Development of a Bioretention Media Mix to Balance Hydraulics and Water Quality

Bioretention devices are a Low Impact Design device used to treat pollutants found in stormwater runoff. Research assessed combinations of manufactured materials readily available in Auckland which create physical and chemical properties of filter media to satisfy objectives for stormwater management. The investigation included: establishing physical characteristics and performance criteria; investigating available materials; particle size distribution testing (PSD); compaction assessment; hydraulic conductivity testing; media chemistry analysis; and water quality testing.

Two commercial media achieved the target hydraulic conductivity with light tamping compaction. Three mixes composed of different fine sands with added compost, satisfied the hydraulic conductivity criteria with wetting and settling compaction or light tamping. These five media were chosen for water quality testing.

For an equivalent of 15 years of pollutant loading, the 90% v/v sand with 10% v/v compost media are capable of removing copper and zinc in synthetic stormwater runoff below 5 µg/L and 10 µg/L respectively, and reduce mass loads by 60% and 70% respectively. Over all, the two commercial media were less effective than the sand based media, however, treatment improved over time as more “runoff” was applied. All five media leached phosphorus over the simulated 15 years. Media chemistry analysis suggests that phosphorus concentrations may be able to be reduced in commercial mixes. The level of phosphorus leaching should be mitigated before media are considered for practical implementation in field trials, unless receiving waters are unlikely to be impacted by phosphorus.