Hydrogeological properties of the soil
Considering the evident relationship between the wellpoint system and the hydrogeological characteristics of the soils, it is useful to describe their properties.
Observing what is generally called “soil” in a section that descends deeply from the ground level, we see that it is made up of three zones:
- superficial zone of predominantly vegetable and organic constitution;
- secondary or evaporation area that does not retain water due to evaporation and filtration to underlying areas;
- saturation zone in which all the interstices are occupied by water which commonly takes the name of “groundwater”.
The upper limit of this saturation zone is called groundwater level and represents the level of static water outcrop. The lower level of the groundwater is generally represented by an impermeable layer; therefore, it’s evident that the groundwater and its movements are conditioned by the shape and position of the impermeable layer that supports it. Of course, going deeper there are other separate layers, as many as the impermeable ones delimiting them. If the aquifer is delimited by two impermeable layers, there is also a pressurized aquifer also known as the “artesian aquifer”. If the upper layer of the artesian aquifer is drilled through a well, the water rises along the well until it reaches a higher level – often close to the ground – called the dynamic level of the deep aquifer.
The main factors affecting the thickness of the groundwater are:
rainfall;
waterways;
irrigations;
drainages.
From a structural point of view, the aquifer is made up of solid particles of various types and shapes, aggregated so to leave empty spaces or interstices that are occupied by water. The presence of these continuous voids in loose soils define a characteristic property called “porosity”.
It depends on factors such as:
size of the granules;
shape of the granules;
arrangement of the granules.
It has been said that it is through these voids that water infiltrates, and the greater or lesser quantity is dependent on them. The property of the soil to be permeated by water is defined as “permeability” and it is quantified with the coefficient K which is measured in cm / sec. It expresses the flow rate of the water in the section of land considered (Darcy). The flow rate of water in the ground is proportional to the hydraulic gradient of the aquifer, so to the height-extension ratio according to the relationship.