Cave water glowing green in UV light (Giant's Hole)

[Pete K]

Hi all.
On the 21st of August I was in Giant's Hole, just upstream from Garlands. I usually use a UV light (only cheap eBay special) to show people how the calcite glows, and I was here showing the same thing to trainee instructors on this date. We noticed that the water around our feet was glowing very green. I'm in here almost once a week doing this with groups at the moment and I can't imagine I'd just not noticed this green glow before.
My question is, what could have been causing the green glow? There is no evidence of anyone doing dye tracing recently. I wonder if it is a farm chemical that has washed in following the heavy rain recently. Maybe the silt in suspension was glowing.
Any ideas? TIA.

 

[Pete K]

From a blog on the European Geosciences Union website:

...groundwater samples might fluoresce due to the presence of microbial matter. In rivers and wastewater systems, the amount of fluorescence at shorter ultraviolet wavelengths (300-350 nm) has been observed to correlate with the amount of oxygen being consumed (the biochemical oxygen demand, or BOD). It is not possible to distinguish between individual or groups of microbial species, and researchers are still investigating exactly what is fluorescing. However, recent research has shown that there is significant potential in using hand-held fluorescent probes to determine microbial water quality in groundwater. Where there might be faecal contamination of groundwater used for drinking water supply, there is great advantage to this method as an immediate reading is possible in comparison with other methods which typically take 18-30 hours.

https://blogs.egu.eu/network/water-underground/2018/02/19/from-groundwater-flow-to-groundwater-glow-why-does-groundwater-fluoresce-in-ultraviolet-light/

 

[droid]

Answered your own question. Thanks for posting, I wasn't aware of this correlation. It might be useful for fishkeepers.

 

[MarkS]

...the amount of fluorescence at shorter ultraviolet wavelengths (300-350 nm)...

Those wavelengths wouldn't even be visible, let alone green...

 

[droid]

Isn't that the wavelength of the UV used to cause fluorescence?

 

[AlexR]

Petes ebay light emits at an unknown wavelength, likely relatively broad and variable from unit to unit as we're not talking about an analytical instrument here. Just guessing, but I would hope it doesn't emit too short a wavelength as those are really not the best to have accidentally shining into your unprotected eye if you value your vision.
It's impossible to say which wavelength excited what, other than emission (green) is around say somewhere between 520-530nm. Fairly large Stokes shift from UV typically seen in highly polar solvent systems such as water.
An enormous variety of organic matter (predominantly delocalised moieties) exhibit fluorescence, e.g. chlorophyll and related compounds. Similarly a multitude of minerals show fluorescence as well, so like you suggested silt (mineral particles) may be the culprit.

 

[Pitlamp]

A bloom of unicellular algae?

 

[Pete K]

Thanks for the replies. The UV light I use is a ?3.50 job with a fair bit of visible light as well as the UV bit of the spectrum (you can see this in the photo). Can't give you any details about wavelengths etc... as it's not a quality lamp, but it does the job with my groups. So, it looks like the green glow is some kind of organic or mineral suspension then, not a farm chemical thank goodness. I'll continue to do some reading up when I get the chance but I'll at least have the bones of a response to the question "why is the water green?" when it inevitably happens again.

 

[2xw]

My bets on algal bloom from all the shite the sheep put in. Not a chemical but I wouldn't drink it

 

[markpot]

I've seen similar in mines in the south peak, we did have one moment in a now inaccessible mine ,the water glowed that green we wondered if our wellies would melt :blink: