The pollutant loading model was developed by Psomas for Los Angeles County Department of Public Works (DPW). DPW is responsible for monitoring and reporting the quality and loading of storm water pollution flowing to the Pacific Ocean. The 3,100-square-mile permit area is divided into six watershed management areas. This program utilizes ArcView GIS and ArcView Spatial Analyst to calculate the total mass of user-specified pollutants resulting from selected storm events by integrating concentration data, land use maps, and rainfall data. In this paper, application design parameters, GIS tool sets, and model results will be presented along with key technical issues.