Soluble Vs. Particulate Phosphorus in Stormwater – Implications for Constructed Wetlands.

Accurate measurement of dissolved versus particulate phosphorus is critical in designing stormwater wetlands BMP’s.  Particulate phosphorus (PP) is often the largest component of the total phosphorus load in stormwater.  In highly-weathered landscapes such as the Piedmont, much of this PP is sorbed to iron oxide coatings on sub-micron sized clay particles.  Since dissolved phosphorus is operationally defined as having passed through a .45 micron filter, this can mean that much of the “soluble” phosphorus measured via standard techniques is actually particulate phosphorus (PP).  Fine-resolution measurement of particle sizes allows us to investigate the mechanisms behind the removal of this PP, since the diameter of these particles determines both their settling velocity (via Stokes’ Law) and the amount of phosphorus sorbed to them (via their surface area).  In this presentation, we put forth a simple mechanistic model of PP removal via gravitational settling of individual mineral particles, based on a continuous particle size distribution.  We then utilize laboratory settling experiments of North Carolina Piedmont stormwater along with flow imaging particle analysis technology to investigate particle settling mechanisms and the relationship between PP and particle surface area.