The eruption of a cold water geyser can be compared to what happens when you open a freshly shaken bottle of sparkling mineral water. As long as the bottle is closed the pressure inside is higher than the pressure outside. Due to this overpressure a high amount of carbon dioxide, also called carbonic acid (CO2), is kept in solution. When you open the bottle pressure releases, CO2 bubbles are formed and expand – the result is a spray of water escaping from the bottle.
The carbon dioxide contained in the Andernach Geyser derives from a magma chamber of the relatively young volcanic area in the East Eifel. Around the peninsula Namedyer Werth large-scale geological faults allow the CO2 to rise up through the usually impermeable slate stone. On its way up, some hundred metres under the earth’s surface, the escaping CO2 meets cold ground water. Due to the low temperature of the water and the high pressure large amounts of CO2 are dissolved in the ground water.
Cold water geysers similar to their better known hot water counterparts need a path to reach the surface and eject water. Like many cold water geysers the Andernach Geyser is a drilled well. When boreholes are drilled in search of mineral springs or CO2 deposits sometimes layers containing CO2-laden water are drilled into thereby providing a direct route for the CO2-laden water to rise to the surface.
CO2-laden ground water flows from several water-bearing faults at different depths into the drilled well of the Andernach Geyser at a depth of 350 m. There the borehole provides an escape for the continuously flowing pressurized water containing CO2 which from there slowly rises to the surface. Due to the weight of the water column the pressure inside the borehole increases with every metre, when the well is completely filled the pressure reaches about 35 bar. The column of water exerts enough pressure on the gaseous CO2 so that it remains in the water. When the CO2-laden water rises to the surface the pressure decreases and the CO2 dissolved in the water is precipitated in form of bubbles. Rising to the surface these bubbles expand. The ground water flowing continuously into the casing of the well and the expansion of the CO2 bubbles displace the water thus causing the water to spring out of the borehole.
As the pressure decreases continuously due to the forming of bubbles the amount of CO2 dissolved in the water drops as well and consequently more and more bubbles rise to the surface, expand on their way up, more and more water sprays out of the borehole.
At a certain point CO2 bubbles are formed which are so big that they occupy the whole borehole, they can only expand upwards and push the complete water column out of the borehole – the cold water geyser erupts.
After the eruption the well recharges with ground water and the whole reaction is repeated in periodical intervals. As we already mentioned this reaction can be compared to the physical reaction which is caused when you open a freshly shaken bottle of sparkling mineral water.