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On a bright late-September afternoon, Mary Bleach showed visitors around the community garden near her apartment in Boston’s Dorchester neighborhood. The sunflowers were nodding their heads in acquiescence to fall, but rust-colored marigolds, pink cosmos, and fuchsia morning glories were still abloom, and a few lazy bees hit them up for nectar. Kale, collards, okra, callaloo (a relative of spinach), tomatoes, onions, herbs, eggplants, beans, peanut plants, and a squash vine with leaves bigger than Bleach’s head entangling 15 feet of chain-link fence—all were still soaking up the fall sun’s rays. Bleach said she lives out of the garden in summer, and she freezes enough to eat well into winter, too.

All this vegetable profusion would soon be gone. Winter was coming, yes, but also heavy machinery to scrape the land level and to haul away the ramshackle chain-link fence and the timbers dividing one plot from another. After more than 25 years, the garden at the corner of Lucerne and Balsam streets was slated for a makeover: handicapped-accessible concrete paths, sturdy fencing, new water service, and reestablished plots with granite dividers.

Boston University toxicologist Wendy Heiger-Bernays and three students had come to check out the site in preparation for a detailed soil contaminant study that would inform the renovation. If the garden’s soil were anything like other Boston soils, it would contain elevated levels of lead—in Dorchester yards, 1,500 ppm of lead is common. In the worst-case scenario, much of the garden’s soil would have to be removed and clean topsoil and compost trucked in.

And those old timber plot dividers? They were pressure-treated lumber of a vintage that was preserved using chromated copper arsenate—although when they were installed, they were considered a safe alternative to creosote-soaked railroad ties, another common landscaping material. In a 2009 study of three other Boston community gardens, Heiger-Bernays and colleagues showed that arsenic can leach from pressure-treated lumber into garden soil, and that polycyclic aromatic hydrocarbons (PAHs) can leach from old railroad ties.

Heiger-Bernays and her students eyeballed the garden’s perimeter. The adjacent houses were Boston’s signature triple-deckers, probably around a century old and layered in old lead-based paint. Long ago, similar houses stood where the garden now grew. Lead-based paint, asbestos, coal ash, and automotive oil from them could still haunt the garden soil. The lot had stood weedy and trash-strewn for years before Bleach and other neighbors reclaimed it in the 1980s.

The students bagged soil samples near the timbers, along the fenceline adjacent to the houses, and in plots throughout the garden. They would take these samples back to Heiger-Bernays’s lab for analysis.

Over the years the garden has been tested for lead and some clean soil brought in. Recently, the city has brought in truckloads of municipal compost almost every year. This black gold not only supplies nutrients to crops, but also dilutes contaminants and binds them to soil particles, reducing the risk of human exposure.