Application of floating vegetative pads (FVP) to improve stormwater quality : a pilot scale study

Abstract

Henderson Creek contributes one of the largest load of sediments & heavy metals (e.g. Copper - Cu and Zinc - Zn) into the Central Waitemata Harbour, Auckland. Cu and Zn particles do not decompose so they are persistent, accumulating on sediments, in filter-feeding shellfish and in plants, and therefore, aquatic health is affected by turbidity and that degrades stormwater pond ecosystem. It is the one of the key item of the Auckland City Council agenda to reduce Cu and Zn in urban storm water detention ponds in order to protect & improve the aquatic ecosystems’ health of stormwater ponds in Auckland Region.

A mesocosm study was conducted at Unitec to investigate the performance of Floating Vegetative Pads (FVP, planted with native plants) in terms of their ability to remove heavy metals, particularly, Cu and Zn and the particulates from the storm water detention pond in Hilwell Drive, Henderson. The eight treatments were compared in this experiment i.e. a floating polystyrene pad on its own (treatment G), a floating polystyrene pad with artificial roots (treatment H), and six floating polystyrene pads with six different native plant species (i.e. ApodasmiaSimilis – treatment A, Deschampsia Caespitosa - treatment B, FiniciaNodosa - treatment C, Hierocloe Redolens - treatment D, Lachnagrostis Billardierei - treatment E, PoaAnceps Blue - treatment F) in six individual buckets).

Storm water samples were collected in the buckets from the studied pond, and then analysed for pH, Cu and Zn. Plant growth of the six native plants used in this experiment were measured by an increase in their wet mass from the start (day 0) until the end of experiment i.e. day 21.

Among all the treatments, B and E treatments removed total Cu (i.e. both dissolved and particulate forms - mg) by 30%. Treatment B and F removed the most total Zn (Zn both in dissolved and particulate forms) by 60% and 50%, respectively. It is not clear why treatment D ended up with more Cu and Zn as compared to the initial values, and this requires further investigation. Although the treatments G and H (i.e. without and with artificial roots) removed Cu and Zn by 20%.

The study showed that treatment E had almost 60% increases in wet mass (i.e. increased from 98.5 to 157.5 g/d). The pH of all treatments except treatment G reduced from 7.35 to 6.45. The drop in pH levels could be due to the bacterial activity happening in the rhizosphere, which releases rhizo deposits and that can drop pH.The treatments E and F had the most area daily Cu-mass removal rates i.e. 0.074 and 0.082 mg/m2/d (i.e. 7.4 and 8.2 mg/100 m2/d), respectively. Whereas, treatment B performed well in terms areal daily Zn-mass removal rate of 0.496 mg/m2/d (i.e. 49.6 mg/100 m2/d).