By Sue Charlelsworth

“Urban geochemistry” is a rapidly evolving science. With the UN Population Division estimating that almost all global population growth in the next 30 years will be in urban areas, it needs to be. Greater concentrations of people in intensely developed areas of asphalt and concrete are creating more “artificial” environments with their own particular issues around contamination and health.

Urban street dust is one of these issues. It’s the material billions of people live in every day, walk on, touch and inhale (3.3 billion people – or more than half the planet’s population). In rough terms, 80% of street dust tends to be just eroded soil from surrounding rural areas, local parks and gardens – but the other 20% is far more hazardous, including excrement, heavy metals and some dust which may be radioactive. It will be increasingly important, therefore, that cities understand the nature of the often toxic dust they are living with, as well as think more creatively about how the problem can be dealt with – beyond the current solution of trying to sweep it up and then send it to landfill.

The atmospheric hazard

Concern over the quality of urban air has lead to legislation on the sources of emissions, concentrating mainly on smoke (for example, in response to the “London smogs” of the 1950s) and the implications of leaded petrol. However, atmospheric particles come from a huge variety of natural and man-made sources and there has been a great deal of research looking at distinguishing what is “natural” and what is controllable. Particle size and chemical composition is what matters. Smaller particles are capable of being breathable, enter the lungs and are not picked up by the body’s clearance mechanisms. Larger particles may cause more irritation but are less serious.

Research suggests that prolonged exposure to ultra-fine particles with traces of typical elements such as arsenic, cadmium, chromium, mercury, manganese, nickel, lead and vanadium can cause sinusitis, asthma, chronic bronchitis, pneumonia, lung haemorrhage, lung cancer and brain haemorrhage. While many studies have shown that emission of such elements as lead have reduced, solid deposits on the surfaces of streets and in people’s homes still retain a record of past contamination which can pose a risk to a city’s inhabitants, and more particularly children.

A recent study looked at the concentrations of metallic and radioactive elements in gully pot sediments, pavement and road dusts in order to assess the potential environmental and health hazards. This showed how mobile street dust can be transported from place to place through standard street gutters and gully pots. It can also be conveyed by passing vehicles and through the process of street cleaning itself. The result is that the contaminated particles are brushed into the air and potentially inhaled or ingested by passing pedestrians or simply deposited further afield.

Sustainable drainage

Long-term solutions are needed if cities are to work as sustainable habitats for the majority of people. One approach with proven potential is sustainable drainage systems (SUDS). Before human beings started to concrete over the countryside, rain fell onto the ground, infiltrated and passed either into groundwater or seeped through the soil into the nearest receiving watercourse. With the advent of urban infrastructure such as roads, gutters and storm sewer systems, the behaviour of water has been constrained by society to be piped out of urban areas as quickly as possible. This hard engineered approach inevitably leads to surface water flooding – and also the opportunity for natural storage and filtration of toxic dust.

SUDs are made up of a variety of devices either used alone, or fitted together as a “train” to attenuate the storm peak and allow water to dissipate slowly. These devices include porous paving systems, grassed rooftops, constructed wetlands, swales, vegetated barriers, retention and detention ponds etc. This approach allows waste materials to be absorbed and processed – with vegetation and microbes making use of the nutrients made available in what were toxic materials – enabling water which flows through the sustainable drainage system to be filtered rather than contaminated.

Whilst such approaches have been embraced for over 25 years in the US, Sweden, France and over 15 years in Scotland, the take-up has been less than enthusiastic in England and Wales. Porous pavements in particular, with their potential for extensive city usage and ability to capture and clean contaminated particles, could have a huge role to play in improving city environments. Future cities will need to look new technologies such as these, as well as to the distant past and the efficiency of “natural” processes, if they are to remain viable and human places to live in.

Many local authorities and people involved with civil engineering are either simply unaware of SUDS or perhaps have heard of it but not sure how they function, or believe they don’t work, are expensive or unsafe. Common fallacies include a belief that SUDS is expensive. When PCs first came out they were beyond the pockets of most. However, as production costs declined and more were sold, the price decreased. The same will be true of SUDS. At the moment, hard engineered solutions are familiar and components are mass produced, hence they are cheaper. In the future the same will be true of SUDS devices when their use becomes commonplace. There is also the belief that porous paving becomes easily blocked. Porous pavements are designed to trap pollutants and so to a certain extent will block. However, PPS are over-designed and so can “block” by up to 70% and still work effectively. There are instances of PPS which were installed 20-25 years ago and which are still performing well.
SUDSnet and CIRIA offer training at various locations around the country (see websites) and Coventry University offers SUDS CPD and a Masters Degree in Sustainable Drainage and River Catchment Management, so there are various opportunities to find out about it.

Regulation change needed

Those who might be willing to try a SUDS approach are hardly encouraged by current legislation. For example the drainage hierarchy in the Building Regulations only requires that builders have considered infiltrating on site, but where this is not possible, the water can go to the receiving watercourse or into the sewer network. As has been previously mentioned, many householders have sealed their frontages, increasing the percentage of impermeable surfaces markedly.

The Pitt Report (2008) has driven the possibility of a change in this attitude by introducing Future Water, legislation which removes the right of builders to plumb new developments directly into the storm sewers. It would also require householders to pay for, and obtain, planning permission before they could seal their frontages. These sorts of changes would bring England and Wales more into line with Scotland which has had these requirements for 10-15 years, hence the reason why they are so far ahead of England and Wales in the SUDS stakes.