OUCC Proceedings 6 (1974)

De Flumine Fluorescente - 
The Trials of Testing

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 Peter Urben

It all started when the grind of the industrial world drove me to read a volume on care and maintenance of water supplies. Discovering that fluorescein is a bacterial metabolite produced by some common soil and water organisms, and presumably eaten by their neighbours, set me thinking.

For water tracing one needs a detectable additive, of low toxicity, negligible occurrence, and chemical stability. Low cost is equally desirable. One further requirement is that it should not be spectacularly detectable the local populace do not always accept their water supply in a strange colour with equanimity. These principles brought to mind phosphates, not widely distributed in nature and largely locked up in the higher forms of life. Save where man is active agriculturally and domestically, the background can be expected to be low, but they are easily detected by an extremely sensitive dour test, though invisible to the naked eye. All the other advantages they possess, with the exception of probable absorption into the geological media of caves.
It was found that in South Wales, phosphates were not detectable in cave water, except in Forth yr Ogof. This may be due to agriculture, washing in the Mellte valley, or indeed to the CDG.

Thus a short system was needed to test whether phosphates introduced into a sink, would reappear at the resurgence. Francis and myself bethought ourselves of Spain. The Ojo del Rio sinks shortly after appearing, and was believed to reappear some 300 metres away and 60 metres lower. With a gradient of this size, a through time of less than an hour is to be expected, if, as seemed likely, a vadose system is present.

Comparison with fluorescein is a good idea. After procrastination and delay by a sprained ankle, I put 15 grams of fluorescein and 100 grams of Na3PO4. into the Ojo del Rio streamway one afternoon and charcoal detectors into the resurgence below Boriza, to extend the sensitivity of the fluorescein. A wet two hours later regular testing of the resurgence showed no trace of either additive. The next day the detectors showed no trace of fluorescein. Another addition of fluorescein was made, 60 grams this time. The detectors remained inaccessible for some days as a result of flooding just after the injection, but when extracted and placed in sodium hydroxide solution, they showed no fluorescein but merely a yellow brown muck.

This raised the question of where on earth the Ojo del Rio was going. Was the Bay of Biscay now bright green? Drastic measures were demanded. 250 grams of fluorescein were put into the sink.

By this time Francis and Mik had dug down to the stream, halfway between sink and resurgence, so that a shorter test route for phosphate was available. I spent two wet hours in the rift carrying the stream, sampling the water and staring at the trout which were hiding from Francis' rod. A connection thus far was proven as the water was green from the fluorescein added five hours earlier and. present at this point in a concentration of rather less than 1 ppm. No phosphate was detected in two hours, and no fluorescein had appeared at the resurgence.

The next morning Mik insisted that the stream above Bolugo was bright green. I was not so easily persuaded and required a third look. It was green - very! The water did not show fluorescein in a litre bottle however, which meant that the concentration was less than 2. x 10-8. A quick tour of bars was needed however, drinking green water to show how healthy it was.We had at last proved the connection between sink and resurgence, but turning the Bolugo stream green for a full day was not the most politic way to do it.

To cap this, the samples from the detectors after the second injection of fluorescein were found to have thrown out a dirty brown precipitate, and were yellow green. Three morals can be extracted from this tale:-

  1. The minimum detectable amount of fluorescein is that which cannot be missed by any peasant within 5 miles.
  2. Fluorescein is at least two orders of magnitude more detectable in a stream than in a laboratory.
  3. All caves in the Sierra de Cuera are phreatic and extremely wet.

Meanwhile, if anyone knows of a nice short vadose sink and resurgence system, I would like to find out whether phosphates will get through an underground streamway.