Floating Treatment Wetland influences on the fate of metals in road runoff retention ponds
2. Materials and methods
2.1. Site description
The study site and instrumentation have been described elsewhere (Borne et al., 2013a). Briefly, the experimental site is a stormwater retention pond located north of Auckland, New Zealand, and collects the runoff coming from a highway. The catchment is approximately 1.7 ha (75% impervious). The retention pond has been bifurcated into two straight-walled parallel sections (∼100 m2 each) with a permanent water depth of 0.75 m, in order to allow a side by side study. The accumulated sediments were dredged before refilling the pond, leaving a thick clay layer at the bottom. In one partition (FTW pond) an approximately 50 m2 FTW planted with Carex virgata (∼17 plants/m2) was installed in December 2010 (summer in the Southern Hemisphere) ( Fig. 1). The other partition remained as a conventional retention pond, to serve as a control (Control pond- Fig. 1).
2.2. Physico-chemical parameters measurement and water column sampling
DO, Eh, specific conductivity and pH were measured during dry weather in both ponds on 8 occasions: one week after the FTW was installed and then at approximately 3-monthly intervals (always at similar time in the morning). Fourteen and six measurements were made for each mission in the FTW and Control ponds, respectively, except during the last mission where seven measurements were made in the FTW pond. Detailed methodology is described elsewhere (Borne et al., 2013a). During the last sampling mission, one sample was collected below the FTW and in the middle of the Control pond and analysed for Chlorophyll A according to method 10200 H (APHA, 2011). Continuous temperature and DO monitoring was carried out below the FTW (at 10 and 40 cm depth) and in the Control pond (at 40 cm depth) at 15 min intervals using a D-Opto logger (Zebra-Tech Ltd, Nelson, NZ). Due to a thick deposit of sediment and bacterial development on the optical window of the permanently installed D-Opto logger in the Control pond in summer, readings became inaccurate and had to be discarded for this period of time.