At the other end of the scale, some rocks have lots of pore space between grains and so there is room for air, water or other liquids and gases. In the top diagram, where there is any material or cement in the rock to fill in the gaps holding the grains together, there will be less pore space.
The bottom diagram shows that the more space between the grains, the greater the porosity. The porosity of a rock varies because of the size of the grains in the rock and the shape of the grains. Another factor that affects the porosity of a rock is whether or not there is any material in the rock or cement to fill in the gaps between pore spaces and hold the grains together.
If a rock has a lot of gaps between grains it is said to have good porosity and a lot of water can fit between the grains. A rock with good porosity can hold a lot of groundwater. For groundwater to be able to get into a rock with good porosity it must also have good permeability. This is an example of a confined aquifer. In this case, the rocks surrounding the aquifer confines the pressure in the porous rock and its water.
This type of well is called artesian. The pressure of water from an artesian well can be quite dramatic. A relationship does not necessarily exist between the water-bearing capacity of rocks and the depth at which they are found.
A very dense granite that will yield little or no water to a well may be exposed at the land surface. Conversely, a porous sandstone, such as the Dakota Sandstone mentioned previously, may lie hundreds or thousands of feet below the land surface and may yield hundreds of gallons per minute of water. Rocks that yield freshwater have been found at depths of more than 6, feet, and salty water has come from oil wells at depths of more than 30, feet.
On the average, however, the porosity and permeability of rocks decrease as their depth below land surface increases; the pores and cracks in rocks at great depths are closed or greatly reduced in size because of the weight of overlying rocks. Water movement in aquifers is highly dependent of the permeability of the aquifer material. Permeable material contains interconnected cracks or spaces that are both numerous enough and large enough to allow water to move freely. In some permeable materials groundwater may move several metres in a day; in other places, it moves only a few centimeters in a century.
Groundwater moves very slowly through relatively impermeable materials such as clay and shale. After entering an aquifer, water moves slowly toward lower lying places and eventually is discharged from the aquifer from springs, seeps into streams, or is withdrawn from the ground by wells.
Groundwater in aquifers between layers of poorly permeable rock, such as clay or shale, may be confined under pressure. If such a confined aquifer is tapped by a well, water will rise above the top of the aquifer and may even flow from the well onto the land surface. Water confined in this way is said to be under artesian pressure, and the aquifer is called an artesian aquifer. Fill a plastic sandwich baggie with water, put a straw in through the opening, tape the opening around the straw closed, do not point the straw towards your teacher or parents, and then squeeze the baggie.
Artesian water is pushed out through the straw. The other type is a confined aquifer that has an aquitard above and below it. An aquitard is basically the opposite of an aquifer with one key exception. By squeezing that sponge we force the water out, similarly, by pumping an aquifer we force the water out of pore spaces. There are lots of terms in hydrogeology, most of which are very simple, but essential.
Here are a few of the big ones and their meanings. Porosity is the amount of empty space in sediments or rocks. I n a soil or rock the porosity empty space exists between the grains of particles or minerals. In a material like gravel the grains are large and there is lots of empty space between them since they have angularity or spherical shape. However, in a material like a gravel, sand and clay mixture the porosity is much less as the smaller grains fill the spaces.
The amount of water a material can hold is directly related to the porosity since water will try and fill the empty spaces in a material. We measure porosity by the percentage of empty space that exists within a particular porous media. Figure 2. Porosity in two different media. The image on the left is analagous to gravel whereas on the right smaller particles are filling some of the pores and displacing water.
Therefore, the water content of the material on the right is less. Source: Wikipedia. Figure 3. Video showing how connected pores have high permeability and can transport water easily. Note that some pores are isolated and cannot transport water trapped within them. Permeability is another intrinsic property of all materials and is closely related to porosity. For example, if particles of gravel are all the same size see Figure 2 , they will stack on top of each other neatly with some gaps.
To use a bucket analogy, stacking the gravel pieces shown in Figure 3 would be like filling a bucket with baseballs. Since the baseballs stack neatly, there are spaces in between the balls, allowing water to flow through them fairly easily. If the gravel pieces are all different sizes see Figure 3 then the smaller grains will fill in the spaces between the larger grains, therefore reducing the overall amount of air space in the mix. Using a similar analogy this would be like filling the same bucket with both baseballs and golf balls.
It would be still be possible to add water to the bucket but the addition of the golf balls would mean there is less room to hold water in the bucket. If grains in rock themselves are porous see Figure 4 giving the rock an appearance of being filled with bubbles, or vesicles, there will be a large amount of air space in the rock. 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.
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