GIS-based nitrogen removal model for assessing Florida's surficial aquifer vulnerability from onsite wastewater treatment systems

Abstract

Florida's aquifer system exhibits varying hydrogeological characteristics such as shallow depth to aquifer and karst features. These characteristics contribute to spatial variability in ground water vulnerability to nitrogen contamination. The vulnerability of ground water warrants vulnerability studies that allow the zonation of areas more or less susceptible to contamination from land use practices. This study provides a method to identify areas vulnerable to contamination by examining the fate and transport of ammonium and nitrate from onsite wastewater treatment systems (OWTS) through a geographic information system (GIS) based modeling approach. Initial concentrations of ammonium and nitrate will be applied to the soil surface as a uniform blanket application input and as a discharge input from existing OWTS application. The contaminant will undergo fate and transport processes as it percolates towards the water table. These processes are represented by a simplified advection - dispersion equation. The simplified equation ignores the effect of dispersion, assumes steady state, and utilizes a subset of equations to describe the nitrification and denitrification processes through considering first-order reaction, sorption processes, and operational parameters. The operational parameters considered in this model include effluent concentration, hydraulic loading rates, porosity, depth to water table, soil moisture, and soil temperature. The spatially variable parameters used in the calculation are incorporated into the GIS-based model to produce zonation maps illustrating Florida's surficial aquifer vulnerability based on the remaining nitrate concentration reaching the water table. The GIS-based model considers two different contaminant transport models, the single step model and the two step model for both blanket application and existing OWTS application of the initial contaminant. The single step model considers nitrification and denitrification as separate processes while the two step model uses the nitrate concentration converted from nitrification as an input concentration into the denitrification process. In addition, results from the existing OWTS application is symbolized with a probability analysis to determine areas most susceptible to nitrate contamination. The resulting maps from the different modeling approaches are classified into vulnerability classes based on the natural breaks in the data. Areas identified in the vulnerability maps will facilitate planners in making informed decisions on groundwater protection and management.