Porous geological formation: Soil layers that allow water to penetrate and circulate through their mass. In other words, they can function as reservoirs (water is stored in the pores) and as a means of transporting water (transporting water through the pores).
The amount of water that can be stored in an aquifer depends on the porosity (n) of the soil formation.
The amount of water that an aquifer can deliver depends on the hydraulic conductivity (k) of the soil formation.
Porosity
Porosity: The ratio of the volume of voids (Vv) to the total volume of soil (VT). Soil porosity is denoted using the letter n.
n= Vv / VT
Exercise: Calculate soil porosity
We get a plastic tank with a volume of 1m3 and we fill it up with sand. We start adding water to the tank and the voids within the sand start to fill up with water. We continue adding water until the water reaches the top of the tank (without overflowing). In total, we added 330 L of water. What is the porosity of that specific sand?
Vv= 330 L= 0.330 m3
VT= 1 m3
Porosity: n= VV / VT = 0.330/1 = 33%
Hydraulic conductivity
Hydraulic conductivity depends primarily on the soil. You mean that the hydraulic conductivity is not constant throughout the mass of the aquifer. In practice, a first assessment is made by soil type (eg fine sand well graded). What accurate estimate is made by pumping tests.
Specific yield: Is the maximum volume of water that can drain (Vy) from fully saturated soil divided by the total volume of the soil (VT)
Sy=Vy/VT
When water drains out a soil some quantity is held on the soil particles due to capillary effects and does not drain out of the soil. This is very obvious in soils containing clay. Clay soils absorb water but it is hard to drain this water out of the soil. On the contrary sandy soil will fill in the void with water which can easily drain out.
A quantity of water is retained by capillaries and other phenomena and does not drain by gravity. Do you mean that in the laboratory we can drain it (oven drying).
Exercise. An aquifer has an area of 6 km2. In the aquifer, there is a drilling system for the irrigation of the wider area. After pumping 3.5 MCM the fall of the aquifer was 2.5 m. What is the specific yield?
Answer: ……
Specific yield is an important parameter in groundwater engineering. Why?
When pumping water out of an aquifer the water is drained out of the formation. Specific yield is an indication of how easy is to drain water out of the formation.
Example: Calculate the specific yield of an aquifer
An unconfined aquifer extends in an area of 6 km2. A system of boreholes allows the pumping of water from the aquifer for irrigation purposes. After a continuous pumping of 3,5 MCM the water table goes down by 2.5. Calculate the specific yield.
Typical values for porosity (n), hydraulic conductivity and specific performance
Ερώτηση: To which category of lands do the following belong:
• A soil material with a grain diameter of 0.63 – 2 mm
• A soil material with a grain diameter of 2 – 63 mm
Ερώτηση: What is the main difference of clay (Clay soils) in relation to other soft soils (eg sands, gravel)