Water Quality Woes in S.W. Florida Linked to Seeping Septic Systems

Brian Lapointe, Ph.D., stands in front of a canal in Cape Coral in Lee County, Florida.

From fecal bacteria to blue-green algae to red tides, southwest Florida’s water quality has declined as the population has increased. Researchers at Florida Atlantic University’s Harbor Branch Oceanographic Institute have been delving deeply into the region’s deteriorating water quality. Several lines of evidence from their multi-year study tracking microbial sources point to septic systems as a contributing source to this decline.

Coastal areas of Florida are particularly vulnerable to pollution from sewage systems due to shallow water tables and porous soils. However, there are about 39,768 “known” and about 57,054 “probable” septic systems in Lee County in Southwest Florida (about 100,000 in total). To identify sources of pollution contributing to water quality problems, researchers studied the couplings between the sewage system, groundwater and surface water by analyzing various parameters.

Working with the Lee County Department of Natural Resources, researchers measured nutrient and bacterial levels and employed a variety of microbial source tracking tools related to fecal indicator bacteria to distinguish the presence of human and animal feces. They used molecular markers for humans, birds and ruminants (cows, deer, goats, etc.) in conjunction with chemical tracers including the artificial sweetener sucralose, pharmaceuticals, herbicides and pesticides. Sucralose, the over-the-counter pain reliever acetaminophen, and the prescription anticonvulsants carbamazepine and primidone have been used as indicators of human fecal contamination. Stable groundwater and phytoplankton isotopes serve as a reliable indicator of nutrient sources and were measured throughout the study and during Red Tide and Microcystis spp. harmful algal bloom events.

The results, published in the journal Science of the Total Environment, revealed the nutrient and microbial couplings between septic systems, groundwater, surface water and harmful algal blooms in the highly modified Caloosahatchee River Estuary on Florida’s southwestern Gulf Coast and coastal waters downstream. These sources of pollution from human excreta have remained relatively constant over time, with location generally being the most influential factor in water quality. This study is one of the few to link harmful algal blooms downstream to nutrient loading from upstream septic systems.

“This connectivity between septic systems, groundwater and surface water is supported by the significant positive correlations between faecal indicator bacteria and tracers of human waste that we observed in surface water during outgoing tides,” said Rachel Brewton, first author and research associate at FAU’s port branch . “For example, enterococci and E. coli were positively correlated with carbamazepine, primidone, and sucralose, while acetaminophen was weakly correlated with both.”

In each watershed, the researchers found several indications that septic systems negatively affected water quality. Shallow water tables in particular indicated that many of these systems, installed prior to current wastewater treatment plant design standards, did not have the physical separation needed to adequately treat human waste. Due to increased seasonal flood levels, many Florida septic tanks may not meet state regulatory requirements. For example, septic tanks can actually stand in groundwater at certain times of the year, meaning they cannot function properly.

“These water quality issues in North Fort Myers are caused by aging sewer systems installed at high densities in areas with shallow water tables. Additionally, the presence of sewers in these residential areas with septic systems can increase the rate of contaminant transfer from groundwater to surface water through tidal pumping,” said Brian Lapointe, Ph.D., senior author and research professor at FAU’s Ports Department. “Based on our previous research, we hypothesized that waste from septic systems was a major source of nutrient and bacterial contamination in North Fort Myers and therefore location would be a more important factor in water quality than temporal factors such as project year or time of year.”

Indeed, evidence of human waste contamination was observed in all three affected watersheds in the study, including high ammonium concentrations in groundwater and surface water, enriched groundwater and phytoplankton nitrogen isotope levels that closely matched wastewater runoff, and elevated faecal indicator bacteria in surface water with the presence of HF183 , a human-associated marker gene, as well as evidence of sucralose, carbamazepine and primidone in ground and surface water.

In addition, the researchers found the presence of herbicides and pesticides in the surface water, providing evidence that stormwater runoff was affecting surface water quality. In addition, evidence from molecular markers indicated that bird feces may also negatively impact water quality at some North Fort Myers sites. These results may be helpful in understanding water quality and harmful algal bloom drivers in other coastal communities.

“Improvements in stormwater management and citizen education campaigns on issues such as not feeding wild birds, proper removal of animal droppings, and judicious use of fertilizers can be other methods to minimize the impact of urbanization on water quality,” Brewton said.

There are many ecological impacts of nutrient loading into the downstream estuary. Coastal red tide nitrogen isotope levels closely matched those of sewage disposal, suggesting human excreta as the cause of these worsening harmful algal blooms. Poor water quality is threatening seagrasses that the endangered Florida manatee depends on for survival, which is especially important given recent unusual mortality events. In addition, the critically endangered smalltooth sawfish utilizes the remaining natural shoreline along the Caloosahatchee River Estuary as essential habitat. Aside from the environmental and human health impacts, these harmful algal blooms can negatively impact the local economy through increased mortality and reduced sales of commercial seafood and by inhibiting ecotourism activities.

“It would be beneficial if coastal areas with a high density of sewage systems and channels were prioritized for septic tank to sewer conversion or other advanced wastewater treatment options,” Lapointe said. “Furthermore, the susceptibility of these systems to localized harmful algal blooms can be reduced by balancing the ecological stoichiometry of the watershed to achieve a nutrient load and nitrogen-phosphorus ratio in which these microalgae do not thrive, resulting in less harmful algal bloom events.” . As other recent research has noted, the magnitude of the red tide problem requires watershed solutions and nitrogen management based on a holistic view that takes into account both oceanographic and anthropogenic processes.”

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From fecal bacteria to blue-green algae to red tides, southwest Florida’s water quality has declined as the population has increased.

Study co-authors are Lisa B. Kreiger, Superintendent, Lee County Department of Natural Resources; Kevin N. Tyre, an aquatic researcher; Diana Balidi, marine biologist and environmental scientist; Lynn E. Wilking, marine biologist; and Laura W. Herren, bioscientist at the FAU Port Office.

This work was funded by US Environmental Protection Agency 319 administered by the Florida Department of Environmental Protection (FDEP Agreement NF047), with appropriate funding provided by Lee County. Additional support was provided by the Florida Center for Coastal and Human Health, which was developed and continuously funded with a grant from the Harbor Branch Oceanographic Institute Foundation and the US National Aeronautics and Space Administration (NASA) Water Resources Program (80NSSC19K1200).

Brian Lapointe, Ph.D., collects water samples in Lee County, Florida.

Red sargassum on a beach in Lee County, Florida.

-FAU-

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