The Oakland A’s planned site for a new stadium is now mostly a parking lot, where colorful shipping containers and tractor-trailers sit in neat rows and diesel fumes and seagulls pack the gusty air.

But underneath the asphalt of Howard Terminal, on the Oakland Inner Harbor, lies a buried legacy of industrial use going back a century. A Chronicle review of regulatory documents found that soil and groundwater on the site contain hazardous and cancer-causing chemicals that would need to be remedied before a ballpark could be built there.

The presence of the toxic material will probably be addressed in the environmental review process that will begin in the coming months. Depending on the concentration and position of the harmful substances, the dirt may have to be removed entirely, altered chemically or further sealed off from human contact, experts say. State regulators will have to sign off on the plans.

Now a 50-acre site for container storage and truck parking, the Charles P. Howard Terminal near Jack London Square was previously home to oil tanks, a manufactured gas plant, a briquette plant where compressed charcoal blocks were made, a coal tramway, an asphalt paving plant and a blacksmith.

The operations created streams of waste: a type of carbon called lampblack, oil tar sludge, spent coke, cyanide, paint and other materials.

Before natural gas became widespread, the manufactured gas plant — which made gas from coal and oil — produced Oakland’s first energy, including for city streetlights. The leftover chemicals still at Howard Terminal are “in the range of what we deal with,” said Richard Sinkoff, director of environmental programs and planning for the Port of Oakland, which owns the land.

“The port is really a patchwork of legacy sites. You’re seeing the history of the city of Oakland,” Sinkoff said. “In the past, we looked at those smokestacks with pride. Now our operations are clean.”

A’s leaders, who have commissioned design plans for the site, are still negotiating a financial deal with the port, with the goal to open the park in 2023. Team President Dave Kaval said he did not yet know how much the cleanup would cost, but that it would be privately financed along with the rest of the project, which will include housing, retail and parks.

“There are going to be a lot of infrastructure costs on the site, whether it’s transportation, whether it’s sea level rise, whether it’s environmental mitigation,” Kaval said. “Those are all things that as part of the project, we’re willing to pay for.”

Elevated levels of chemicals that remain in the site’s soil and groundwater, such as polynuclear aromatic hydrocarbons, are known to cause cancer. Volatile organic compounds, also at the site but in much lower concentrations, have been linked to birth defects and can damage the central nervous system.

“They’re very nasty materials,” said Dan Hirsch, retired director of the Environmental and Nuclear Policy Program at UC Santa Cruz. “People are scared of radiation. They should be just as scared of these chemicals.”

In recent decades, the site has been capped with asphalt that ranges from 4 to 30 inches in depth. After the California Department of Toxic Substances Control declared in 2000 that the site posed an “imminent and substantial endangerment” to public health and the environment, the city and port have been obligated to monitor the groundwater and keep the cap safely intact.

The chemicals found years ago were not eliminated or reduced but walled off with the cap, said Russ Edmondson, spokesman for the state agency.

“Thus, the contaminants presently on the site would be the same as indicated by previous investigation, to the extent they have not degraded naturally,” he said.

Because of the chemical hazards, Howard Terminal is considered a “brownfield,” one of more than 450,000 sites across the country where pollutants and contaminants make redevelopment difficult, according to the U.S. Environmental Protection Agency.

It’s not yet clear how the chemicals will be dealt with ahead of stadium construction. Sinkoff said the parties are still in the early stages of planning, but that a complete removal of contaminated dirt would be unlikely.

“One of the things we know from past experiences is that hauling off all your soils is one of the most expensive things you can contemplate. This is the challenge of infill development and brownfield sites,” he said. “Digging up an entire site and moving the soil to a landfill has major implications for cost, trucks and landfill capacity.”

The public will get to weigh in on the remediation strategy, Kaval said, and the A’s will ensure human health and the environment are protected.

“It’s a relatively common situation,” he said. “There’s a defined process. There’s agencies set up to make sure it’s done appropriately.”

Professional sports stadiums are often built on contaminated property, said Lenny Siegel, executive director of the Center for Public Environmental Oversight and the former mayor of Mountain View. Once construction is complete, he said, the ballpark itself may not be a concern, but areas in adjacent communities could be.

“The owner has an interest in having a relatively clean site, but you have to check and see whether anything is bleeding from the site,” Siegel said. “Because it’s on the bay, you worry about the impact of sea level rise and more storms causing the contamination to get away from the cap.”

Right now, groundwater flows toward San Francisco Bay. But in the coming decades, its direction could change as it bumps up against a rising sea, said Kristina Hill, a UC Berkeley associate professor of landscape architecture, environmental planning and urban design. Groundwater flooding is already happening in locations around the East Bay, Hill said.

“People think that flooding is something that comes sideways in from the ocean, but this kind of flooding actually bubbles up through the soil over a long period of time,” she said, creating “soupy shorelines.”

Since the groundwater can contain harmful chemicals, people in the surrounding community should push for the installation of additional monitoring wells to track the changing dynamic “because people’s lives are at stake,” Hill said. The site’s immediate neighbors are industrial users, but a few blocks east is the growing commercial and residential area around Jack London Square, and the team wants to build additional housing near the stadium.

Last month, the A’s and the Department of Toxic Substances Control entered into an agreement — standard for development projects — that allows the team to investigate hazardous substances at the site and ensures that the baseball franchise reimburses state regulators for the costs of their oversight.

For the huge redevelopment project, it’s not just contaminants in the soil that could be a concern, but also the stability of the site. Tidal mudflats covered the land before it was filled beginning in the late 1800s.

One of the first actions the A’s engineering consultant, Engeo, said it will take is collecting soil samples and conducting “cone penetration tests,” which involve drilling up to 100 feet underground. The objective is to fill in gaps of the existing geotechnical data, the firm said.

Kaval said the development, which includes building parking garages, retail and commercial offices in addition to the stadium, is an opportunity to finally clean the land.

“One of the benefits of this project is that the site would be remediated,” Kaval said. “Not only are the health impacts handled in an appropriate way, but sea level rise and other resiliency issues would be addressed.”

Kimberly Veklerov is a San Francisco Chronicle staff writer. Email: kveklerov@sfchronicle.com Twitter: @kveklerov