Warning: This page is an edit and testing page use by the wiki authors. It is not a content page for the Manual. Information on this page may not be accurate and should not be used as guidance in managing stormwater.

Stormwater pollutant concentrations and event mean concentrations

Models are used to estimating pollutant loading from stormwater runoff and reductions in loading associated with implementation of best management practices (BMPs). An important input in these models is the pollutant concentration in runoff. For example, the Simple Method, which is commonly used to estimate pollutant loading, utilizes the following equation.

$$L = 0.227 P P_j R_v C A$$

where

L = Load of a pollutant in pounds per year;
P = Rainfall depth per year (inches);
Pj = Fraction of rainfall events that produce runoff;
Rv = Runoff coefficient, which expresses the fraction of rainfall which is converted into runoff. Rv = 0.05 + 0.009(I);
I = Site imperviousness (i.e., I = 75 if site is 75% impervious);
C = Flow-weighted mean concentration of the pollutant in urban runoff (mg/l); and
A = Area of the development site (acres).

This page provides a discussion of pollutant concentrations in stormwater runoff, including a review of literature and recommended values to use in stormwater models.

## Determining the appropriate storm concentration

Pollutant concentrations vary within individual storms.

## Defining event mean concentration

Event mean concentrations are typically used in stormwater models and calculations of pollutant loading. The simplest definition of event mean concentration (emc) is the average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L).

Although this seems conceptually simple, accurately estimating an emc for a given pollutant is challenging. Several factors affect emc, including the following.

• the method for calculating emc
• land use
• temporal variability (e.g. season)
• the pollutant and methods for sampling and analysis

These are briefly discussed below.