Pollutant loads

What is the difference between measuring the concentration of a pollutant and knowing what the load of the pollutant is?  

Imagine you have a gallon of water and put five tablespoons of salt in it, the resulting concentration would be five tablespoons per gallon (5 tbs/gal). Now imagine you have a two-gallons of water and add ten tablespoons of salt, the resulting concentrations would still be (5 tbs/gal). Although in the second case, the concentration of the pollutant is the same as it was in the first case, the total amount of the pollutant (load) is twice as high.  

Consider now a stream in an area where flow is low during summer and high during winter. Let’s say winter flows are five times higher than summer flows and the nitrogen concentration is the same during the winter and summer. If you only considered the concentration you might conclude that there was no difference in pollutant levels through the year. However, there is five times more water in the stream during the winter, so there is five times more nitrogen being transported during the winter.  The stream contributes five times the nitrogen load then it contributes in the summer, even though concentrations are the same.

Calculating pollutant loads
Loading is a simple function of concentration and flow. Loading can be reported in a number of different units and can be calculated as shown in the table.  L = F ´ C ´ D
Where:  L = load  Note: This is a simple example of load calculations;
however loads are often complicated by other factors
that influence the conditions of the watershed.  The
best case scenario is to determine loads from actual
data but more often models are used because there is
not sufficient data to support direct load calculations.
  F = unit conversion factor 
  C = concentration 
  D = discharge 

Concentration units

Flow units

Conversion factor

Load unit





#/100 mL




Click-Here to download a spreadsheet tool that estimates a variety pollutant loads and HERE for a tool specific to septic systems.
Additional Resources 
  1. Characterize the Watershed - Estimating Pollutant Loads
  2. Discharge Monitoring Report (DMR) Pollutant Loading Tool
  3. Load Duration Curve Online provides an interactive process to develop duration curves, which identify pollutant loading capacities relative to a target concentration
  4. STEPL on-line is a mapping interphase program that generates watershed information; once you have this information you can calculate loads using models (STEPL and Region V Models)
  5. Tracking TMDLs (Guide for evaluating proposed restoration plans) 
  6. Pollutant load estimation for water quality projects (USEPA: NPS Monitoring Program Technical Notes)