Dying for a swim

For years, the turquoise waters of Harpur Hill have hidden dangerous secrets. With its vibrant blue hue, the pool looked more like a Mediterranean idyll than a disused quarry in Derbyshire, United Kingdom – which perhaps explains why swimmers could often be seen paddling across its surface on hot summer days. But as the warning signs around its edge made clear, they did so at great risk to their health. As well as being treacherously deep and strewn with debris, dead animals and other hazards, the quarry’s waters are highly toxic, with an alkalinity comparable to that of ammonia. Now the local council has taken a bold step to further dissuade people from taking a dip: they’ve dyed the ‘blue lagoon’ black!

The pool’s original colour was a result of the site’s former life as a limestone quarry. Over years, the quarry stone and various chemicals pollutants reacted with the water to produce a beautiful but caustic alkaline solution, with a pH value of 11.4 – just two places below bleach.  Despite numerous warnings explaining the dangers of coming into contact with such a toxic solution, people continued to visit the quarry during warm summer weather, spurring the local government into action. By dying the water black, the pool’s picturesque beauty was deliberately spoiled, in the hope that, by making the risks more visually obvious, many injuries, illnesses and even deaths will be prevented.

The pH scale measures the concentration of hydrogen ions in a solution – the higher the concentration of hydrogen ions, the more acidic the solution. On the pH scale, a solution with the lowest value of 0 has the highest possible concentration of hydrogen ions, while pH 14 means that a solution has a lower concentration and is highly alkaline.  Water in public swimming pools is kept at a pH level between a neutral 7 and a slightly alkaline 8.

While Harpur Hill’s toxicity is the result of human activities, there are many naturally occurring caustic lakes around the world. Lake Natron in Tanzania, for example, owes its blood-red waters and high pH value of 10.5 to sodium carbonate, which is deposited by mineral-rich hot springs in the Southern Ewaso Ng’iro River. Regular eruptions of the Poás Volcano in Costa Rica, meanwhile, have spewed sulphur into two lakes near its summits, producing sulphuric acid with 0 pH value – certainly not somewhere you’d want to go for a swim!