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.

Stream Loads

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 x C x D

L - Load
F - Conversion Factor
C - Concentration
D - Discharge

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 m odels are used because there is not sufficient data to support direct load calculations.

Concentration Units Flow Units Conversion Factor Load Unit
mg/L cfs 5.39 lbs/day
#/100 mL cfs 284.7 #/second