TAXPAYERS are facing a £1.2 million bill to clean up a toxic loch in order to stage a single event at the Glasgow Commonwealth Games in 2014.

The money is needed to improve the water quality at Strathclyde Loch in North Lanarkshire for the initial 1500m open-water swim in the Triathlon.

The loch was chosen for the endurance challenge, which also includes a 40km bike ride and a 10k run, despite a long history of toxic algae making the water unsafe for swimmers.

In 2010, its high levels of blue-green algae – which can cause skin blisters, vomiting and abdominal pains – forced the cancellation of Scotland's biggest open-air swim, which had attracted 2000 entrants.

Last year, a £150,000 project by the Scottish Environment Protection Agency (Sepa) and Scottish Water looked at ways of fixing the issue in time for the Triathlon in July next year. That work has now led to a solution – but at a further cost of up to £1.2 million.

The key element will be a £500,000 temporary dam across the loch's southern tip. The 265m "water quality boom" will isolate enough water to let the triathletes swim two laps of a 750m circuit in safety.

More money will be spent on access works to enable the dam's installation. New pumps and an ultraviolet water treatment system may also be needed.

The area will be used for just two days – for the individual and team triathlon events.

North Lanarkshire Council and the Scottish Government have each agreed to contribute £600,000.

The dam is to be dismantled a few months after the games.

The organisers of Glasgow 2014 were sufficiently worried about the water quality at Strathclyde Loch to start canvassing alternative sites in March last year.

A few months later, in June, 57 of the 70 people who took part in an open-air swim at the loch fell ill with stomach cramps and diarrhoea, with some testing positive for the winter vomiting bug.

The loch had to be closed to "immersion sports" – kayaking, water skiing, windsurfing and swimming – for six weeks.

Four days after the June outbreak, Alex Salmond expressed his personal worries about the loch to the Government group he chairs on the delivery of the Games, saying it was "critical" it be cleaned up.

North Lanarkshire Council said the final costs would depend on bids received for the works, and whether a new pumping system and UV treatment was used, with keys decisions being made later this year.

Asked why a toxic loch was chosen for the Triathlon in the first place, a Glasgow 2014 spokeswoman said: "The water quality problems are not unique to Strathclyde Loch – blue-green algae is a problem in the majority of water spaces in the UK."

A Scottish Government spokesman added: "This loch is a fantastic venue and all partners are committed to delivering high-quality triathlon events there, so this approach will help inform our thinking on longer-term loch improvements."