Not all Chicago-area grocery shoppers were prepared on Saturday when the city’s plastic bag ordinance took effect.

“I forgot,” said Andrew Leth, shopping at the Jewel-Osco on 4660 W. Irving Park Rd. “It didn’t even cross my mind. I usually just run into the store, so I don’t think of the bags.”

Because of the additional cost, Leth decided to just carry his groceries back to his car. Across the city, other shoppers who, like Leth forgot to bring their own bags, filed out of stores with groceries packed in whatever alternative a store offered. In the case of Jewel, it was a thicker plastic bag, which shoppers were urged to reuse, or traditional paper bags.

Jewel-Osco, the city’s largest grocery chain, and other large retailers, are among the first string of stores required to stop using the familiar lightweight plastic bag to shoppers.

Under the ordinance, passed in May 2014, affected retailers “shall provide reusable bags, recyclable paper bags or any combination thereof.” Supporters on the council say they wanted to cut down on pollution.

The ordinance is being phased in. First to comply are stores of more than 10,000 square feet. Next Aug. 1, the ordinance will apply to stores below that size, provided they are part of chain of at least three stores. Smaller “mom and pop” stores are exempt.

Saturday, Jewel-Osco cashiers often asked customers if they had brought their own bags — and many had, coming ready with cloth bags or previously used plastic bags.

Those who didn’t have their own bags had a few options. Reusable non-plastic bags could be purchased at Jewel for $1.49. Customers can also opt for a 10-cent black plastic bag. There also are reusable white plastic bags that are given without charge. These thicker white bags, Jewel-Osco says, hold up to 22 pounds and can be used more than 125 times. Jewel says they comply with the ordinance.

So what’s the difference between the black and white reusable plastic bags?

“None,” said Sean, a manager of a North Side Jewel-Osco.

In the parking lots, several customers said they used whatever bag the cashiers handed them. Some customers said that lack of clarity in bag options slowed their checkout.

“Not everyone brought bags and now they have to buy bags and didn’t know which ones to get for the best price,” said Terra Tarjan, who was shopping with her young son at the Jewel-Osco at 4042 W. Foster Ave. “So that made it a bit choppy.”

Customers remained optimistic, however.

“Give it a week or so and people will start understanding the process,” Leth said.

Jewel-Osco says shoppers who bring in their own reusable bag can enter a weekly raffle for $25 in store credit.