Pollutant Concentration is added on the Pollution tab of an Inflow.

Runoff from each area can contain a concentration for each pollutant specified for the area type. Concentration is measured in mg/L, and does not vary in time in the inflow area. The default set of pollutants can be entered through the Pollutants ribbon menu item. 

To specify the pollutant concentrations for an inflow area, open the area's properties dialog and select the Pollution tab.

Initial/background pollutant concentration in Stormwater Controls

Water already in ponds (below the Starting Water Level) can have an initial background pollutant concentration.  This can be specified as part of the Stormwater Control if a Starting Water Level is specified. It may also be left as zero values.