Pollution concentrations are expressed as pollutant mass per unit volume of water (e.g. milligrams per liter).
The Concentration of each pollutant in an Inflow can be specified on the Pollution tab. This value is the amount of pollutant contained in the runoff from the inflow.
Inflow concentration values remain constant throughout the analysis and do not vary with time.
The Background Concentration of each pollutant in a Stormwater Control can be specified on the Pollution tab. This is the value below which the pollution concentration within the stormwater control cannot fall during analysis. No further removal occurs when concentration reaches this level, even if a Pollutant Removal Method is specified.
The background concentration only applies to the water inside the stormwater control.
For some stormwater controls (such as Ponds and Tanks) there is an Initial Depth property. If specified, then the background concentration also applies as an initial concentration at the start of the analysis.