Media playback is unsupported on your device Media caption Richard Black takes a 'green' tour of the London Olympic site

For an Olympic Games claiming to be the "greenest ever", in a city intending to be the "greenest ever", taking place under a government vowing to be the "greenest ever", there's precious little green to be seen when I pitch up at the 2012 venue in London.

The dominant hues are grey - the sky, where wintry clouds pregnant with snow stretch from horizon to horizon, and the completed arenas - and the browns of the building trade.

In all honesty, it still looks more like a building site than a verdant park.

Image caption Otters may be on site as the Games begin

But come the summer - assuming the British skies deliver one - things should be a lot different.

Visitors will not only be able to marvel as the likes of Usain Bolt and Victoria Pendleton scream round the tracks of their various venues.

They will also be able to take a quiet moment, if they choose, to commune with kingfishers and otters on the newly pristine River Lea that runs through the site.

Damaged neighbourhood

Not living too far away, I've visited the area many times down the years - and boy, was it a tip.

Along the river lay dozens of small industrial sites, many of them derelict, and none of them easy on the eye.

Nor were they easy on the ecology of the river and its banks.

"Whether it was in the last few years or a century ago, you've had chemical storage, soap factories, tanneries - all the industries that people don't like in their backyards, that had moved out to east London," says Rob McCarthy, the Environment Agency's Olympics account manager.

"There were heavy metals, hydrocarbons, arsenic and cyanide at certain levels in the ground."

One arm of the years-long partnership between the agency and the Olympics Delivery Authority (ODA) has been a clean-up on a massive scale.

Junk was first removed by hand. Then a giant piece of machinery known as a soil washer was installed on site.

Altogether, two million tonnes of soil has been through the machine and stripped of its contaminants. The cleansed material has been used on site - amounting to about 95% of the total needed.

Image caption Giant soil-washing machines acted as cleansing stations around the site, processing two million tonnes

"You get a win-win out this re-use of soil on site because you don't have the lorry movements back and forth," says Chris Smith, chairman of the Environment Agency.

"You don't send vast quantities of semi-contaminated soil to landfill sites around the country, and you can have a much more sensible process that re-uses and recycles."

And potentially a much cheaper process too, as everything that goes into landfill sites these days has a tax attached.

Murky waters

Aquifers underneath the industrial sector were also polluted.

We hung our hats too firmly on a wind turbine Sir John Armitt, ODA chairman

Water is still being pumped out, cleaned, and pumped back down again.

As a result, the Lea itself, which used to be one of the most polluted rivers in the country, is now much cleaner than it was.

But the river has been transformed in other ways too.

Last century, it became fashionable to try and constrain flood-prone rivers by channelling them between concrete banks. The technique has limited success, and it takes away any notion of a river being a functioning ecosystem.

Many engineers these days prefer to build more natural defences, to widen out the river basin so there are areas that can safely flood, and slow the water with robust plants - which is also good for nature.

Here, the Lea's cross-section has been radically re-shaped to give a shallower run-off area for floodwater. The banks are now wetlands of rush and reed and ooze that will find favour with insects, birds, frogs and small mammals.

Image caption The changing shape of the Lea valley - old (dotted line) and new, with wetlands and natural flood defences

The wildlife is being given a helping hand by a "kingfisher wall", pitted with nest-sized holes, while an artificial otter's holt (the name for the animals' dens) lies beneath an embankment - though otters have yet to make an appearance.

About 750 nesting boxes for birds and bats are dotted around the site.

Further modifications have reduced the risk of floodwater travelling down the course of the Docklands Light Railway, as has happened in years past. The Environment Agency estimates that the threat of flooding has been removed from 4,000 houses.

Kite-flying?

The word "legacy" coursed through the UK's Olympics bid - and the legacy here should be a new park of more than 100 hectares, the biggest new urban park in the country for a century.

Image caption Plans for a big wind turbine were abandoned, leaving a few "token" small ones around the site

Even more important on the ecological front is that the Games site along the Lea will make a "wildlife corridor", linking Hackney Marshes to the north with the Thames, reconnecting populations of animals and plants that have been separated for a century by concrete and industry.

But what about the Games' wider green credentials?

Its low-carbon ambitions took something of a knock two years ago when plans to build a wind turbine on site were scrapped.

That meant a downscaling of the pledge to produce 20% of the site's energy from local, renewable sources.

The current target is about 11%, to be fulfilled by solar panels and the two small power stations on site.

They can use gas - not renewable - or biomass, which can be renewable; and they produce hot water and electricity at the same time, "co-generation" as it is known, which is considered an efficient approach.

"We hung our hats too firmly on a wind turbine to provide 7-8% of that," admits Sir John Armitt, ODA chairman.

"And for commercial reasons and safety reasons, for a whole variety of reasons, it was just not tenable to put this turbine up.

"My personal view is that you won't see wind turbines in urban environments because there are too many constraints around them."

Instead, says Sir John, the money that would have gone on the wind turbine - a few million pounds - will instead go to local communities for investment in home insulation, which could, he says, "produce a bigger win" in terms of reducing carbon dioxide emissions.

Indian files

Another "green" aspect of the site is water.

Swimming pool water, recycled sewerage and rainwater will all be used for things like flushing toilets, reducing the overall water footprint.

Image caption The whole feel of the river has changed, from closed-in and littered to open and accessible

Where possible, Olympic buildings have been designed to use energy-intense materials as sparingly as possible.

With the Velodrome roof, the original plan called for a steel beam structure requiring 2,000 tonnes of steel.

"And the engineer and contractor got together and realised they could do it with a cable net structure that used 100 tonnes of steel," recalls Sir John.

Other materials have been selected under the scrutiny of the Commission for a Sustainable London, which has aimed to ensure that wood, for example, comes from sustainably harvested stock.

The limits of the approach became clear a few weeks ago when one of the commissioners, Meredith Alexander, resigned in protest at the "wrap" for the stadium being procured from Dow Chemicals.

Dow now owns the Bhopal factory in India, where a leak of poisonous gas in 1984 killed many thousands of people and caused long-term health problems to many more; and Ms Alexander felt that even though Dow did not own the factory at the time of the disaster, it was wrong for the Olympics to deal with the company.

The commission had only been empowered to decide whether the wrapping material was sustainable, and nothing wider.

Despite hitches over turbines and wraps, the majority of environmental and sustainability planning appears to have gone to plan.

But there are limits to the greenery. Most obviously, there is no way of curbing the influx of tens of thousands of athletes, backup staff and spectators, and the carbon dioxide that will be produced by all the flights involved.

To that extent, no Olympics can be completely "green".

But London appears to be in with a fair chance of winning itself a green medal to go alongside all the golds its athletes are hoping to secure.

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