More blue water - why is the Nil Diya Pokuna blue?

On my last visit to Sri Lanka, I was keen on exploring some lesser-known attractions and decided to visit Nil Diya Pokuna (නිල් දිය පොකුණ) located close to Ella in the Uva Province. I was impressed and fascinated by the massive underground cave complex and the blue water pond at the end of the 850m hike through the cave. This was the second time I saw clear blue water in Sri Lanka, the first being in a limestone quarry.  

The usual reason for ponded water to appear bright blue or turquoise in colour is the fine particulates that selectively scatter light through water (the same reason why the sky is blue). In the case of the limestone quarry the fine particulates are minute calcite crystals and in the case of glacial lakes they are finely ground rock particles known as glacial flour. 

Nil Diya Pokuna has a very interesting geology, with several different rock types present around and within the caves, and I wanted to understand what gives the water its blue colour. Caves of this scale are usually formed by the action of weathering and erosion of sedimentary rocks such as limestone. However, this region of Sri Lanka consists of primary of metamorphic rocks. This blog post by Dr Jayasingha describes the geological origins of the cave complex containing Nil Diya Pokuna. According to it, the caves have been formed by the initial dissolution of Marble, which leads to weakening of rock joints and bedding planes and subsequent collapses of the other rock masses creating the large underground caverns. 

Marble is formed by the metamorphosis of limestone, and its dissolution would lead to the release of calcite crystals. There are stalactites formed at several places within the cave, as seen in the photos below, that confirm the occurrence of marble or limestone dissolution. Therefore, it is reasonable to conclude that the reason for the blue coloured water in Nil Diya Pokuna is the calcite crystals that are accumulated in the water as it flows through the joints and fissures in rock containing marble or limestone before making its way into the pond. Below are some photos from my visit:

Stalactites in the cave indicating marble or limestone dissolution
 

Evidence of weathering and staining in the rock

Visible bedding planes and smooth joint surface of a possible collapse leading to cave formation

Blue water and more stalactites

High water levels were blocking off some more expansive areas of the cave

The water was a little murky due to recent rains