A new ArcGIS-based software of uncertainty analysis for nitrate load estimation.

Domestic waste water treatment using Onsite Sewage Treatment and Disposal Systems (OSTDS) (also known as septic systems) is a source of nitrogen contamination in groundwater and surface water. Estimating nutrient load from septic systems as a storm water source to surface water bodies is important to reduce nutrient load in support of Total Maximum Daily Load (TMDL) programs. Software packages of nitrate load estimation using geographic information systems (GIS) have gained popularity, not only because a GIS is efficient to process and integrate spatial data but also because skills required for applying GIS-based software are widely available. Recently, Rios et al. (2013a) developed an ArcGIS-Based Nitrate Load Estimation Toolkit (ArcNLET) to simulate nitrate transport in groundwater and to estimate nitrate load from septic systems to surface water bodies. However, the load estimates are inherently uncertain, and impacts of the uncertainty on TMDLrelated decision making have not been well studied. Uncertainty analysis for GIS-simulated results is of particular importance, because GIS is frequently used as a decision support system. Conducting uncertainty analyses within GIS environments has become popular, owing to recent development of GIS tools and technologies for coupling GIS with methods and tools of uncertainty analysis. This paper presents a new software package, ArcNLET-MC (MC stands for Monte Carlo), developed by Rios et al. (2013b) to analyze uncertainty in nitrate