Numerical Modeling of the Effect of Tidal Dynamics on Nutrient Release from Septic Tanks and its Impact on Florida's Groundwater and Coastal Zones

Coastal environments in Florida are becoming increasingly contaminated, particularly with nitrogen pollution from septic systems, which poses serious threats to ecosystems and human health. This study investigated the influence of tidal dynamics on nutrient release from septic tanks and its impact on groundwater and coastal zones using a coupled 1D WAVE (Water and Agrochemicals in the Soil, Crop, and Vadose Environment) model for the vertical flow and transport through the vadose zone and 1D Advection-Dispersion horizontal model for the flow and transport through the saturated zone. Nielsen's analytical solutions were employed to evaluate groundwater velocity variations in response to tidal propagation, with a specific focus on the vertical beach solution. The study analyzed variations in water levels and groundwater velocity at different distances from the coast. The linear flux term in Nielsen’s equation represents the hydraulic gradient from the aquifer to the coast controlling the groundwater flow to the tidal waterbody (e.g., sea, lagoon, or tidal rivers). Groundwater fluctuations influenced concentration levels near the waterbodies, with lower concentrations during peaks of the groundwater variation and higher concentrations during negative peaks. The consistency of this relationship vanishes with increasing distance from the waterbodies. After 10 days of simulation, concentration levels stabilized and reached a tidally averaged state. Several parameters, including tidal range, effective porosity, hydraulic conductivity, linear flux term, and distance from waterbody to soil column, were subjected to sensitivity analysis, and the net nutrient flux was calculated for each of these parameters. Higher tidal ranges, hydraulic conductivity, and linear flux terms were found to facilitate greater nutrient transport to the waterbody. Higher effective porosity, on the other hand, reduced nutrient movement, resulting in lower net nutrient flux to the waterbody. Furthermore, the closer the septic tank is to the waterbody, the greater the net nutrient flux to the waterbody, and the farther the septic tank is from the waterbody, the less nutrient flux will reach the waterbody. These findings have important implications for nutrient transport from septic systems to groundwater and coastal zones, which will aid in the development of environmental management strategies to protect Florida's water resources.