Recommendations for rain garden media design typically specify high sand content and low organic matter content to promote infiltration and avoid excessive ponding.
This design is effective at infiltrating stormwater and removing solids, heavy metals, phosphorus, and some species of nitrogen; however, the aerobic and low carbon conditions inhibit denitrification, the microbial reduction of nitrate to nitrogen gas which is released to the atmosphere.
As a result, rain gardens typically release nitrate in effluent.