EPA Groundwater Page. National Groundwater. The Groundwater Blog. The Groundwater Foundation. The pore space of an aquifer is the spaces or voids between the solid material. The porosity of the aquifer is the volume of void space to the total volume, typically expressed as a percentage. Effective porosity is the space available for fluid to flow , and is calculated as the volume of interconnected void space to the total volume, typically given as a percentage.
Porosity is highly dependent upon the arrangement, the shape, and the size of the material. However the diameter of the grain size does not affect porosity, since it is the ratio of open space volume to total volume. Porosity is a description of how much space there could be to hold water under the ground, and permeability describes how those pores are shaped and interconnected.
This determines how easy it is for water to flow from one pore to the next. Larger pores mean there is less friction between flowing water and the sides of the pores. Smaller pores mean more friction along pore walls, but also more twists and turns for the water to have to flow-through. A permeable material has a greater number of larger, well-connected pores spaces, whereas an impermeable material has fewer, smaller pores that are poorly connected.
Permeability is the most important variable in groundwater. Permeability describes how easily water can flow through the rock or unconsolidated sediment and how easy it will be to extract the water for our purposes.
The characteristic of permeability of a geological material is quantified by geoscientists and engineers using a number of different units, but the most common is the hydraulic conductivity. The symbol used for hydraulic conductivity is K. The materials in Figure Unconsolidated materials are generally more permeable than the corresponding rocks compare sand with sandstone, for example , and the coarser materials are much more permeable than the finer ones.
The least permeable rocks are unfractured intrusive igneous and metamorphic rocks, followed by unfractured mudstone, sandstone, and limestone. The permeability of sandstone can vary widely depending on the degree of sorting and the amount of cement that is present.
Fractured igneous and metamorphic rocks, and especially fractured volcanic rocks, can be highly permeable, as can limestone that has been dissolved along fractures and bedding planes to create solutional openings. The surface of most silicate mineral grains has a slight negative charge due to imperfections in the mineral structure. For porosity to affect permeability these bubbles need to be interconnected as shown in Figure 4.
Otherwise, water would never be able to reach the air space and the rock would not be very permeable. A wiffle ball, seen in Figure 5, has holes around the outside and is hollow in the middle. If a bucket was filled with wiffle balls instead of baseballs and then filled with water, the bucket would hold more water than the bucket filled with baseballs because the water would flow through the wiffle balls.
Pumice is a type of rock that has these properties i. Porosity refers to how much air space is in the soil or rock. Permeability refers to the ease with which water can travel through the material. In many cases, rocks or soil with high porosity will also have high permeability. The water will pass quickly through the air spaces in the rock or soil.
For the water to travel through the air spaces, those air spaces must be connected. When rock or soil that has high porosity has air spaces that are not connected, the rock or soil has low permeability even though it has high porosity.
A bucket filled with ping pong balls that is then filled with water would not hold very much water. Even though the ping pong balls have air spaces inside of them, the water cannot travel through the ping pong ball. Now imagine pouring water on a block of Swiss cheese.
Swiss cheese has a lot of holes in it, but these holes do not extend all the way through the entire block of cheese. Even though the cheese has high porosity, it does not have high permeability because the water would not flow through.
What is mean by ground water? Groundwater is the water found underground in the cracks and spaces in soil, sand and rock. It is stored in and moves slowly through geologic formations of soil, sand and rocks called aquifers. What material has a high porosity and low permeability? A good example of a rock with high porosity and low permeability is a vesicular volcanic rock, where the bubbles that once contained gas give the rock a high porosity, but since these holes are not connected to one another the rock has low permeability.
Which soil has the highest permeability? The soils with the best permeability also have the greatest porosity. Larger grains with smaller surface areas drain the best. Clay has a small grain but a large surface area, which means it bonds and doesn't drain. Permeability in sand is high. Does sand have high or low permeability? Generally speaking, sand is more porous than clay because sand particles are larger and the pore spaces between the particles are also larger. However, typically speaking, a sand is both more permeable and more porous than a clay - but only typically.
What factors control the porosity of an aquifer? Several factors can affect porosity.
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