“Permeable” is “Porous,” but “Porous” May Not Be “Permeable”
Outside of math, A = B may not necessarily mean B = A.
A pore is a space in rock, soil, or unconsolidated sediment that is not occupied by mineral matter and that allows the passage or absorption of fluids.
The word comes from Latin porus, Greek poros,where it means “passage.” A thing that is porous is having pores, and porosity is the condition or situation of being porous.
There are several properties of aquifers that determine how much water can be drawn from them. Porosity is the percentage of rock or soil that is void of material and thus can be filled with water. The larger the pore space or the greater their number, the higher the porosity.
Porosity in Percentage
Material |
Primary Openings |
Equal-size spheres (marbles)
Loosest packing
Tightest packing |
48
26 |
Soil |
55 |
Clay |
50 |
Sand |
25 |
Gravel |
20 |
Limestone |
10 |
Sandstone (semiconsolidated) |
10 |
Basalt (young) |
10 |
When a fluid permeates a medium, it spreads, penetrates or flows throughout that medium. In Latin, per means through and meare means to pass. A permeable substance is one than is able to be permeated.
If pore spaces in sediment or bedrock are interconnected, then water can travel through those easily. This material is said to be permeable. Permeable rock units make good aquifers.
The property of permeability is related to porosity.
In qualitative terms, permeability is expressed as the capacity of a porous rock or soil to transmit a fluid.
Large interconnected pore openings are associated with high permeability, while very small unconnected pore openings are associated with low permeability. Sand and gravel with large interconnected pore openings have high porosity and permeability.
Clay tends to have high porosity, but the very small openings tend to inhibit the passage of water. Therefore, clay displays low permeability. |
