An Iron-Enhanced Rain Garden for Dissolved Phosphorus Removal

Document Type

Conference Proceeding

Publication Date



As a stormwater management technique, rain gardens can reduce runoff volume and retain contaminants such as metals found in urban stormwater runoff. Due to organic material in rain gardens such as plants, mulch, and compost, however, rain gardens have been shown to release dissolved phosphorus, which is typically targeted for removal from stormwater. As a result, rain gardens can act as a source of dissolved phosphorus and cause a corresponding increase in the dissolved phosphorus concentration in runoff. This project assesses the performance of an iron-enhanced rain garden with regards to its ability to retain dissolved phosphorus. Studies have shown that iron-enhanced sand filtration has the ability to remove over 90% of dissolved phosphorus from stormwater runoff. A new rain garden design incorporating a layer of sand mixed with iron shavings at 5% (by weight) beneath typical rain garden media was tested under laboratory conditions for its ability to retain dissolved phosphorus. The performance was compared to a control rain garden that contained a layer of only sand beneath typical rain garden media. Preliminary results indicated that the iron-enhanced rain garden consistently retained approximately 90% or more of the dissolved phosphorus load after more than 61 m of water had infiltrated. The control rain garden initially retained over 50% of the dissolved phosphorus load. However, after the same depth of infiltrated water, dissolved phosphorus retention dropped significantly and ultimately became negative.