Septic system waste pervasive throughout Florida’s Indian River lagoon

For more than a decade, fertilizer leaching and associated stormwater runoff were thought to be the major drivers of harmful algal blooms in Florida’s Indian River Lagoon. Despite the numerous residential fertilizer ordinances passed since 2011, water quality, harmful algal blooms, and seagrass loss, which has resulted in mass deaths of the threatened Florida manatee, have continued to worsen.

There are more than 300,000 septic systems permitted in six counties adjacent to the 156-mile-long Indian River Lagoon, which makes up 40 percent of Florida’s eastern coast, and in Indian River and Martin counties, septic systems represent more than 50 percent of wastewater disposal. Five inlets allow the lagoon’s waters to drain into the ocean, potentially impacting another important Florida ecosystem.    

To determine if septic systems in Indian River County contribute to nutrient enrichment of groundwaters and surface waters that discharge into the central Indian River Lagoon, researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute assessed water quality at 20 sites in four Indian River County sub-drainage basins.

For the study, published in the journal Marine Pollution Bulletin, they measured stable nitrogen isotopes in groundwater, surface water, and macrophyte tissue to identify nitrogen sources impacting the Indian River Lagoon. Sucralose, an artificial sweetener that is not completely broken down by septic systems or wastewater treatment plants, was used as a human wastewater tracer, and fecal indicator bacteria density was used as an indicator of wastewater pollution.

Septic system waste pervasive throughout Florida's Indian River lagoon
Margaret “Maggie” Vogel (standing) and Marie Tarnowksi pictured collecting water samples in Indian River County for the study. Credit: Laura Herren

Results reveal that nitrogen enrichment of all sub-drainage basins in this study supports that even “properly functioning” septic systems contribute nitrogen to surficial (shallow) groundwater. Furthermore, shallow ecosystems without a significant source of flushing and dilution, such as the central Indian River Lagoon are more susceptible to inputs from contaminated groundwater. Evidence shows that this issue is likely widespread in the Indian River Lagoon, including its canals, tributaries and rivers.

Groundwater had significantly higher dissolved nutrient concentrations, nutrient ratios and more enriched stable nitrate isotopes than surface waters, indicating septic system-enriched groundwater as a nitrogen source to adjacent surface waters. This finding has implications for nutrient loading and pollution, as submarine groundwater discharge is a primary mechanism for nutrient and microbial transport to coastal waters.


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