[LUM#14] A Grain of Salt in the Eternity of the Pharaohs
Salt for preservation? Not always… And that is precisely the problem facing the pharaohs Osorkon II and Psusennes I, whose legacy is now under threat. Their tombs, located in Tanis in the Nile Delta, are suffering damage 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*, has set out to investigate.
“Obelisks, enormous blocks of granite, and stone colossi barely peeking out of the sand that stretches as far as the eye can see.” Welcome to the royal necropolis of Tanis, located in the heart of the Nile Delta, the eternal resting place of Pharaohs Osorkon II and Psusennes I. Tombs dating back 3,000 years are now endangered by the infiltration of saltwater, which is causing the deterioration of the engravings and paintings that adorn them, as well as the erosion of the blocks and joints that make them up. A tragedy for history, for which the solution is far from obvious.
In the middle of the desert and nearly 25 km from the sea, the origin of this brackish water is a mystery. To solve this mystery, Séverin Pistre, a researcher at the Hydrosciences Montpellier laboratory, set off for Egypt, with many questions and a few hypotheses in mind. “It all started with a conservation expert at the Louvre Museum who, faced with this conservation issue, thought it necessary to study the water at the site level. That is to say, surface runoff linked to rain, but also the likelihood of a groundwater table located beneath the tombs, ” he explains.
The Testimony of the Sand
To test these hypotheses, the hydrogeologist started from scratch—or almost: he began with just a few handfuls of sand collected from the site. Deeper down, there was yellow sand corresponding to the ancient dune. “We call it ghezira. It’s a type of sand typical of these sites that is highly permeable to water and in which the water table could be found.” Closer to the surface, there is brown sand “that is highly impermeable, allowing rainwater to flow over it and run off into the water table, thereby raising its level,” explains Séverin Pistre.
To verify the presence of this groundwater table, the researcher went deeper—exactly 27 meters down—using piezometers. These small devices, equipped with electrodes, can detect the presence of water underground and measure its level. This work was facilitated by the presence on-site of old boreholes dating back to an operation conducted by Egyptian authorities about fifteen years earlier. “Thanks to several measurement points, we were thus able to confirm the presence of the aquifer less than a meter below the tombs. From there, we were also able to study the direction of water flow , for example. ”
The proof is in the electricity
Although the culprit seemed obvious, the salinity of the groundwater still needed to be verified. A simple lab test would have done the trick. Except that in the land of the pharaohs, no samples of any kind are allowed to leave 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 better the current flows—it’s a very reliable method. And in this case, I came across extremely salty water, sometimes very close to seawater.” A result confirmed by analyses conducted in a Cairo laboratory.
At this point, there is no longer any doubt: it is indeed this salty groundwater that, through capillary action, rises up to the tombs and causes them to deteriorate. But how can this phenomenon be prevented? According to the researcher, there are several ways to address this problem. The first involves “dismantling the tombs in order to slide a sheet of lead beneath the water table before rebuilding them. “It’s effective but very heavy, both literally and figuratively,” the researcher points out. The second option is to lower the table, “in other words, to bring it down to a depth where the salts can no longer reach the tombs through capillary action. This requires a nearly continuous pumping system.”
The essence of the mystery
While the mystery of the water has now been solved, that of the salt remains a complete enigma; for how can we explain its presence in this arid, desert-like area? “When aquifers are close to the sea, as in Mauguio for example, we observe a phenomenon known as a ‘salt wedge.’ This means that seawater, which is denser than freshwater, gradually seeps under the aquifer, but here we are too far from the sea for that to happen, ” explains Séverin Pistre.
Other hypotheses are being considered, such as the possibility that these saltwater deposits have been present for a long time—perhaps trapped there thousands of years ago—or that the phenomenon is related to evaporation. “Together with our colleagues at the Montpellier Geosciences Laboratory, we are conducting geochemical analyses of the isotopes contained in these saltwater deposits to try to determine their origin. We still have more digging to do, ” the researcher concludes. Proof that even in the 21st century, the mysteries of the pharaohs have certainly lost none of their appeal.
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*Hydrosciences Montpellier (UM – CNRS – IRD)