Development of a Simple Phosphorus Model for a Large Urban Watershed: A Case Study

Often, the initiation of a total maximum daily load (TMDL) program is delayed until intensive monitoring data can be collected— even in watersheds where large historical data sets exist. This paper provides a case study of a modeling effort that utilizes available historical data to fulfill an intermediate goal of a TMDL program for the Passaic River Basin. The subject model is developed to simulate total phosphorus concentrations (and loads) within the basinʹs effluent-dominated streams. The model is based on the assumption that the primary process controlling in-stream total phosphorus concentrations is the dilution of the cumulative upstream effluent load-which was computed on a continuous (daily) basis. Model comparisons indicate a generally good fit to long-term river-monitoring data at several key sites. Model results, and data analyses, suggest that secondary processes have a relatively minor impact on total phosphorus (TP) concentrations in this relatively large, urbanized system. This finding is consistent with a previous QUAL2E model study of the system, and consistent with the relatively conservative behavior of TP reported in many medium-to-large river systems throughout the United States. Model results are used to facilitate TMDL planning efforts for a major water supply reservoir in the basin.