GIS-based nitrogen removal model for assessing Florida’s surficial aquifer vulnerability

Florida’s aquifer system exhibits spatially variable hydrogeological characteristics including shallow depth to aquifer and karst features. These characteristics contribute to groundwater vulnerability to nitrogen contamination and thus warranting vulnerability studies that allow zonation of areas into different levels of susceptibility to contamination from land use practices. A geographic information system (GIS)-based nitrogen fate and transport model (GIS-N model) was developed to assess aquifer vulnerability to contamination by examining the fate and transport of ammonium and nitrate from onsite wastewater treatment systems (OWTS). The GIS-N model analyzes fate and transport of nitrogen through the unsaturated zone using a simplified advection–dispersion equation. Operational inputs considered in this model include wastewater effluent ammonium or nitrate concentration, hydraulic loading rates, and OWTS locations. The GIS-N model considers two different modeling approaches: single step and two step. The single-step model considers a denitrification process assuming all the ammonium is converted to nitrate before land application, while the two-step model uses ammonium as an input and considers nitrification followed by denitrification. The resulting maps were classified into vulnerability zones based on the Jenks’s natural breaks in the data histogram. It was revealed that groundwater vulnerability from OWTS is sensitive to the depth to water table, first-order reaction rates, and parameters controlling the time and amount of conversion. Nitrate concentration is highest in areas with shallow water table depth. The vulnerability maps produced in this study will facilitate planners in making informed decisions on placement of OWTS and on groundwater protection and management.