Bioretention

Dynamic Study of Heavy Metal Fates in Bioretention 

This research began January 2003 and is now completed.  Pot prototypes were built to simulate the conditions of natural grass growth in bioretention.  Of interest is to evaluate the fate of heavy metals that are captured by bioretention soil media.  Mass balances are used to investigate the amounts of metals that are taken up by grasses in a bioretention situation.  

Key to managing heavy metals in bioretention is understanding their fate in these facilities.  Metals are captured by the soil and other media during runoff treatment events.  The metals may be taken into the vegetation in the time between events.  Four heavy metals, copper, cadmium, lead, and zinc are investigated, with three different grass species, and using a bioretention media consisting of 30% soil, 20% mulch and 50% sand.  Two heavy metal loadings were investigated.

The results show average removals of metals exceed 90% by the bioretention media at both low and high loadings.  The metal contents in the shoots and roots of the three plants vary greatly in different growing phases and the metal concentrations in the shoots decrease from bottom to top.  The results of mass balance calculations show the fates of input metals are 87.5-96.9% captured in soil media, 0.5-3.3% accumulated in plants and 2.0-11.6% not captured by bioretention media. The total metals captured by plants are relatively low due to the lower biomass yields.   Based on field biomass yields and laboratory metal concentrations in plants, it appears possible and practical to achieve metal removals of up to 25% of input by grasses.  If 20% of input metals is accumulated by plants, the lifetime of bioretention cell will be extended by 1.25 times, which show that plants have an important role in prolonging the lifetime of bioretention systems.  

A manuscript describing experimental results and implications is in press.

Bioretention Installations