Green infrastructure in urban areas can help to improve the quality of water in urban areas. Urban vegetation provides storage and interception of rainfall at the source and can reduce diffuse pollution by enhancing sediment retention. Also, trees and woodland are effective at intercepting aerial drift of pesticides that would otherwise enter watercourses.
The provision of high quality water is essential for health and survival, and the quality of the water in our rivers, lakes and estuaries is an indicator of the quality of the local environment. Good quality waters also have recreational value, enhance biodiversity and help reduce the costs and environmental impacts, such as from energy use, of treating water. Over the last few decades, legislation has increasingly addressed water pollution, and encouraged large-scale investment that targets polluting processes in industrial sectors. This has brought substantial improvements in the biological and chemical quality of rivers, as some of the most acute sources of pollution, such as sewage treatment works and sewer overflows, have been addressed.
However, the quality of water that flows through an urban catchment can be severely impacted due pollutants and detritus collected from high speed runoff from urban surfaces and reduced infiltration of precipitation. Additionally, many urban areas have combined sewerage and stormwater collection systems from which overflows, due to high rainfall events, adversely affect water quality.
The majority of water bodies in the UK currently fail to meet the targets of the EU Water Framework Directive (WFD) due to diffuse pollution and other pressures. Therefore the use of green infrastructure as a mechanism for improving water quality has been suggested as a cost effective and environmentally friendly option to improve water quality.
The sewerage and waste water infrastructure supporting our towns and cities is mostly old. Investment is needed in the supporting infrastructure to prevent decline in water quality and reduce the risk of local flooding. Water resources are already stretched in some parts of the country and increased consumption and housing will increase the demand on this resource.
However, under climate change projections annual rainfall is set to increase in some areas of the UK, and as a result, flooding events are predicted to increase and this is likely to lead to further decline in water quality. Therefore measures are required in order to mitigate the effects of this increase. The use of sustainable urban drainage systems (SUDS) to intercept and filter flood waters would reduce the quantity of water reaching water treatment facilities.
Green infrastructure helps to treat grey water and sewage through the use of reed beds, living machines and other technologies. However, there are issues of social perception with regards to using SUDS for urban drainage due to questions of safety for children. Also, there are maintenance concerns for ponds and swales which over time may become clogged with silt. Therefore SUDS for improved water quality and reduced flooding require maintenance and appropriate safety measures such as warning signs and lifejackets.
SUDS can be used in control of pollution and for sediment retention and green roofs also provide pollutant retention potential. The surface runoff from extensive green roofs on multi-storey buildings have been found to have lower chemical variables in than from the conventional roof. However, total phosphorus concentrations in runoff can be significantly higher than the conventional roof due to leaching from the growth media. The results highlight the need to manage green roofs carefully to minimise leaching of nutrients and other contaminants while maintaining their ability to support plant growth.
The use of trees in urban and peri-urban areas can provide significant water quality benefits. Urban trees provide all of the functions associated with SUDS, including the storage and interception of rainfall at source, filtration of pollutants in the canopy, and infiltration at the root zone, along with amenity and ecological benefits. Urban forests are likely to provide more benefits through water quality protection than flood control. The potential benefits of unmanaged and well-managed woodlands to water quality have been formally recognised by local government in Great Britain and country forestry strategies reflect the potential of woodland to deliver Water Framework Directive (WFD) objectives, including highlighting opportunities for woodland to reduce the impact of diffuse pollution from agriculture and urban activities (alongside flood management). These benefits have also shaped European policy, with a specific Resolution on Forests and Water adopted by the Fifth ministerial conference for the protection of forestry in Europe. This recommends action across Europe to better co-ordinate policies on forests and water, and to incorporate an economic valuation of water-related forest services.
The benefits of protection forests and sustainable forest management for water quality are increasingly recognised as a key ecosystem service, and woodland is being created to safeguard the water environment. While there may be water trade-offs in terms of the potential for forests to reduce water yield, these are usually more than compensated for by the water quality and other ecosystem services provided by forests, for carbon, landscape, biodiversity and recreation.
Water in rivers and streams in the UK must conform to the EU Water Framework Directive. The Water Framework Directive has the following key aims:
- Water protection to all waters, surface waters and groundwater
- Achieving "good status" for all waters
- Water management based on river basins
- "Combined approach" of emission limit values and quality standards
- Getting the citizen involved more closely
- Streamlining legislation.
The Directive requires the production of a number of key documents over six year planning cycles. Most important among these is the River Basin Management Plans, published in 2009, 2015 and 2021.
Under the EU Water Framework Directive (WFD), water quality assessments are being published using a tough methodology. WFD monitoring, known as classification, is risk-based and focuses where there is likely to be a problem.
The impact of a three-fold increase in woodland cover from 9% to 27%, within Greenwood Community Forest, Nottinghamshire, was estimated to reduce annual recharge and runoff by 11% (over a 24 year period).
Urban development in the River Quaggy catchment, Lewisham, London has increased the likelihood of flooding events and the traditional mechanism for reducing flood risk was to widen the river channel. This had proven unsuccessful and therefore it was suggested that green spaces along the river network be used to store water during peak flows and also facilitate a more natural and attractive meandering river. This reduces storm water run-off to Lewisham Town Centre as well as providing improved recreational parks for local communities and more variation in habitats that encourages greater biodiversity.
The Mersey Forest and Weaver Valley Initiative. There has always been the risk of flooding on the Weaver particularly in Northwich and this is likely to increase with global warming. Restoring floodplains and recreating wetlands in the Weaver Valley will reduce flood risk in downstream areas and improve water quality. Habitats such as reed beds, fens, ponds wet woodland and grasslands can be created.
Defra (2008). Future water, the Government’s water strategy for England(PDF-2070K). Department of Environment, Food and Rural Affairs, London.
Willis, K.G. (2002). Benefits and costs of forests to water supply and water quality(PDF-204K). University of Newcastle. Report to Forestry Commission.
Further information on water quality and can be gained from the Environment Agency.
Further information on flood risk alleviation can be obtained from CABE – Using green infrastructure to alleviate flood risk.
Forest Research’s analytical laboratory offers a high quality water analysis service for the private sector and research organisations. Regular water analysis can be carried out for research projects with the latest state of the art dual view spectrophotometer giving extremely low limits of detection suitable to assess drinking water quality for cations contaminants.