It's not raining cats anymore

The mystery of why panthers were dropping dead from the trees in the Florida Everglades was solved fairly quickly. The mystery of how they were being poisoned with mercury took a little longer.

Scientists from PNNL's Marine Sciences Laboratory were part of a team of researchers involved with the initial assessment of mercury contamination in the Florida Everglades project between 1993 and 2000.

MSL, which has been a pioneer in mercury sampling and analysis techniques, specializes in finding extremely low levels of mercury in samples.

"It is so easy to contaminate a mercury sample. For many years, most of the data we saw was contaminated from poor sampling and analytical techniques. Now we're realizing just the degree of cleanliness required to do these analytical techniques," said Brenda Lasorsa, who managed the project for PNNL.

Because of PNNL's expertise in analyzing different chemical species of mercury, it acted as quality assurance lab for all the labs involved in the U.S. Environmental Protection Agency's assessment project. PNNL researchers analyzed a percentage of all samples done by other labs to verify accuracy, and they helped resolve data quality issues.

Just as they had suspected, researchers found industrial mercury from nearby urban areas being volatilized into the air, blown into the Everglades and rained out. While inorganic mercury is volatile, it is not easily absorbed by animals or human cells. But scientists found methyl mercury, a neurotoxin, in the dead panthers

. "We had thought the methyl mercury was occurring in the animals," said Lasorsa, "but we were surprised to find that sulfur-reducing bacteria in the sediment were responsible for producing it." Scientists found that less oxygen in the Everglades water caused by drainage canals disrupting the natural through-flow of water, high sulfur-reducing bacteria production fed by fertilizer run-off from surrounding agricultural areas and mercury blown in from Florida's populated east coast, caused the Everglades to produce high levels of methyl mercury. "This situation is the worst possible scenario," Lasorsa said. "Inorganic mercury transformed to methyl mercury was going up the food chain from invertebrates in the water to the fish, the raccoons and, finally, the panthers."

"This started with the population boom in Florida in the 1920s and 30s, but it has only been in the last ten years that our detection limits for methyl mercury were good enough to see it was in the water," Lasorsa said. "Because of new clean techniques developed at MSL in the late 1980s and improved upon during the 1990s, we are now able to see inorganic and organic mercury at the sub part-per-trillion level. Detecting accurate low level concentrations helps us better understand how the mercury evolves and travels up the food chain."

The U.S. Army Corps of Engineers is studying the assessment data to devise a restoration plan for the Everglades. On contract with the South Florida Water Management District, PNNL's role might expand in the final project. In addition to quality monitoring, Lasorsa's team hopes to be involved in advising on how to mitigate the effects of the restoration, monitoring the wildlife and ecosystem and restoring natural vegetation.