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How to easily detect the presence of water in rocks

Sandstone, whether as a natural cliff or a building material, contains water in its pores. This water transports dissolved salts, which can cause surface erosion due to crystallization, creating interesting geomorphological shapes or causing the disintegration of monuments. So far, there has been no method that could reliably and cost-effectively detect water at the sandstone subsurface. However, scientists from the Faculty of Science came up with a simple but effective solution.

If we look at the usual distribution of water below the surface of sandstone or other porous rocks, we would observe almost completely dry material in the first few centimetres. If any water is present in this surface zone, it is in the form of vapor or it appears only sporadically. Deeper, the pores between the sandstone grains are still filled with air, but the occurrence of liquid water in this wet zone is already continuous. Under certain conditions, liquid water may reach the rock surface, e.g. after rains, but it usually dries after several days due to sunlight and wind.

However, the system of water flow in such porous materials in nature is more complex and difficult to research: the compactness of the material and the presence of dissolved salts limit the use of several methods common, for example, in pedology. The proposed method is based on the dissolution of a powder dye.

The device, which detects the borderline between dry and wet zones, consists of a stainless-steel needle, which is coated in a polyurethane adhesive that serves as a fluorescein carrier - a dye that changes colour from red to green upon contact with liquid water. A hole with a diameter of at least 2 mm is drilled into sandstone or other material where we are interested in the depth of the liquid water presence. This time is sufficient to dissolve the dye which comes into contact with the liquid water, while also being short enough so that the dye is not affected by air humidity present in the dry zone.

Scientists tested the device on various rocks in the Bohemian Paradise, in the archaeological city of Petra in Jordan, in tuffs in Utah, on buildings constructed of sandstone blocks or in the area of Prague Castle. This method will help in the study of the evolution of moisture in the surface zone of porous rocks and thus in a better understanding of the weathering processes or the formation of various sandstone formations, which we have written about in the previous section.

Weiss, T., Mareš, J., Bruthans, J., Slavík, M., inventors; Charles University, Faculty of Science, assignee (2018): The vaporization plane depth measuring method and the apparatus for performing this method (in Czech). Dec 10 2018. Czech Republic Patent Application No. PV 2018-689.

Weiss, T., Mareš, J., Slavík, M., Bruthans, J. (2020): A microdestructive method using dye-coated-probe to visualize capillary, diffusion and evaporation zones in porous materials. Science of The Total Environment. https://doi.org/10.1016/j.scitotenv.2019.135339

Tomáš Weiss

Published: Jan 24, 2020 10:05 AM

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