Alison Duffy [Dundee, UK], Juliette O'Keeffe 1, Elizabeth Prosser 2, Kate Heal 2, Dean Bowie 3, James Dalrymple 3
Urban Water Technology Centre, Abertay University1, University of Edinburgh 2, Green Blue Urban3
Introduction Large trees provide financial and environmental benefits, whilst also contributing to the well-being of 80% of urban populations (Armour, et al, 2012). In terms of climate change, planting large species trees will regulate microclimates, filter rainwater, attenuate noise, improve air quality and sequester carbon (UNEA, 2011). According to Warwick and Charlesworth (2013) one urban tree sequesters approximately 100kg of carbon per year. Armosa et al (2013) claim that tree pits reduce runoff by 60% when compared to asphalt. Sustainable drainage systems (SUDS) are common in Scotland. However, it is argued that many have not resulted in quality of place (TDAG, 2014). The general position of the Scottish Environment Agency is that whilst SUDS are required for new developments and redevelopments, the retrofit of proprietary SUDS is not recommended unless the functions of treatment, infiltration and flow attenuation are fulfilled.
A monitoring programme in collaboration with Dundee City Council for a novel SUDS proprietary product which is combined with a large species tree pit has been in place since 2012. The system, retrofitted to a car park and discharging to a combined sewer system, has been engineered to provide a healthy environment for tree root growth and prevent pavement damage (Lucke et al 2011). Benefits over and above those stated above include: stormwater attenuation which increases over time as the tree matures; downstream SUDS footprint reduction; reduced pumping and treatment costs in combined sewer areas; and hydrocarbon removal. The system consists of various underground components; of note are the SUDS troughs that act as a storage reservoir during times of drought. The top section of the troughs is lined with a semi-impermeable membrane and filled with Leca (expanded clay balls) to filter runoff before entering the system.
Research Undertaken The study investigates the robustness of the system and aims to validate that it is fit for purpose. The paper will discuss retrofit limitations, initial monitoring results for water quality, tree growth and vigour. The system has been adapted from sheet to piped flow in order to collect car park runoff entering the system and treated runoff leaving the system. Initially, grab sampling was used to collect water samples but limitations of this technique (reactive sampling following rainfall) meant that it was difficult to respond quickly to storm events. Flow loggers and automatic samplers are now installed at the inlet and outlet. Parameters analysed include: Heavy metals (Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Zinc); Total Petroleum Hydrocarbons; pH; BOD, Suspended Solids; Conductivity; Chloride; Phosphate; Nitrogen. A control tree has also been planted in nearby public open space to enable regular health monitoring for both trees by recording tree heights, crowns and girth sizes.
Results To date average concentrations for all parameters except chromium discharged from the system are within the EU Environmental Quality Standards for Priority and Dangerous Substances (Directive 2006/11/EC). On average BOD is reduced by 40%, P 16%, N 76%, TPH 100%, Arsenic 100%, Copper 26%, Nickel 66%, Lead 31% and Zinc 10%. Cadmium concentration has always recorded below the limit of detection. Chromium concentration increases on average by 50% which exceeds the threshold by 42%. A one off analysis of settled sediments in the trough water, the membrane and Leca was undertaken in July 2014 to give an indication of uptake of pollutants in these materials / components. From the results it is evident that the Leca and membrane are retaining pollutants associated with sediments from entering the system -- the filter media more so than the clay balls. Sediments that are settling on the base of the trough are also retaining pollutants. It is too early to comment on tree health but measurements taken to date initially indicate that the tree in the tree pit is growing at a faster rate than the control tree. System hydraulics has still to be analysed in detail to ascertain attenuation performance.
Conclusions and Recommendations From a water quality perspective, overall the system is performing well and complies with EU thresholds for water quality standards apart from chromium. This is a brownfield site that is located where a textile mill operated and there may be historical influences to consider. There is the likelihood that legacy contaminants such as chromium remain in the environment as this used to be a key component for the dyeing process. The next steps to validate the system are to quantify attenuation performance and continue water quality and health monitoring. 1. Armsona. D., Stringerb. P., Ennosa. A.R. (2013). The effect of street trees and amenity grass on urban surface water runoff in Manchester UK. Urban Forestry & Urban Greening 12 (2013) 282Â–286.
2. Armour, T., Job, M. and Canavan, R. (2012). CIRIA Report C712: The benefits of large species trees in urban landscapes: a costing, design and management guide. London.
3. Lucke, T., Johnson, T., Beecham, S., Cameron, D., Moore, G. (2011). Using permeable pavements to promote street tree health, to minimize pavement damage and to reduce stormwater flows. 12th International Conference on Urban Drainage, Porto Alegre/Brazil, 11-16 September.
4. Trees & Design Action Group (TDAG) (2014). Trees in Hard Landscapes A Guide for Delivery - Consultation Draft. May 28. www.tdag.org.uk
5. UK National Ecosystem Assessment (2011) The UK National Ecosystem Assessment Technical Report. UNEP-WCMC, Cambridge.
6. Warwick, F., Charlesworth, S. (2013). Sustainable drainage devices for carbon mitigation. Management of Environmental Quality: An International Journal, Vol. 24 Iss: 1, pp.123 - 136