Septic system-groundwater-surface water couplings in waterfront communities contribute to harmful algal blooms in Southwest Florida

As human population growth has expanded in Southwest Florida, water quality has become degraded with an increased occurrence of harmful algal blooms (HABs). Red tide (Karenia brevis) originating offshore, intensifies in near-shore waters along Florida's Gulf Coast, and blue-green algae (Microcystis spp.) originating in Lake Okeechobee is discharged into the Caloosahatchee River. These HABs could be enhanced by anthropogenic nitrogen (N) and phosphorus (P) from adjacent watersheds. North Fort Myers is a heavily developed, low-lying city on the Caloosahatchee River Estuary serviced by septic systems with documented nutrient and bacterial pollution. To identify sources of pollution within North Fort Myers and determine connections with downstream HABs, this multiyear (2017-2020) study examined septic system- groundwater-surface water couplings through the analysis of water table depth, nutrients (N, P), fecal indicator bacteria (FIB), molecular markers (HF183, GFD, Gull2), chemical tracers (sucralose, pharmaceuticals, herbicides, pesticides), stable isotopes of groundwater (delta N-15-NH4, delta N-15-NO3) and particulate organic matter (POM; delta N-15, delta C-13), and POM elemental composition (C:N:P). POM samples were also collected during K. brevis and Microcystis spp. HAB events. Most (> 80%) water table depth measurements were too shallow to support septic system functioning (< 1.07 m). High concentrations of NH4+ and NOx, up to 1094 mu M and 482 mu M respectively, were found in groundwater and surface water. delta N-15 values of groundwater (+4.7 parts per thousand) were similar to septic effluent (+4.9 parts per thousand), POM (+4.7 parts per thousand), and downstream HABs (+4.8 to 6.9 parts per thousand), indicating a human waste N source. In surface water, FIB were elevated and HF183 was detected, while in groundwater and surface water sucralose, carbamazepine, primidone, and acetaminophen were detected. These data suggest that groundwater and surface water in North Fort Myers are coupled and contaminated by septic system effluent, which is negatively affecting water quality and contributing to the maintenance and intensification of downstream HABs.

Automated summary

As the population in Southwest Florida increases, water quality has deteriorated, leading to a higher occurrence of harmful algal blooms (HABs). These HABs are mainly caused by red tides originating offshore and blue-green algae originating in Lake Okeechobee, potentially intensified by anthropogenic nitrogen and phosphorus inputs from adjacent watersheds. In the heavily developed city of North Fort Myers, septic system effluent contaminates groundwater and surface water, negatively impacting water quality and contributing to the maintenance and intensification of downstream HABs.