Mosaic seeks Riverview gypsum-stack extension; neighbors warn of environmental risks

Mosaic asked the Hillsborough County zoning hearing master on April 27 to expand a phosphogypsum stack at its Riverview facility and add roughly 1,412 acres to an existing planned-development district, a request that drew both community support and sharp opposition.

Shelley Thornton, Mosaic’s director of land‑use permitting, told the hearing master the company needs a lateral extension of the East Stack to provide “approximately 16 years” of storage capacity. Thornton said the proposed East Stack extension would involve about 178.5 acres of new disturbance within the stack footprint and would affect degraded wetlands and streams. To compensate, Mosaic proposed a multilayered mitigation package that includes purchasing and placing a permanent conservation easement on about 347 acres of land south of the Alafia River, and engineered restoration of North Archie Creek.

“Because the resources being impacted are already degraded, the mitigation strategy is intentionally robust and multilayered,” Thornton said. “The goal here isn’t just replacement, but also improvement.”

Mosaic’s environmental witness, Santino Provenzano, described state and federal rules that require engineered stack design, daily and periodic inspections, and professional engineering oversight. Planner Tina Ekblad said the applicant is seeking a limited flex of about 19.3 acres of natural‑preservation land into light‑industrial use to accommodate the extension and committed to filing a future‑land‑use amendment and recording an amended declaration of restrictions as a condition of approval.

Supporters at the hearing described Mosaic as a long‑standing local employer. Marlise Talbert Jones, vice president of the Progress Village Civic Council, said she has toured Mosaic sites and supports the expansion because of mitigation commitments and the company’s community involvement. “I am in support of them doing the expansion,” she said.

Opponents focused on safety and long‑term environmental risk. Theresa Gordon, a Riverview-area resident, cited the 2021 Piney Point discharge and broader concerns about gypsum stacks, saying, “All gypsum stacks are ticking time bombs.” Another resident flagged historic enforcement actions and sinkhole events at other Florida sites.

Mosaic’s counsel and technical witnesses countered that stack operations are tightly regulated and that the Riverview facility is subject to federal, state and county oversight, including the phosphogypsum management rule and site inspections. Mosaic also said visual management, an annual monitoring program and decades of photographic records document maturing buffers around the stack.

The hearing master took public testimony and will file a written recommendation within 15 business days. The county attorney’s office noted that applications heard tonight are scheduled to be considered by the Board of County Commissioners at a subsequent land‑use meeting (staff papers cited 06/09/2026 as the likely date), where final decisions will be made on the record.

What’s next: the hearing master’s written recommendation will be added to the record; Board of County Commissioners action is scheduled later; state and federal permits would be required before construction could proceed.

Provenance: testimony and presentations beginning with the applicant’s presentation and spanning the public‑comment period (topic introduction: SEG 2560; topic finish: SEG 3590).