Constructed Ponds for the Treatment of Urban Stormwater - Biotic Processes Influencing the Removal of Nitrogen, Phosphorus and Carbon

Stormwater ponds for the treatment of urban runoff are being increasingly used as ‘treatment’ technologies in the management of urban stormwater world wide. Although stormwater ponds have been proven to be effective at reducing the concentration of nutrients from urban stormwater, the exact processes leading to this reduction remains largely unknown and unquantified. With stormwater management fast becoming a managed and mitigated water quality issue, the importance of our understanding on how these ecosystem function increases. A stormwater pond system designed and built for the treatment of urban runoff in 2001 was used as the study site for the PhD investigation. The primary goal of the PhD study was to investigate and quantify biological processes leading to N, P, and C load reduction within the inlet and outlet ponds at the Bridgewater Creek Stormwater Treatment System (BWC System) in Brisbane, Southeast Queensland Australia. The BWC System consists of 6 interconnected ponds spanning a total surface area of 0.8 ha, and draining 198 ha catchment consisting almost entirely of residential, single lot development. The initial sedimentation basin (Pond 1) and the final polishing pond (Pond 6) of the BWC System were the focus of the investigation. Based on rainfall runoff rates, 29.5% of the catchment feeding into the stormwater treatment pond system was classed as ‘directly connected impervious area’, with each storm event delivering a mean 18.3ML of urban runoff. Data showed that storm events generating greater than 9.89 mm of rainfall produced runoff volumes greater than the storage capacity of the BWC System, thus triggering the high flow bypass channel. During the study period (2003-2005), 56.9% of all generated stormwater bypassed the BWC System, attributed to the small size of the pond system compared to its catchment drainage area. Within Ponds 2-6 of the treatment system, stormwater was retained for up to 9 days. Nutrient load reduction within the pond system was calculated at 50%, and 16% for TN and DOC respectively, but TP loads increased by 30% within the pond system. The export of TP loads greater than input for the given events within the BWC System was attributed, in part, to sediment resuspension and transportation due to the lack of significant macrophyte stands to stabilize sediment within the system. Possibly a result of this, the BWC System rarely achieved TP water quality objectives, set by the Brisbane City Council, but TN, Org-N, NH4, NOx and PO4 objectives where achieved more than 40% of the time.m Using Principal Component Analysis, three factors where found to significantly influence the variability of water quality within the pond system on a temporal scale. That being, ‘storm events’, ‘phosphorus dynamics’, and/or ‘DIN dynamics’. These factors effectively describe the abiotic or biotic variables driving water quality within the ponds at any given point in time. Key physicochemical parameters driving water quality in the ponds included pH, redox potential and DO. Water quality was similar between pelagic and littoral zones of both ponds within the BWC System. The phytoplankton community within the BWC System was characteristic of polluted freshwaters, dominated by mixotrophic green algae. Cyanobacteria blooms within the ponds was minimal, with one bloom of Anabaena spp. occurring in Apr-04. Phytoplankton production rates were indicative of eutrophic freshwater environments (0.6-20 gC m? ² day? ¹), with production limited by light. Bacterioplankton production far exceeded that of phytoplankton production (8-210 gC m? ² day? ¹) during all incubations, defining the pond system as net heterotrophic. These high rates of bacterioplankton production coupled with the phytoplankton production suggested that the mineralsation of gross particulate organic matter entering the system during stormevents may be providing the nutrients nessasary to fuel such high production rates. Within Pond 1, nutrient movement across the benthic/water column interface were calculated at 0.73, 0.37 and 0.47 mg m? ² day? ¹ for TN, TP and DOC. In Pond 6, there was a gross loss of nutrients from the water column to the sediment zone for TN and TP (0.24 and 0.03 mg m? ² day? ¹) but not for DOC (sediment to water movement, 0.17 mg m? ² day? ¹). As in Pond 6, if sunlight can penetrate the benthos and gross particulate matter is low, Benthic Macro Algae can inhabit the benthos and enhace nutrient loss from the water column. During the mesocosom experiments the Ceratophyllum/epiphyton complex exhausted ammonia concentrations within the incubation jars and reduced PO4-P to background concentrations. Attached epiphyton and phytoplankton community displayed similar characteristics in PO4-P concentration reduction from the water column, with biotic uptake within the incubation jars limited to 0.03 and 0.02 mg L? ¹ day? ¹ respectively (0.14 mg L? ¹ day? ¹ for Ceratophyllum/epiphyton complex). From these incubation experiments, the important role of submerged macrophyte communities in nutrient reduction from the water coloumn was proven. The research presented in this thesis justifies the use of stormwater ponds for urban stormwater management. The ability of submerged macrophytes at enhancing nutrient removal from stormwater was demonstrated. Phytoplankton and Bacterioplankton production and associated inorganic N and P uptake within the pond system did not correspond to measured and calculated storm flow inputs, indicating a large ‘unmeasured source’ – assumed to be the input of large particulate organic matter into the system during storm events.