When a firestorm consumed the Hawaii town of Lahaina last year, killing 100 people, it left behind a toxic wasteland of melted batteries, charred propane tanks, and miles of debris tainted by arsenic and lead.

Crews have already removed some of the most hazardous items, shipping them out for disposal on the mainland. Now begins the even more formidable task of collecting hundreds of thousands of tons of additional debris and soil — enough to cover five football fields about five stories high. Even as excavators began filling dump trucks this month, the question of where it should all ultimately go remained unanswered.

For now, the county has chosen a “temporary” dump site in Olowalu, a few miles south of Lahaina on the West Maui coastline. There, just up the hill from a vital coral reef and an important ecosystem for manta rays, residents worry that dumping dangerous waste into the area could create a fresh disaster.

“It hurts,” said Foster Ampong, 65, who has family members who lost homes in Lahaina and spends much of his time in Olowalu helping other relatives farm taro. “I’m very much worried about the future of Olowalu…”

The fire that swept through Lahaina burned through more than 2,000 buildings, leaving little more than cinder blocks, car husks and piles of ash behind. To prepare for rebuilding, crews have started clearing plots of land, with excavators digging down six inches to remove contaminated soil.

County leaders want the task done with urgency. In the midst of the rainy season, storms already have sent runoff into the ocean, turning the water along the coastline cloudy. Meanwhile, thousands of people who lived in Lahaina remain without permanent residences and are eager to get rebuilding underway. Many have left the island in search of stability.

Richard Bissen Jr., the mayor of Maui County, said that he agrees that the Olowalu site should not be permanent. But for the sake of the Lahaina survivors, he said, he does not want to delay work at the temporary location as part of a rebuilding process that is expected to take years.

“While there is no easy path forward, I must navigate these contentious issues with the greater good as my foremost sense of kuleana,” Mr. Bissen said at a meeting this month, using a Hawaiian word to emphasize his sense of responsibility…

One of the chief concerns about the wildfire debris is the presence of heavy metals in the ash and soil. While perhaps not an acute health threat, prolonged exposure is a concern, said Peter Guria, who supervises hazardous debris issues at the federal Environmental Protection Agency in the region. To prevent clouds of toxic dust from swirling through the air, crews have sprayed a soil stabilizer across much of Lahaina…

The coast of Olowalu is popular with snorkelers and filled with abundant sea life. In 2017, the coral reef offshore became a focal point for protection by the nonprofit Mission Blue, which advocates to protect the ocean.

The organization said the reef acts as a sort of nursery to enhance reefs on other islands nearby. It also supports a large population of manta rays.

“It’s environmentally precious,” said Tom Gruber, an adviser to Mission Blue. “It’s like Yosemite. You wouldn’t put a toxic waste dump upstream of Yosemite…”