In a letter sent Monday to county commissioners and other officials, Mosaic representatives said they want a settlement within 90 days.
The $618 million figure represents the decline in value of the Altman Tract if the company is not allowed to mine it, said Mosaic spokesman David Townsend.
Before the County Commission voted 4-3 to deny the company permission to extract phosphate, the 2,048-acre tract had a long-term value of about $631 million, he said. Townsend said a new appraisal places the land's value at $13 million if mining is not allowed.
"If we can't promptly resolve this through dialogue, we have triggered the start of our judicial remedies," Richard Mack, the company's senior vice president and general counsel, said in a statement Monday.
It is no small sum for a county whose spending for all programs and services next year will be about $530 million.
Assistant Manatee County Attorney Bill Clague, the county's legal point man on this case, declined to comment on the letter Monday.
Commission Chairwoman Jane von Hahmann, who voted against the Mosaic mining expansion, said she is not surprised by the legal threat. The company already mines land nearby the Altman Tract.
"I knew going in that we would probably be looking at a lawsuit," she said. "I still think we were fully within our rights."
The land lies within the watershed of the Peace River, one of the region's primary sources of drinking water. Area environmentalists and some county experts have argued that mining operations on the property could hurt the quality and quantity of that water.
According to county land-use policy, wetlands should be preserved whenever possible.
If a lawsuit is filed, the company will seek compensation under the state's Bert Harris Act, which prohibits local governments from placing an "inordinate burden" on property rights.
County Commission approval was the last regulatory hurdle for Mosaic to proceed.
It is not clear how the politics of the debate will change in November, when two new commissioners are sworn in.
Both von Hahmann and Amy Stein, who also voted against the Mosaic plan, were defeated in the Republican primary in August. They will be replaced by Bradenton Beach City Commissioner John Chappie and Palmetto Mayor Larry Bustle.
Both men said Monday they do not know how they would have voted Sept. 16.
"I need to have a little more time to digest what has happened," Bustle said. "I wouldn't presume to second-guess how the vote went."
Mosaic estimates it can easily extract 6.2 million tons of phosphate from the property. The value of the mineral, a key component in farm fertilizer, has been about $50 per ton or less in recent years, but it recently spiked to more than $400 per ton, according to Mosaic.
The company had hoped to mine more than 1,500 acres on the 2,048-acre Altman property. In the process, about 400 acres of wetlands would have been destroyed.
Company representatives said they would set aside more than 500 acres of wetlands, restore the destroyed wetlands after mining and create an additional 100 acres of new wetlands.
Mosaic has also promised to build a new fire station and park in Duette.
That was not enough for a majority of commissioners, who wanted to deny mining on an additional 107 acres of wetlands, which contain about 700,000 tons of mineable phosphate.
Environmentalists have also filed suit in connection with the case. The Sierra Club, ManaSota-88, the Gulf Restoration Network and People for Protecting Peace River have filed a federal lawsuit against the U.S. Army Corps of Engineers for granting Mosaic a permit to mine the Altman Tract.
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