Dissolved phosphorus in stormwater runoff - sources and management strategies

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Schematic showing analysis for different forms of phosphorus in water. Filtered phosphorus is considered to represent dissolved phosphorus, while unfiltered phosphorus represents all phosphorus. Particulate phosphorus is the difference between filtered and unfiltered.

This page provides a discussion of dissolved phosphorus in stormwater runoff, its sources, and strategies for managing dissolved phosphorus. While the focus is on urban runoff, the basic principles are applicable to agricultural runoff.

Definitions

Phosphorus in water is often classified as dissolved (soluble) or particulate (attached to or a component of particulate matter) phosphorus. This nomenclature is somewhat ambiguous, however, as dissolved phosphorus consists of multiple forms of phosphorus, including phosphorus attached to other materials.

• Dissolved phosphorus is typically identified as phosphorus passing through a 0.45 micron filter. It is this dissolved fraction that is considered to be most bioavailable and most difficult to treat.
• Reactive phosphorus is the phosphorus associated with the test for orthophosphate. It consists mostly of orthophosphate but includes a small fraction of other forms.
• Soluble reactive phosphorus is a measure of orthophosphate, the filterable (soluble, inorganic) fraction of phosphorus, the form directly taken up by plant cells.
• Bioavailable phosphorus is the sum of immediately available phosphorus, which can be transformed into an available form by naturally occurring processes.

References for phosphorus forms and testing includes the following.

• US EPA
• Lake Erie Algae
• City of Boulder, Colorado
• Water Research Center