Blue-green infrastructure can have an important role to play in protecting water quality, as some of the solutions below demonstrate.
Natural or Constructed Wetlands
Lakes and reedbeds can help improve the resilience of freshwater systems by slowing flows and settling out sediment. They can be very effective at removing nitrates and any pollutants bound to sediment, and have been used successfully to treat a range of wastewaters, including agricultural discharge, municipal effluent, landfill leachate and stormwater.
Wetlands are typically not very effective at removing phosphorus (Richardson and Craft, 1993), although in some experiments, phosphate removal was enhanced by the addition of ochre (Heal et al, 2005). Similarly, nitrate removal can be enhanced by providing additional carbon in the form of methanol or more natural substances (Gersberg et al, 1983).
Metal contamination can also be addressed by reedbeds or constructed wetlands. In the UK, the Coal Authority has a duty to manage the effects of past mining, including acid mine drainage which is often associated with high levels of metals like iron, lead and cadmium. Reedbeds can offer a polishing service to the treatment train, which and are often preceded by settling lagoons and aeration facilities.
These are areas of vegetation protecting vulnerable water from polluted run-off, or working as part of a SuDS (Sustainable Drainage System) treatment train to reduce pollutant loads before discharge. In the example given by Heal et al (2009), the filter strips at a motorway service station successfully reduced pollutant levels discharged to a local watercourse (removal levels of 70%-90% for potentially toxic heavy metals). The grass filter strips were found to be particularly effective.
Upland Water Purification
There has been a move towards investment in upland areas in order to improve the colour and clarity of water. Rather than spending significant sums downstream, some water companies are focussing their efforts on the headwaters of the catchment, preventing sediment erosion by restoring the peat’s soil and vegetation structure. Activities such as grip-blocking (removing artificial drainage channels) can also provide flood risk benefits by attenuating storm flows -although this is a complex area that needs careful modelling (Lane & Milledge, 2013) to determine whether the attenuation benefits are offset by worsened antecedent soil conditions.