Algal blooms and excessive plant growth commonly plague lacustrine  systems due to the buildup of phosphorus.  While phosphorus is not a principal component in  highway runoff, there may be areas where WSDOT is required to reduce phosphorus levels in its  runoff, such as areas where TMDLs have been developed.  Conventional stormwater BMPs do  not capture phosphorus very efficiently, generally having capture rates of less than 50 percent.   BMP designs that target phosphorus removal are needed to meet regulatory mandates in areas  where highway runoff drains to lacustrine systems.  

Continue to evaluate phosphorus removal as part of experimental BMP evaluations.  Conduct technical literature searches to evaluate whether precipitation (as  phosphates or using flocculants such as Catfloc, chitosan, or polyacrylamide) or ion exchange (anionic exchange) unit operations for phosphorus capture is feasible for highway runoff and  linear transportation systems.  Examine low impact development methods, such as bioretention  and pervious pavement, for their ability to reduce phosphorus loadings in runoff.  After completion of Ecology Embankment (EE) monitoring, evaluate the phosphorus capture data and,  if applicable, petition Ecology for use of the EE as a phosphorus control BMP.

Many stormwater BMPs, including source controls such as fertilizer restrictions, have been evaluated for their ability to reduce phosphorus discharges to surface  waters.  Other than infiltration, none have been shown to be particularly effective, and some may  actually export phosphorus, such as compost filters.  The phosphorus control menu in the  Stormwater Management Manual for Western Washington relies heavily on sand filtration,  which tends to have very intensive maintenance requirements.  Historically, stormwater BMPs  were not designed with phosphorus removal as a primary objective, but as a “side benefit”.  The options for phosphorus removal need to be expanded so that sand filtration is not specifically needed.