A

Lime in the water cycle

Spring limestone

Lime precipitation in a spring creek

The groundwater is saturated with lime - in lime-carbonic acid equilibrium. The CO 2 content of the groundwater is always higher than it corresponds to the equilibrium with the atmosphere. So, in the spring CO 2 escapes to the air, the pH value rises and the water in the spring becomes calciferous. This leads to the deposition of lime in the course of the stream. This is particularly evident in the case of a piece of wood from the stream, where half of the lime shell has been removed (figure).

In the soil, the biological decomposition of organic matter produces further CO 2 , which can dissolve in the percolating water. This increases the dissolution of lime in the soil. Interim balance : So we have a water that has been enriched with CO 2 in equilibrium with the soil air and has thus increasingly dissolved lime. The content of CO 2 in the water is higher than would be possible in equilibrium with atmospheric air.

Formation of dripstones

If this water is saturated with lime (calcite saturation) and then emerges from the

ground/rock in an underground cave, for example, CO

2 is lost in gaseous form to the

cave air. The process described above is reversed:

Summarized, simplified : Calcite precipitates out of the water and dripstones are formed.

Chemical equation: Increased dissolution of lime due to CO 2

Chemical equation: Precipitation of lime by CO 2 removal

The same process can occur with water saturated with lime in a spring. CO 2 outgasses and is lost to the air. Lime precipitates and forms so-called spring limestone.

However, this only happens in a few wells, since the emerging waters are usually not saturated with lime and the outgassing CO 2 then does not lead to lime precipitation.

Lime is contained in rocks of the earth's crust.

Limestone

consists

mainly

of

calcite,

with

a

small

proportion

of

aragonite.

Dolomite

is

a

carbonate

mixture

of

CaCO

3 and

MgCO

3 .

Limestone

and

dolomite

were

formed

by

precipitation

of

lime

and/or

sedimentation

of

calcareous

shells

of

microorganisms

(e.g.

foraminifera) in the primordial oceans.

However, many rocks are free of lime, for example basalt and granite.

Contact with water leads to the dissolution of lime from the rock.

Lime

dissolves

physically

in

the

water

(see

calcite

saturation).

However,

the

solubility

is very low, so that only low concentrations of calcium would be expected in the water.

The presence of carbonic acid in water increases the solubility of lime in water. The natural CO 2 content in rainwater leads to an acidification of the water to pH 5.6. Due to anthropogenic gases (e.g. SO 2 , NO X ), which form acids together with water, the pH of rainwater can be even lower. In the soil, this promotes the dissolution of minerals and lime. Summarized, simplified :

Karstification Karstification often occurs in limestone / carbonate rock. Water with dissolved carbonic acid penetrates fissures and crevices in the rock. As a result of the dissolution of the lime, fissures become crevices in geological time periods and some of the crevices become large cave systems. An example of such a cave system is the Hölloch in the Mahdtal (Kleinwalsertal, Austria) in the area of Hoher Ifen and Gottesackerplateau. The measured passage length of this cave is 12,900 meters (http://www.hoelloch.de/index.php). The entrance maw to this cave, the Hölloch, goes 76 meters into the depth. The karstified rock (Schrattenkalk) of the Hohen Ifen and the Gottesacker drains through this cave system. The water comes back to the surface in three larger and some smaller karst springs.

Figure: Hölloch, 76m deep , access to the karst cave

Stream Shrinkage

Surface water in karst areas can seep into the crevices of the karst, then flow directly underground through these cavities, sometimes forming underground lakes and rivers. If entire streams or rivers disappear underground, this is called stream swallowing (swallow hole), seepage (laminar flow), or sinking (turbulent flow). ( uncertainty about the correct translation ) Well-known is the Danube sinkage near Immendingen (Baden Württemberg, Germany), where at low water the whole river seeps into the ground. At high water, an outflowing residual volume of water remains in the riverbed. The water returns to the surface in the Aach spring (see below).

Karst springs

In karst aquifers, the water flows underground at high flow velocities, comparable to surface waters. When the water returns to the surface in karst springs, they have a very high spring discharge. Germany's largest karst springs 1. Aach spring (Radolfzeller Aach, flows into Lake Constance) Average discharge 8,590 liters per second (1,300 to 24,000 L/s) (Most of the water comes from the Danube sinkhole near Immendingen) 2. Blautopf (Blau, flows into the Danube) Average discharge 2,280 liters per second (250 to 32,670 L/s) 3. Rhume spring (Rhume, flows into the Leine) Average flow rate 2,000 liters per second 4. Largest spring area in Germany Pader springs (Pader, flows into the Lippe) All springs together average 5,000 liters per second (3,000 to 9,000 L/s)

Estavelle

An Estavelle is both a stream sinkage and a karst spring. At low water levels, it acts as a stream sinkage and absorbs the surface water that flows underground. At high water levels, the groundwater body is filled to such an extent that the estuary overflows and becomes a karst spring. Estavelle during its time as a stream sinkage (Schwarzwasserbach in Kleinwalsertal)

See chalk

In lakes, intensive photosynthesis can lead to an increase in the pH value due to CO 2 consumtion. If the lime-carbonic acid equilibrium is exceeded, lime precipitation occurs in the water and lake chalk forms. In Lake Constance, lime deposits are often found on the leaves of aquatic plants (biogenic decalcification). White calcareous sediments in the shore area of Lake Constance are known as "Wysse".