A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 3 - Streams

Pollutant Concentrations Versus Pollutant Loading

Before discussing data interpretation it may help to understand the difference between measuring the concentration of a pollutant and knowing what the load of the pollutant is. Imagine you have a liter of water and put five tablespoons of salt in it, the resulting concentration would be five tablespoons per liter (5 Tbsp/L). Now imagine you have a two-liter jug of water and add ten tablespoons of salt, the resulting concentrations would still be five tablespoons per liter (5 Tbsp/L). Although in the second case, the concentration of the pollutant, in this case salt, is the same as it was in the first case, the total amount of the pollutant – the load – is twice as high. Sometimes this pollutant load is more important information than the pollutant concentration.

Consider now a stream in the Puget Sound area where flow is low during summer and high during winter. For example, let’s say winter flows are ten times higher than summer flows and the phosphorus concentration is the same in the stream during both winter and summer sampling periods. If you only considered the pollutant concentration you might be tempted to conclude that there was no difference in pollutant levels through the year. BUT, there is ten times more water in the stream during the winter, so there is ten times more phosphorus being transported by the stream during both winter. During the winter the stream contributes ten times the phosphorus load it contributes in the summer, even though pollutant concentrations are the same.

Likewise, think about a lake that has two inflowing streams. Stream A has ten times the flow of stream B, but both have similar concentrations of nutrients. Stream A is contributing ten times more nutrients to the lake than Stream B – it constitutes ten times greater load.

Although State standards and pollution indices are by necessity based on pollutant concentrations, in the cases of streams, pollutant loads provide much more comparative information for assessing the level of impact. Pollutant loads are a function of pollutant concentrations and streamflow. Pollutant loads can be calculated for nutrients, TSS, and fecal coliform bacteria. Chapter Five provides a detailed example of how to calculate a pollutant load.

The next section provides information about how to develop a stream monitoring program.

Stream Temperature | Stream Dissolved Oxygen | Stream pH | Stream Nutrients | Stream TSS and Turbidity | Stream Fecal Coliforms | Return to Table of Contents