Nutrients that fueled Pahokee algal bloom came from the basin inside the dike

Posted 5/7/21

The toxic algal bloom in the Pahokee marina is gone.

But … could it happen again?

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Nutrients that fueled Pahokee algal bloom came from the basin inside the dike

Posted

PAHOKEE – The toxic algal bloom in the Pahokee marina is gone. The most recent tests by the Florida Department of Environmental Protection show water quality is good ... for now.

But questions remain.

What fueled the massive blue-green mass of cyanobacteria that released toxins at more than 100 times the level deemed safe by the World Health Organization?

Could it happen again? The marina has a history of seasonal algal blooms.

The Palm Beach County Water Utilities Department sampled water from the marina on April 30. Tests showed medium levels of E. Coli bacteria and high levels of nitrogen and phosphorus in the water in the boat basin.

Nitrogen levels in the boat basin were more than double the level of Nitrogen in water sampled from Lake Okeechobee just outside the marina.

This map shows where the samples were taken.
This map shows where the samples were taken.

The phosphorus levels were also troubling. Phosphorus levels in the lake water vary -- with lower phosphorus levels in the marshes where the plants clean the water and higher levels in the center, deeper water, especially if the muck on the bottom of the lake has been stirred up. On average is around 140 parts per billion. The phosphorus level in the area with the boats was 432 ppb.

Cyanobacteria, also called blue-green algae, feed on phosphorus and nitrogen. Some cyanobacteria can “fix” nitrogen from the air. Microcystis aeruginosa, the dominant species found at the Pahokee marina in the DEP tests, is not a nitrogen fixer. It must have nitrogen in the water in order to reproduce into an algal bloom.

Where did the excess nitrogen, phosphorus and the E. Coli come from?

The source of that nutrient load must be inside the basin, according to the U.S. Army Corps of Engineers. Not only is that area inside the Herbert Hoover Dike, there is a seepage wall within the dike that prevents water flow through the earthen berm.

According to the corps geologist, on May 7 the Lake Okeechobee water surface elevation was 13.95 ft above sea level. To the east in the town of Pahokee, USACE wells show groundwater levels that are about 5 to 6 feet below land surface, or at elevations of about 11 feet above sea level. That means that the lake currently is recharging the aquifer, and groundwater flow direction is to the east/southeast.

In order to reach the Pahokee marina, groundwater from farmlands would have to flow west and uphill, and even then it would be blocked by the seepage barrier, the geologist explained.

The seepage barrier within the Herbert Hoover dike is about 3 feet thick and extends through the dike and into the earth below down to about 30 to 40 feet below sea level. The wall was constructed as part of the Herbert Hoover Dike repairs.

The seepage wall was constructed to prevent the lake water from flowing east/southeast through the dike, eroding the dike. Before the seepage wall was constructed, this was a problem when the lake level got higher due to the water pressure against the side of the dike.

In the unlikely event water outside the dike in Pahokee  rose higher than the lake level, the barrier would also prevent water from flowing west/northwest through the dike.

Another source of nutrient load could be runoff from fertilizer. That didn’t come from the dike, according to Erica Skolte of the U.S. Army Corps of Engineers. The grass on the dike is a Florida-friendly variety that does not require fertilizer.

“Construction contractors may use fertilizer on the Herbert Hoover Dike to re-establish grass for erosion control immediately following construction, but the corps maintenance team and Goodwill contractors do not apply fertilizers,” she explained. The last time work was done on the dike near Pahokee was in 2012.

PAHOKEE -- The seepage wall within the Herbert Hoover Dike was completed in 2012.
PAHOKEE -- The seepage wall within the Herbert Hoover Dike was completed in 2012.

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