There are more than 300,000 septic systems permitted in six counties adjacent to the 156-mile-long Indian River Lagoon (IRL) in Florida. Researchers assessed water quality and measured stable nitrogen isotopes in groundwater, surface water, and macrophyte tissue to identify nitrogen sources impacting the 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. Results showed that nitrogen enrichment of all sub-drainage basins in this study supports that even ‘properly functioning’ septic systems contribute nitrogen to surficial (shallow) groundwater.