|Thursday, May 6, 2021
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When you’ve travelled along Autopista TF 1, between the south of Tenerife and Santa Cruz, have you ever noticed a little train outside a mine entrance at Junction 14 to Fasnia? It’s a green diesel engine pulling two mineral trucks. Tenerife today is not exactly littered with railways, so what on earth are they doing there? And what was being mined?

Well the answer to the mining question is that, odd though it may seem to us English who are familiar with coal mines, tin mines, iron mines and lead mines, it was water that was mined on Tenerife.

The mine, or gallery, entrance at Junction 14 is not genuine and there was never a railway there. There’s no information board, which is a pity, but in fact the train and the mine are reconstructions built as a memorial to the water mines of the island. I’ve been told an astounding fact, that there are more mines on Tenerife than in the whole of mainland Spain.

With regard to railways, there used to be many hundreds of kilometres on the island, but nearly all out of sight, underground, burrowing into the central core of the island on a slight uphill gradient toward ever-retreating water sources, and removing the spoil to be tipped outside the mine entrance.

There are some railway relics still to be seen, but you have to keep your eyes open. Wooden railway sleepers pop up surprisingly here and there; by the side of a dusty track in Adeje, or forming the edge of a flower bed in a café in Puerto de la Cruz, while near Guimar railway lines with their 60cm gauge still cross the road from the disused mine to the spoil dump. And a solitary length of railway line survives beside the main road between Santa Cruz and Bufadero.

Moving on to water, unlike Britain, on the island of Tenerife there are no underground lakes or watercourses to produce springs, all fresh water is the result of snowfall and rainfall. In the past, when the island’s population was smaller and there was less demand for water, there were occasional springs and small streams in places where water from melting snow ran out from the rocks. But of all the water resulting from snow and rainfall, 72% of it is lost through evaporation and run-off, while the remainder soaks into the ground and between vertical belts of impervious rock called ‘dikes.’ These are what are tapped by the mines.

To the north of the east-west mountain ridge across the island is the curious phenomenon of ‘horizontal rain,’ or the belt of cloud that frequently forms when warm air from the sea cools on rising to 1500-1800 metres. This is most evident in the Orotava valley where the cloud is contained within the high valley sides and cools to deposit moisture. The cloud has its own name ‘panza de burro,’ or ‘the donkey’s stomach,’ due to its soft, pale grey, fluffy appearance.

However, snow is the preferred precipitation because it doesn’t run away so quickly as rain on the surface, instead it melts slowly, giving the water a chance to soak into the ground. In relatively recent years the accumulation of water has been improved by the realisation that tree planting helps water retention enormously, both in preventing erosion and holding on to moisture in the soil.

Of course there is a problem. There are not tens of thousands of people on Tenerife as in the past, there are now millions, including, and especially, tourists, all demanding showers, swimming pools, and golf courses to keep green by watering when the surrounding areas are dry and brown, not to mention more intensive agriculture.

Geologically speaking, Tenerife is basically a volcanic dome with a ‘phreatic level’ (the level at which underground water reservoirs or cavities can be reached by wells or mines), that is getting lower. It follows that any mine draining water from one part of the island’s water table inevitably drains it from the whole. So how is the phreatic level doing?

In the 1970’s there were about 150 water mines, or galleries, and even though supply was exceeding demand at that time, there was concern about the future of the water supply because the phreatic level was dropping by about five metres per year. What must the level be now, forty years later?

There are no working galleries today, and they are all sealed off due to the danger of accumulation of poisonous gases. As technology improved, extraction of water was then carried out by sinking wells or shafts, and the ‘agua dulce,’ or sweet water, was pumped to the surface. But even a lot of the shafts have had to be abandoned as the phreatic level dropped and the rock dried out. Whenever this happened a new shaft had to be sunk at a lower altitude. Sometimes a shaft cannot be sunk any further and it has to be abandoned when the saline water level is reached. This occurs at the lowest levels, when descending percolating fresh water mingles with sea water that penetrates the porous rocks, making it unpalatable for drinking and unsuitable for domestic use or for agricultural purposes.

Collecting surface water from rainwater and snow run-off is a problem, because, as well as evaporation, the porosity of the volcanic rock can mean that dams and reservoirs cannot be guaranteed to retain water. Not far from Santa Cruz the reservoir Embalse de los Campitos failed from the moment it was built; all there is at the bottom is a small area of lush green grass, even though the reservoir is shown blue on maps to represent water. Another reservoir, at the Charca de Tahodio, does collect and hold water from the Anaga hills, but it is totally inadequate for the needs of the capital city.

The natural water table, the phreatic level, although it rises with each winter snowfall, inevitably falls lower and lower with continually increasing consumption. The galleries at higher levels went out of use a long time ago, and new ones that were constructed were done so at a greater cost, this was because they had to penetrate deeper into the massif of Tenerife. The law of diminishing marginal returns played itself out; more investment for smaller amounts of water. The shafts and wells are still being bored deeper and deeper but they all have to stop short of the bad water level.

There are solutions to the problem; desalination of sea water and purification of the ‘bad’ underground water are two answers for the time being, but if I was Achaman, the supreme god of the Guanches looking over Tenerife, I’d be worried.


I’d like to thank Carlos Quintana Gonzalez de Chaves and Miguel Angel Noriega Aguero for providing infor-mation and taking me to see the sites mentioned in the article.