Keeping Florida's groundwater safe from rising sea levels

A simple truth about Florida's southern coastline is that seawater always wants to reach further inland. Denser than freshwater and more abundant, seawater usually gets its way, seeping into the groundwater supply and mucking it up. Forcing it back can be difficult. The porosity of the Biscayne Aquifer, which supplies most of the freshwater for a large portion of South Florida in the US, from Palm Beach to the Florida Keys, doesn't help matters.

Geologically speaking, at only a million to ten thousand years old, the limestone making up the Biscayne Aquifer is a baby, and it isn’t great at keeping seawater out. "If you hold a piece in your hand, you can basically see through it. It's something like 50-80% porosity," says Michael Sukop, an associate professor at Florida International University in Miami, US, who has been studying the aquifer.

Miamians have dealt with seawater intrusion for years, using hundreds of salinity control structures placed along the canals that originally drained the Everglades — the largest subtropical wetland in the United States. The control structures work with gravity to raise the inland water level in the canals and push back the seawater underneath. Over the years, this method has been fairly successful, but as sea levels rise, many of these salinity control structures won't be able to drain.

With a greater than 90% chance that the global mean sea level will rise at least eight inches (0.2 meters) or more by the year 2100, pumping freshwater out of the aquifer for consumption by Southeast Florida’s 5.5 million residents is a necessity that will only compound the problem.  

Biscayne aquifer limestone. Image courtesy United States Geological Survey and Michael Wacker.

To combat future seawater intrusion, scientists must first learn more about the aquifer and how it works — something that has been elusive even after many years of study. To get a better look at the aquifer's secrets, Sukop and his colleagues worked with Kevin Cunningham of the United States Geological Survey (USGS).

The USGS plays a pivotal role in keeping tabs on the country's water resources, collecting and sharing data that help states and municipalities better understand what's going on with the water supply. With access to current and historical data collected from numerous wells in the Biscayne Aquifer region, the USGS is a valuable resource when it comes to anticipating changes in the region's groundwater supply.

Sukop's team worked with Cunningham to analyze computed tomography (CT) scans of the aquifer's porous material collected from beneath Miami. The scans provide a good picture of the aquifer's construction. They then used computational fluid dynamics simulations to glean flow data from every pore in the rock. 

To visualize how the groundwater flows through karst limestone, Sukop and his colleagues called upon the Texas Advanced Computing Center at The University of Texas at Austin, US, an XSEDE resource and home to the Stampede supercomputer. Using fluid dynamics calculations, CT data, and imaging techniques, they created a 3D visualization of groundwater flowing through the limestone sample. The hope is that understanding how water moves through the aquifer will provide insight into ways to keep seawater out.  

"We're hoping to be able to make better predictions about the effect of sea-level rise and the interaction with the groundwater system to prevent seawater intrusion into the major well fields," Sukop says. "We're trying to learn the truth about the permeability of the Biscayne Aquifer, and it's been something that's been really elusive for a long time," Sukop adds. "I think we're getting closer to it. The available XSEDE resources are remarkable."

- Amanda Aubuchon