and why does it eject water so regularly?

The “Brubbel”, as the cold water geyser in Wallenborn is known, ejects water regularly, every 30 – 40 minutes or so, caused by carbon dioxide gas rising from the ground below.

Carbon dioxide collects in a cavity. When the cavity is full, it suddenly empties itself and the carbon dioxide rises into the bore hole above. The gas bubbles that form make the water bubble. Another way to explain it is that carbon dioxide rises into the water until a critical volume is reached. Gas bubbles then form which rise and make the water bubble. This process repeats itself regularly.