DURHAM, NC – A new study of water chemistry in Florida’s largest groundwater aquifer suggests denitrification occurring in it, and agricultural fertilizer runoff flowing into it, have larger impacts than previously thought on nitrogen loading in the region’s waters.

Nitrogen loading is linked to harmful algae growth, low dissolved oxygen levels, fish kills and other problems, and is a concern in many Florida rivers, estuaries, springsheds and wetlands. Denitrification is a naturally occurring process that transforms nitrate from pollution sources into biologically unavailable nitrogen gas.

The new study by researchers at Duke University, Florida International University, the University of Florida and the U.S. Geological Survey in Tallahassee finds the amount of denitrification occurring in the Upper Floridian Aquifer is much larger than past estimates have suggested, and is helping to reduce nitrogen loading in downstream waters.

At the same time, however, it may be muddying efforts to correctly identify the source of nitrogen pollution entering the aquifer.

“Denitrification disguises the isotopic signature of nitrate from inorganic sources such as fertilizer so it looks like – and is easily confused with – nitrogen from organic sources such as human and animal waste. If you don’t take into account the effect of denitrification, you’ll get the source wrong,” said Jim Heffernan, assistant professor of ecosystem ecology and ecohydrology at Duke University’s Nicholas School of the Environment.

“Until now, the prevailing assumption has been that the amount of denitrification going on in the aquifer is negligible. Our study is the first to estimate the magnitude and variability of it, on this large of a spatial scale, from direct measurements,” Heffernan said.

The Upper Floridan Aquifer underlies much of Florida and southern Georgia. Nitrate concentrations in the limestone karst aquifer and its springs have risen sharply over the past half-century.

Because previous isotopic studies seemed to implicate organic sources, most water-quality management and policy efforts in the region have focused primarily on reducing nitrogen inputs from septic tanks and municipal and agricultural wastewater. Inorganic nitrogen from fertilizer was believed to play a smaller role.

Heffernan and his colleagues analyzed water samples from more than 100 springs flowing from the aquifer. By employing a novel analytical method that uses dual noble gas tracers – neon and argon – to predict nitrogen gas concentrations, they were able to show that denitrification-enriched inorganic nitrogen accounted for 32 percent of the estimated total nitrogen input across all the sampled springs, despite relatively low average input rates at many individual springs.

“Denitrification’s effect was much larger, and more varied from spring to spring, than previously assumed,” Heffernan said. “That’s a key finding we might have missed if we’d only done this study in a couple of springs.”

Uncertainty about amount of denitrification and how it varies from spring to spring complicates the jobs of water-quality managers, who need reliable estimates of nutrient loading and where it’s coming from if they are to set effective total maximum daily limits (TMDLs) for nitrate levels in the region’s groundwater system.

“Managers in the St. Johns River district, for example, are currently in a TDML process for a number of springs in their area, but to do this they need to know if the nitrogen is coming primarily from farms or cities,” Heffernan said.

The study’s findings and new testing methods should help them and scientists studying nutrient levels and denitrification in other aquifers estimate these variables more accurately.

“Aquifer denitrification is a potentially huge component of regional and global nitrogen budgets – possibly contributing the equivalent of 16 percent of all land-based nitrogen inputs – but scientists have an extremely limited number of direct measurements of it and existing models for estimating it are simple and poorly constrained,” Heffernan said. “Hopefully, this study puts an important new tool in our scientific toolbox.”

The study appears in the open-access, peer-reviewed journal Biogeosciences athttp://www.biogeosciences.net/9/1671/2012/bg-9-1671-2012.html.

Heffernan’s co-authors on the study are Andres Albertin and Matt Cohen of the University of Florida, Megan Fork of Florida International University, and Brian Katz of the United States Geological Survey in Tallahassee. Heffernan conducted the research while he was a faculty member at Florida International University. He joined the Duke faculty earlier this year.

Note: Jim Heffernan can be reached for additional comment at (919) 681-4193 orjames.heffernan@duke.edu.