Simulation of Nitrogen Transport in Surficial Aquifer and Estimation of Nitrogen Load from Septic Systems in the Indian River Lagoon Area, Florida

摘要

Onsite sewage treatment and disposal system (OSTDS) or septic systems, is one of the significant sources of nitrogen in groundwater and surface water, poseing serious threats to human health and the environment. The ArcGIS-based Nitrate Load Estimation Toolkit (ArcNLET) has been used to simulate nitrogen transport and estimate nitrogen load from septic systems to surface waterbodies through the surficial aquifer. It is based on a simplified conceptual model of groundwater flow and nitrogen transport. This study utilizes the ArcNLET tool for the Main-South Canal (MSC) drainage basins of Indian River County (IRC) area which discharge to the Indian River Lagoon (IRL), FL. The purpose of this study is to estimate the nitrogen load from 12,735 septic systems to the IRL and other surface waterbodies in MSC drainage basins. This study found, all the waterbodies (574 in number) including the IRL have been receiving a total nitrogen load of 13,742 lbs/yr. Twenty out of the 574 watebodies receive 50.5% (6,802 lbs/yr) of the total load. Among the 12,735 septic systems, only 358 septic systems contributed nitrogen load directly to the IRL, which is 2.01% (271 lbs/yr) of the total load. It is also found that the estimated ground water load of nitrogen to the southern drainage basin is greater than the estimated septic tank load to the main drainage basin. The estimation of nitrogen load exhibits spatial variability, which is useful to facilitate decisions on the sewering of residential areas served by OSTDS to reduce nitrogen load to the canals and the IRL. The modeling results may change when new data are available for model calibration.