Nearly half of the 2,100-square-mile Everglades is choked with phosphorus, a primary ingredient of farm fertilizers, that is bound up in muck soils where it poses a significant threat to restoration, a new study by U.S. EPA has found.

Findings from the 11-year study, released today from the agency's Everglades Ecosystem Assessment Program, show that phosphorus in wetland soils exceeded 500 milligrams per kilogram of soil (mg/kg) across 24 percent of the publicly owned Everglades, up from 16 percent in 1995-96.

Meanwhile, across nearly half of the Florida wetland -- including significant portions of Everglades National Park -- soil-based phosphorus remained above 400 mg/kg, the federal government's threshold for what is considered healthy for native plants, fish and other wildlife.

Besides phosphorus, the study also examined mercury concentrations in both water and fish, as well as sulfate enrichment and soil loss due to generally drier conditions throughout the lower range of the Everglades.

Scientists described their findings on mercury as mixed. While concentrations of the heavy metal in mosquitofish, a primary indicator species, had dropped since 1995-96, the metal persists at elevated levels across 65 percent of the Everglades and continues to pose a threat to other wildlife that consume fish as well as humans who eat a fish-heavy diet.

The findings on phosphorus may prove more challenging to restoration managers because it is so strongly linked to runoff from the Everglades Agricultural Area, where much of the nation's sugarcane is grown, and ranching operations north of Lake Okeechobee, the Everglades headwaters.

'Legacy pollution'

While efforts in recent years to curb agricultural runoff have helped reduce phosphorus in Everglades surface waters, the new data suggests that "legacy pollution" associated with past poor environmental practices remains a major obstacle to meeting restoration goals.

"It's important to remember that prior to around 1990, nothing was done to control phosphorus from stormwater going into the Everglades," said Dan Scheidt, a senior scientist at EPA's regional laboratory in Athens, Ga., and a co-author of the study.

And despite billions of dollars being spent by both Florida and the federal government to trap and treat polluted stormwater in the upper portion of the watershed, the Everglades continues to absorb far more phosphorus that it can process.

Scheidt said that to meet state water quality standards, phosphorus inputs should not exceed 42 metric tons per year. Yet in 2005, scientists found that the system took in roughly 147 metric tons of phosphorus. The result is diminished water quality and the loss of native plant species that cannot live under such conditions.

"Everybody's goal is to get better," Scheidt added. "It's a big problem at this point, and we're not there yet."

Click here to review the complete EPA study.