When Colin Walthall goes to a shopping center, he often leaves without a single purchase. Instead, he walks out with empty soda bottles, discarded napkins and other pieces of litter — and a clipboard recording what he picked up.

Walthall, an employee of environmental nonprofit group Keep Prince William Beautiful, hopes to see hundreds of volunteers join him in surveying the environmental impact of Prince William County shopping centers. To encourage volunteers to participate, he has added an incentive: Of all the people who spend half an hour walking around a shopping center and rating it, one each month will win a $50 restaurant gift certificate.

Volunteers for Keep Prince William Beautiful visit a shopping center with a list of points to check. Without speaking to the property manager or store employees, they walk around outside. Then they give the shopping center scores on a long list of assessment criteria. Are the buildings marred by graffiti? Are trash cans covered so that litter does not blow away? Is liquid waste running into nearby streams? Are there potholes in the parking lot?

The organization’s staff members review the volunteers’ reports. When they see a shopping center flagged for a problem that might be a county violation — such as not using garbage receptacles or recycling bins properly — they report it to the county.

When the same shopping centers score low on aesthetic criteria multiple times, Keep Prince William Beautiful staff members give the property managers a call. “We say, ‘Hey, we’ve surveyed you a couple times. You lack in these areas,’ ” Walthall said. Then comes the positive message: “We have volunteers. We have the resources to plant some trees and shrubs. And we’d like to.”

Often, the surprised shopping center owners agree to the free landscaping help.

By the organization’s count, Prince William is home to more than 90 shopping centers. At its high point, the group’s five-year-old surveying program covered 78 of them in one year, hitting several multiple times. But the program has dwindled; Walthall said that just 10 to 15 surveys have been completed since July.

His goal is to hit 50 more shopping centers by the end of June. That’s where the restaurant gift certificate idea came in. Each time a volunteer completes a survey, he or she is entered in the monthly drawing for the prize.

Walthall recommends the surveying to people who are looking for purposeful exercise or seeking a way to rack up community service hours. He and the group’s executive director, Kiliaen Anderson, walked around the perimeter of Manassas Mall on Thursday morning to assess the property using the survey. Anderson picked up two empty bottles and a McDonald’s bag, but overall, they gave the mall nearly perfect marks.

“If you don’t see cigarettes around the stop signs, you know they’re actually sending someone out to clean up,” Anderson said in the parking lot.

Walthall noted a truck parked at a loading dock behind the mall, not far from a grate with the warning “Dump no waste! Drains to waterway.”

“It’s drains like this that are a problem,” Walthall said, worried that store employees might change trucks’ oil there and let excess oil run into the water system. “That doesn’t go through any purification system,” he said.

He praised the same store, though, for keeping its hazardous materials in a cage and using well-designed trash cans.

Anderson said that such considerations make a big difference, even if residents do not actively keep an eye out for them.

“Shopping centers represent the community. When they start to go downhill, it really gives you a sense of how the community feels about itself. It’s less safe. You start to see graffiti,” Anderson said. “People are not taking pride in the place.”