Describing Nitrogen Sources

Concentrations, Loads, and Yields

Each source of nitrogen entering Puget Sound has both a concentration and a load.

Nitrogen concentrations are the mass of nitrogen per volume of water within a particular source. These are usually measured as parts per million (ppm) or milligrams per liter (mg/L).

Nitrogen loads are defined as the mass of nitrogen per unit of time. These are usually measured in pounds or kilograms per day, and are calculated by multiplying concentrations by flow, such as the treatment plant effluent flow or river streamflow.

A small wastewater treatment plant with a high nitrogen concentration and a low flow rate could have a smaller nitrogen load than a larger plant with lower nitrogen concentrations and a higher flow rate. The same concept applies to loading from rivers and streams.

To compare nitrogen loading from rivers of different sizes, we can also “normalize” the load by dividing this load by the river’s drainage area or watershed area. This gives the nitrogen yield, which is expressed as a load per unit area per time, such as tons per square mile per year (ton/mi2-yr) or kilograms per square meter per year (kg/m2-yr).