SOUTH PORTLAND — City officials have asked an energy company to revise its proposal to build a liquid propane gas depot at Rigby Yard because it would violate a zoning regulation that blocks additional tank farms.

NGL Supply Terminal Co. had begun site plan review to install six 60,000-gallon above-ground tanks at the rail yard, an industrial site off Main Street (Route 1), between the Cash Corner and Thornton Heights neighborhoods.

However, in a port city that already has 87 massive oil or gasoline fuel storage tanks, 1990s zoning prohibits the installation of additional above-ground tanks that hold more than 25,000 gallons of anything, either individually or in total.

NGL wants to build the depot because it must move its railside storage terminal on Commercial Street in Portland, where the state is expanding the International Marine Terminal near the Casco Bay Bridge.

To build the proposed depot, NGL could either seek a zoning waiver or text amendment, or it could partner with landowner Pan Am Railways to qualify for exemption from local zoning under federal interstate commerce laws, said Stephen Puleo, city planner.

“We’re waiting for the applicant to decide how they want to proceed,” Puleo said.

NGL is a subsidiary of Tulsa, Oklahoma-based NGL Energy Partners, parent company of Downeast Energy. NGL’s Maine representative didn’t respond to a call for comment.

NGL is looking to move because the Maine Department of Transportation is expanding the International Marine Terminal, which is owned by the city of Portland and managed by the Maine Port Authority. The state has purchased four acres of land from Unitil where NGL’s terminal is located and plans to build a multimillion-dollar cold storage warehouse, state officials said.

Because NGL is a tenant on the land, the state must pay for the company’s move, but it likely won’t be helping NGL navigate zoning hurdles at Rigby Yard, said Nate Moulton, director of the MDOT’s railroad program.

“We’re keeping tabs on the situation, but we don’t anticipate getting involved,” Moulton said Friday. “We think (Rigby Yard) is an ideal spot for (NGL), but it’s up to them to get the permits and approvals.”

Moulton said he didn’t know whether NGL might pursue help from Pan Am, which would benefit from increased and improved freight handling at Rigby Yard. A Pan Am representative didn’t respond to a call for comment.

The Rigby Yard proposal would increase NGL’s fixed storage capacity by 80,000 gallons – from six tanks that hold a total of 280,000 gallons at its Portland site, to 360,000 gallons at Rigby Yard, city officials said.

Rigby Yard also would allow NGL to bring more rail cars into a secure, fenced-in terminal with security cameras, sensors and hydrants. The four-acre terminal in Portland has track capacity for eight cars, while the 10-acre terminal at Rigby Yard would have track capacity for 24 to 30 cars, Puleo said.

City officials took a closer look at NGL’s proposal after some citizens raised concerns about it.

“It doesn’t do anything to preserve the health, safety and general welfare of the city or its inhabitants,” said Eben Rose, who lives on Buchanan Street. “It just exposes it to greater risk.”

NGL representatives have touted a clean safety record. City officials have said that bulk storage of liquid propane doesn’t pose a threat as long as appropriate safety measures are met. They also have said NGL’s depot proposal would be an improvement over the way liquid propane is handled now at the 245-acre rail yard.

As many as 100 rail cars loaded with liquid propane pass through Rigby Yard daily with little local security or oversight, said Fire Chief Kevin Guimond. Some tank cars unload their cargo directly to delivery trucks operated by AmeriGas, another liquid propane retailer. Under federal law, companies aren’t required to register cargo with communities along railways while it’s in transit.

In recent months, because severe snowstorms greatly limited rail travel throughout the region, NGL had as many as 70 tank cars stranded at Rigby Yard because they were unable to reach and unload at the storage and distribution facility on Commercial Street in Portland, Puleo said.

If NGL’s depot were approved at Rigby Yard, it would be the third liquid propane storage facility in the city, including one on Lincoln Street, near Interstate 295, and another on outer Broadway, near the Scarborough line.

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CORRECTION: This story was updated at 3:18 p.m. on April 13 to clarify that there are two active liquid propane storage facilities in South Portland, on Lincoln Street and outer Broadway.