Oliver Koit - Why does the Tuhala Witch’s Well Boil Over?
In this One Minute Lecture, Oliver Koit, a Master’s student from the Tallinn University School of Natural and Health Sciences explains why the Witch’s Well at Tuhala, Estonia boils over.
In this One Minute Lecture, Oliver Koit, a Master’s student from the Tallinn University School of Natural Sciences and Health explains why the Witch’s Well at Tuhala, Estonia boils over.
The Tuhala karst area is situated on the Harju plateau, on the lower course of the Tuhala river. There the carbonate rocks from the Ordovician era are covered by clay-rich glacial sediments with a thickness of 2-5 metres.
This soil with very low water conductivity acts as a watertight layer on top of groundwater, which means groundwater can access the ground only through the places with no surface, or a very thin surface.
These spots at a karst area are called suffusion sinkholes, which occur as groundwater helps erode the underlying layers, after which the top layers cave in.
A spring at the bottom of one of such sinkholes is the location they set up the Tuhala Witch’s Well during the first half of the past century.
The entry and exit of the karst area of Tuhala are directly connected. The Tuhala underground river, which stars near Ämmaaugu and ends at the Virulase valley, uses a well developed network of natural channels to flow at up to 670m/h.
When the water level is low to medium, the water exits the karst system at the Veetõusme springs. The spring underneath the Witch’s Well is connected to the system, but the water does not reach the surface there.
When the water level rises, the pressure at the final springs rises as well. For the Witch’s Well to boil over, the groundwater must reach the pressure it takes to push it 59.39 metres over the sea level.