Estimation of Nitrogen Load from Septic Systems to Surface Waterbodies in Indian River County, FL

Excessive nitrogen loading to surface water bodies has resulted in serious environmental, economical, ecological, and human health problems, such as groundwater contamination and eutrophication in surface water. One important source of nitrogen in the environment, especially in densely populated coastal areas in Florida, is due to wastewater treatment using onsite sewage treatment and disposal systems (OSTDS) (a.k.a., septic systems). Moreover, due to the population expansion, nitrogen loads from septic systems are expected to increase. Therefore, sustainable decision-making and management of nitrogen pollution due to septic systems are urgently needed. In this thesis, two software are used to simulate the whole process of nitrogen (ammonium and nitrate) transport starting from septic systems to finally reach the surface waterbodies. One software is VZMOD, and the other one is the ArcGIS-based Nitrogen Load Estimation Toolkit (ArcNLET). VZMOD is seamlessly integrated with ArcNLET in the way as follows. VZMOD is firstly used to simulate the flow and nitrogen transport in the vadose zone, which is between drain field infiltrative surface and water table, based on the assumption of steady-state, one-dimensional vertical reactive transport with constant incoming fluxes of water, ammonium, and nitrate. The ammonium and nitrate concentrations, given by VZMOD at the water table, are then used as the inputs to the modeling of ammonium and nitrate fate and transport in groundwater in ArcNLET, considering heterogeneous hydraulic conductivity and porosity as well as spatial variability of septic system locations, surface water bodies, and distances between septic systems and surface water bodies. In addition, the key mechanisms controlling nitrogen transport, including advection, dispersion, and denitrification, are also considered in ArcNLET. The study sites of this thesis research are the Main-South Canal (MSC) drainage basin and the City of Sebastian located in Indian River County in southeast Florida. Surface water bodies (e.g., rivers and streams) and groundwater at the two site discharge to the Southern Indian River Lagoon, where the ecological and biological integrity has deteriorated in the last several decades due to the decline in water quality caused in part by nitrogen pollution. There are in total 12,741 septic systems in the MSC area, while in the City of Sebastian, the number of septic systems is 4,883. The process of simulating nitrogen reactive transport from septic tanks to surface water bodies consists of the following three steps: (1) based on the site-specific data, such as DEM, waterbodies, septic locations, hydraulic conductivity and porosity, forward models of VZMOD and ArcNLET is developed, (2) based on the measured data of system state variables, such as water level and nitrogen concentration, the forward models are calibrated, and (3) the calibrated models are used to simulate nitrogen plumes and to estimate nitrogen load from the septic systems to surface water bodies. Considering the modeling ability and the site complexity, two questions, (1) what are the nitrogen characteristics of these two sites, (2) can my model be able to capture these nitrogen characteristics, have been investigated in this study, and the major findings are as follows: (1) The simulated nitrogen plumes and load estimates exhibit substantial spatial variability in the both sites, and the depth from drainfields to water table is important to nitrogen reactive transport, especially the ammonium nitrification to nitrate. (2) Ammonium and nitrate loads for the Main-South Canal drainage basin are largely located in the south to the South Canal drainage basin. Along the ditches and canals, the ammonium concentration is lower due to the small distance between water table and drainfields. There exists a region located in the southeast drainage basin where ammonium loading is high. (3) Incomplete nitrification process is exposed under the vadose zone while the denitrification process is mostly complete in the saturated zone in the Main-South Canal area. (4) The nitrification process is largely complete under the unsaturated zone while the denitrification process is incomplete in the saturated zone in the City of Sebastian area. (5) Reduction ratio is lower while nitrogen loading to surface waterbodies per septic system is larger in the City of Sebastian area than in the Main-South Canal area. (6) The flow model calibration in the City of Sebastian area is not as satisfactory as in the Main-South Canal area, because of the simplified assumption that water table is a subdued replica of topography used in ArcNLET is not satisfied at the study site. These results can be used to support the on-going Basin Management Action Plan. More efforts, such as investigating the soil condition (e.g. micro-bacteria content, dissolved oxygen or dissolved organic carbon and pH) and specific septic system environment, are also needed to verify these results and to develop more insights about the nitrogen processes in the study areas.