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How a massive sinkhole dumped 200 million gallons of radioactive water into a Florida aquifer

Last month, a 300-foot deep sinkhole opened up at a phosphate plant in Mulberry, Florida. The sinkhole, which measures some 45 feet in diameter, poured 215 million gallons of wastewater directly into a major Florida aquifer. But homeowners and nearby residents who draw drinking water from that aquifer weren’t notified of the breach for more than three weeks.

The reason? Current laws don’t require notification until there is evidence that the water migrated off-site. The manufacturer, Mosaic, told the Florida Department of Environmental Protection, the EPA, and Polk County officials, but wasn’t required to post anything publicly about the incident.

“Should there be any indication of offsite migration of contaminated groundwater, rules require the notification of affected parties,” the DEP’s Dee Ann Miller said in an email to the Tampa Bay Times. “However, to date there is no evidence of offsite movement or threat to offsite groundwater supplies.”

That’s quite different than what happened in 1994, as the Tampa Bay Times notes, when a similar sinkhole opened at a different phosphate plant. The Florida legislature changed the law in 2005, giving plant owners 10 days to notify the Florida Department of Environmental Protection, and DEP 30 days to notify consumers. Said notification period kicks in only after contamination has been found outside the initial area, which means Florida residents who draw water from the aquifer legally still weren’t entitled to know their drinking water might have been compromised.

A bit of background may be helpful here. According to the US Geological Survey (PDF), Polk County’s hydrology is composed of three layers — a surficial aquifer provides most of the groundwater for the country, with a confining layer and a lower aquifer system below that. The upper, surficial layer of the aquifer system ranges greatly in thickness, from several feet in some areas of the county to more than 200 feet thick in others. The image below, from Wikipedia, shows a typical aquifer with a similar distribution of layers.

The terrain in this part of Florida, known as Bone Valley, is a unique mix of sand, phosphate pebbles, and clay. These substantial phosphate deposits are considered a major distinguishing feature of the area and have been mined for commercial fertilizer production for decades. The problem in Polk County is the area also contains large amounts of limestone, dolomite, and gypsum — all materials that are soluble in water. This makes the entire region prone to sinkholes, caves, and other similar rock formations. Combine sinkhole-friendly strata with extensive mining, and you’ve got a recipe for water pollution.

The radioactivity comes into play because the phosphate deposits we mine for fertilizer are weakly radioactive to start with, at roughly 100 ppm. The phosphate ore is treated with sulfuric acid, creating a byproduct known as phosphogypsum. Phosphogypsum is somewhat more radioactive than the raw ore, since the treatment process concentrates the radioactivity that was present initially. Phosphogypsum is stored in huge “stacks,” but since it emits radon initially, the stacks are covered in water. Over time, as the water evaporates, the stack forms a thick crust that seals at least some of the radon gas and prevents the phosphogypsum from blowing in the wind and creating dust storms. The sinkhole that opened in Tampa short-circuited this process by draining the water directly into the water supply.

For now, Mosaic is offering free water testing to concerned residents and has already set up a recovery well pumping 3,500 gallons of water per minute out of the affected area, to try and recapture some of the contaminated material. But this particular stack will continue to leak into the aquifer until the hole is physically plugged, which could still take months. Bone Valley currently contains roughly one billion tons of phosphogypsum in these various stacks. No current plan exists for repurposing the material, though there have been various proposals to use it for road pavement, a landfill cover, or for artificial reefs.

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