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

Howard S. Litwack; Joseph L. DiLorenzo; Poshu Huang; and Tavit O. Najarian, M.ASCE

Abstract

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 basin’s 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 rivermonitoring 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.

ASCE Journal of Environmental Engineering, Vol.132, No.4, April1, 2006.