Earth

How underground waters move

Karst springs greatly depend on surface rainfall for different reasons. Considering that the speed of waters in the vadose zone is as high as that of surface streams, the increase in the hydraulic head when great quantities of water enter a cave can exert a high pressure on waters in the phreatic zone that are literally pushed out with a piston flow, a term that clearly explains the mechanism involved. The flood wave that follows the arrival of infiltration waters will reach after a period of time equal to the time it takes to physically transport these infiltrated waters, while the piston flow makes the flood wave spread like an energy wave (called a pressure pulse) rather than a kinematic one. The speed at which a flood wave spreads is therefore much higher than the speed of water, and is taken to be equal to the speed of sound: hence, in some cases, the propagation of a flood wave is practically instantaneous. This implies that a rainfall on the surface can produce an almost immediate flood at the spring, which takes place a few hours or a few minutes after the rainfall.  The outcoming of this type of floods is fast and has no premonitory signs, such as a progressive rise in water level or increase in  the speed of flow. Some caves situated close to the water table are known to fill up in a few minutes after relatively small showers (eg. The Peyrejal cave in South France): to explore these caves it is compulsory to look at reliable weather forecasts and to have a thorough knowledge of the system.
The arrival of a flood wave as a result of direct propagation is on the contrary often pre-announced by signals such as a gradual rise of water level or an increase of the flow rate, often accompanied by a heavy beating sound against the walls, whistles or rumbling caused by the ejection of air from the conduits that are getting flooded (a famous example is the Masera cave on Lake Como, when the spring gets saturated it is announced by a loud rumble that is clearly audible in the nearby town, so its inhabitants rush out to enjoy the show).
It is interesting to compare the discharge during a flood (with a graph known as a hydrograph) in a circulation through pores aquifer (typical of porous rock) and in a karst aquifer: the presence of big karst conduits makes the flood wave arrive faster and more concentrated in time so that its peak is much higher, for equal precipitations.
A flood wave is usually accompanied by an increase in the content in ions in solution in the water and transportation of additional materials. The relationship between the discharge and the variations in the content in ions in solution (calculated by measuring the electric conductivity) during a flood in a karst aquifer indicate that an increase in electrical conductivity is related to the piston effect on waters that have remained in the phreatic zone for a long time (and are therefore rich in carbonates), while a decrease in ions in solution is related to the direct arrival of infiltrated surface waters: hence, the analysis of these waters allows us to highlight the arrival of a dynamic flood wave rather than a ‘physical’ one. A chemical analysis of water and the study of the discharge of a spring are rather complex, but are essential to understand the flood mechanisms in a karst system.