The quarry originally mined limestone deposited in this area in the Carboniferous, ~350 million or so years ago. The limestone isn’t pure though, there are various impurities in it due to fluids passing through and precipitating various minerals after it was formed.
Once the quarry was closed, it began to refill with groundwater and interact with the atmosphere. The combination of minerals being exposed to air and oxygen is not always a good one. Minerals often react with the oxygen and give off side effects. One side effect was the blue color in the upper image; almost looking pleasant and leading to the nickname “the blue lagoon”.
Unfortunately, another product of this reaction was alkalinity. The lake is highly alkaline, with a pH of 11.3, comparable to ammonia.
The caustic waters are dangerous to swim in, but the color was so inviting people would ignore the signs and enter anyway. In fact, in 2009, a young boy died while swimming in this quarry.
Finally, the local government may have figured out a way to stop people from swimming. They changed the color of the water; dying it black, as seen in the lower image. Swimming in black water just isn’t as appetizing as swimming in bright blue waters, and it turns out, fewer people seem to be hopping in these days.
Unless its well known to be safe, swimming in quarries isn’t the greatest idea. Lots of weird chemistries and rock hazards. Hopefully this solution will work better than the signs.
Image credit: Rachel Thomas/BBC News:
Earlier photos and articles: