The Boston Harbor sewer outfall relocation involves one of the most advanced coastal monitoring efforts ever undertaken, measuring numerous chemical, physical, and biological parameters over greater than 500 square kilometers and totaling over 500,000 data records per year. Geography is critical to separating the effects of the outfall relocation from those of numerous other natural and man-made processes affecting the area. ENSR has implemented an advanced client/server GIS combining ArcInfo with a central large-scale Oracle database to manage the data volume and complexity. ENSR has also used interactive ArcView GIS presentations to interpret and communicate this remarkable natural data set.