal year, is on pace to exceed $2 million. The money goes to water quality and stormwater control programs.

“It’s a little more [revenue] than we thought we would take in,” said Patrick Lacefield, spokesman for County Executive Isiah Leggett (D). “The paradoxical thing here is that the executive wants us to take in less money.”

There is uncertainty about how much paper and plastic shoppers actually used before passage of the tax, making it difficult to gauge exactly how effective the tax has been. But environmental groups that conduct periodic cleanups of county stream banks report a significant reduction in the number of bags recovered.

The tax covers virtually every retail business in the county. But that doesn’t mean that shoppers who bag milk and eggs in their reusable totes will use them for the shoes they buy at Bloomingdale’s.

“I understand where they’re coming from with it,” said Mathieu Schulman, 23, a veterinary technician, who was shopping at the Giant on Arlington Road in Bethesda. “Five cents — it’s a way to protect our environment. In a way I see that as good. . . . Mostly, I’ve grown into it, so I see it as a good thing.”

But for other kinds of purchases, the plastic habit has been harder to break.

“I do take my bags to the grocery store, but when I come to a store like Macy’s, I think we shouldn’t have to pay for the bag like that,” said Lillian Day, 67, of Potomac, on a recent visit to the Westfield Montgomery Mall. “I don’t mind in a grocery store, but in these kind of stores when you’re buying a garment, I think it should be wrapped.”

Montgomery County Council member Roger Berliner, who supported the tax, said it may be time to tweak it to cover only stores that sell food and drink.

“The supermarket case is a slam dunk. But I don’t think you and I go into Nordstrom’s with our reusable bags,” said Berliner (D-Potomac-Bethesda) “There isn’t that same nexus to our consciousness. . . . Let’s make sure we don’t create resentment in pursuit of a worthy objective.”

Montgomery officials said they worked aggressively to raise public awareness of the tax, with Webinars, television ads, outreach to businesses and 100,000 free reusable bags. On Black Friday in November, county staff worked the long lines at the malls and big box stores.

But Lacefield allowed that the county will have to do more. “We may well need to go back out again, and in fact we’re planning to do that,” he said.

San Francisco and several other California cities have banned plastic shopping bags, and the issue is under debate in other localities. Prince George’s County pursued a five-cent bag tax, but the measure died last year in the General Assembly.

The District imposed the region’s first bag fee in 2010, one more narrowly cast than Montgomery’s, covering only businesses that sell food and alcohol. The city claimed an 80 percent drop in bag usage during the first year: from 270 million to about 55 million. It earned about $1.8 million in fees in fiscal 2011, about half of what officials projected.

But no one is really sure how many plastic and paper bags were used before the taxes took effect.

Montgomery established 82.9 million bags as its benchmark, a figure officials said they derived from the D.C. data and then adjusted for the number of retail establishments in the county.

The District pulled its estimate of 270 million bags from a study conducted by Seattle, a city selected for its comparable size. But District officials have backed off from that number and now acknowledge that they’re not sure.

“We really don’t know,” said Jeffrey Seltzer, stormwater administrator for the D.C. Department of the Environment. “We truly believe it [the impact] is significant, but getting precise empirical data hasn’t been done.” He said the city has commissioned a study to get a better fix on bag usage.

Beth Mullin, executive director of the Rock Creek Conservancy, said the tax has clearly helped. The annual spring cleanups in April 2011 and 2012 show a 25 percent drop in the number of bags recovered in Montgomery, from 5,274 to 3,957.

“Our cleanup leaders report that in many areas their sites are significantly cleaner than in past years, and our numbers back that up,” Mullin said in an e-mail.

Michael Honig, vice president of the board of the Muddy Branch Alliance, whose members regularly pick up trash along county streams, said the difference is unmistakable.

“This is truly a welcome relief,” he said. “And I would rate it a rousing success.”

Farah Mohamed contributed to this report.