A grain of salt in the eternity of the pharaohs
Salt to preserve? Not always... And that's the problem facing the pharaohs Osorkon II and Psousannes I, whose eternity is now under threat. Their tombs, located at Tanis in the Nile delta, are deteriorating due to the presence of salt water in their masonry. Where does this water come from? A mystery that Séverin Pistre, a researcher at Hydrosciences Montpellier*, set out to investigate.
"Obelisks, huge blocks of granite and stone colossi poking their heads out of the sand as far as the eye can see. Welcome to the royal necropolis of Tanis, in the heart of the Nile delta, the eternal home of the pharaohs Osorkon II and Psusennes I. These 3,000-year-old tombs have been threatened by the infiltration of salt water, which has led to the degradation of the engravings and paintings that adorn them, as well as the deterioration of the blocks and joints that make them up. A tragedy for history, with no obvious solution.
In the middle of the desert and almost 25 km from the sea, the source of this brackish water is a mystery. It was to solve this mystery that Séverin Pistre, a researcher at the Hydrosciences Montpellier laboratory, set off for Egypt, with many questions and a few hypotheses in mind. "Initially, it was an expert curator at the Musée du Louvre who, faced with this conservation problem, thought it necessary to study water on the scale of this site. In other words, the surface run-off linked to rain, but also the likelihood of a water table located beneath the tombs", he explains.
The testimony of sand
To test these hypotheses, the hydrogeologist started with little or nothing: a few handfuls of sand collected on the site. At depth, a yellow sand corresponding to the ancient dune. "We call it ghezira. It's a sand typical of these sites that is very permeable to water and in which the water table could be found". Further out, we find a brown sand "which is very impermeable and on which rainwater can circulate and run off to the water table, raising its level", describes Séverin Pistre.
To verify the presence of this underground water table, the researcher went a step further. To a depth of exactly 27 metres, he used piezometers. These small devices use electrodes to detect the presence of water in the subsoil and measure its level. This work was made easier by the existence of old boreholes on the site, dating back to an operation carried out by the Egyptian authorities some fifteen years earlier. "Thanks to several measuring points, we were able to confirm the presence of the water table at a depth of less than one metre beneath the tombs. From there, we were also able to work on the direction of water flow , for example."
Proof with electricity
If the culprit seemed obvious, the only thing left to do was to verify the salty nature of the groundwater. A simple laboratory analysis would do the trick. Except that, in the land of the Pharaohs, no sample whatsoever leaves the country. So Séverin Pistre continued his investigation on site, equipped with a conductivity meter. "This device measures the electrical conductivity of water. The saltier the water, the stronger the current, so it's a very reliable method. In this case, I found extremely salty water, sometimes very close to seawater. A result confirmed by analyses carried out in a Cairo laboratory.
At this stage, there's no doubt that it's this salty water table which, by capillary action, is rising up to the tombs and causing them to deteriorate. But how can this phenomenon be prevented? For the researcher, there are several ways of dealing with the problem. The first is to " dismantle the tombs and slide a sheet of lead under the water table before rebuilding them. This is effective, but very cumbersome, both literally and figuratively", stresses the researcher. The second option is to lower the water table, "in other words, to bring it down to a depth where the salts can no longer reach the tombs by capillary action. This requires a quasi-permanent pumping system".
The salt of mystery
While the enigma of water has now been solved, the enigma of salt is still a mystery: how can we explain its presence in this arid, desert area? When the aquifers are close to the sea, as in Mauguio, for example, we observe a phenomenon known as "biseau salé". In other words, seawater, which is denser than freshwater, gradually seeps under the water table, but here we're too far from the sea for this to happen," explains Séverin Pistre.
Other hypotheses include the ancient presence of this salt water, which could have been trapped millennia ago, or a phenomenon linked to evaporation. "With our colleagues from the Géosciences Montpellier laboratory, we are carrying out geochemical analyses of the isotopes contained in these salt waters to try to determine their origin. We still need to dig deeper," concludes the researcher. Proof that, even in the 21st century, the mysteries of the Pharaohs have lost none of their salt.
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*Hydrosciences Montpellier (UM - CNRS - IRD)