As part of the NPDES permitting process, Flow Science performed a comprehensive analysis of the dilution available in Old River for the City of Tracy’s Wastewater Treatment Plant’s effluent outfall. The project encompassed the use of two field studies to calibrate a flow and dilution model for the area.In the field study, an inert tracer (rubidium chloride) was injected into the plant effluent. Water samples from the next few days were analyzed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to accurately measure the resultant concentration of the water in Old River. This study was performed twice to evaluate the effects of the placement of the South Delta flow control barriers.Results from the field study were used to calibrate the effect of the barriers in the Fischer Delta Model (FDM). The FDM was applied to examine the tidal and seasonal fluctuations in the flow of Old River. These calculated flows were then used in a computer dilution model to closely examine the long-tern averages and short-term fluctuations in the dilution pattern in Old River.