CHICAGO — Despite the bitter wind, Kim Wasserman showed me around La Villita Park. Occupying 21 acres in the middle of this city’s largest Mexican-American neighborhood, called Little Village, the park used to be a brownfield and illegal dump. Back then, the site leached toxins into hundreds of nearby basements. Sickened residents protested for years. The federal cleanup, finally completed in 2012, became the largest urban Superfund project in America.

Ms. Wasserman, executive director of the Little Village Environmental Justice Organization, then helped lobby the city for the park.

She pointed out where residents got the playground, ball fields, skate park and community gardens they wanted. The $19 million park now hosts city-sponsored sports programs and free concerts. During warm months, Ms. Wasserman said, formerly incarcerated young residents from Little Village help keep an eye on La Villita, discouraging gangs from moving in. “The community feels ownership of the place,” she said.

One result in this city known for its murder rate: next to no violent crime in the park, according to Ms. Wasserman.