Awesome Aquifers B

[FueL]

Porosity = amount of pore space. The greater the porosity, the more water the rock can hold. Permeability = how quickly water percolates through the rock. This depends on both the size of the pores and how interconnected they are.

A good example of this is gravel and sand. Sand has a higher porosity, but a lower permeability.

[TYG]

What are some general statements that can be made about porosity and permeability? For example, something like, "Well sorted sediment is more permeable than unsorted sediment." (not sure about the accuracy of that statement, though)

[FueL]

What are some general statements that can be made about porosity and permeability? For example, something like, "Well sorted sediment is more permeable than unsorted sediment." (not sure about the accuracy of that statement, though)

Thinking back to my earth science days, I think both porosity and permeability can apply to your statement. Sediment of uniform size has a higher porosity than unconsolidated sediment.

Here's another one: porosity doesn't change with particle size. Sand will have the same porosity as silt, given that all the particles are the same size.

Sorry I couldn't be of more help.

[TYG]

no, it was really helpful, thanks.

Another question:
If gravel and sand are both unconsolidated rock, and gravel is larger than sand, how is sand more porous? Generally, isn't larger sediment more porous than finer sediment, granted they are relatively sorted, since there are larger spaces between the larger sediment?

[Skink]

At regionals, two concepts were porosity between two materials, and permability between two materials...whats the exact difference? (yes i know, i fail...Hence the name)

The reason they ask the two together is for this reason...people tend to confuse the two. A lot. Make sure you read and understand FueL's post.

What are some general statements that can be made about porosity and permeability? For example, something like, "Well sorted sediment is more permeable than unsorted sediment." (not sure about the accuracy of that statement, though)

Uhhhh...who needs general statements? Okay, let's see...
One you can't make is to try to relate them like this:
As porosity increases, so does permeability.
There are exceptions to that rule. For the strata you're using, it probably holds true, but don't assume because you know what happens then.

The substances I've used in this event all tend to be around 37-40% porous. It's not a general rule because you may be using different rocks 'n' things, but it helps to have an idea where your strata are--especially any mixed ones--if you haven't bothered to measure the porosity yourself yet.

Sediment of uniform size has a higher porosity than unconsolidated sediment.

And it always applies? Suppose the uniform size statum fits together really well and has small pores (so low porosity) while the unconsolidated stratum is full of odds and ends that leave large pores available to store water. I mean, I can see it going both ways...or maybe I'm splitting hairs.

If gravel and sand are both unconsolidated rock, and gravel is larger than sand, how is sand more porous? Generally, isn't larger sediment more porous than finer sediment, granted they are relatively sorted, since there are larger spaces between the larger sediment?

...uh, because it isn't? The study guide says sand holds approximately 20% and gravel 25%. My layers run at least 10% higher than that on average. Did you find some special sand or something? Try it and report your numbers back. I'd be curious to hear what you got.
By the way...are you using sand in your model? I can't stand the stuff. It's messy, it gets everything, sticks to everything, clogs everything...

[FueL]

And it always applies? Suppose the uniform size statum fits together really well and has small pores (so low porosity) while the unconsolidated stratum is full of odds and ends that leave large pores available to store water. I mean, I can see it going both ways...or maybe I'm splitting hairs.

No, there are exceptions like you said, but for the most part it holds true. I made a mistake there, I meant mixed, not unconsolidated, but oftentimes with loose mixed sediment the smaller grains will fill up the pore spaces between the larger grains.

...uh, because it isn't? The study guide says sand holds approximately 20% and gravel 25%. My layers run at least 10% higher than that on average. Did you find some special sand or something? Try it and report your numbers back. I'd be curious to hear what you got.

That's strange, the book I'm reading says that typical gravel holds 32% while sand holds 42%. This website also shows that sand has a higher porosity than gravel.

If gravel and sand are both unconsolidated rock, and gravel is larger than sand, how is sand more porous? Generally, isn't larger sediment more porous than finer sediment, granted they are relatively sorted, since there are larger spaces between the larger sediment.

Well, first, theoretically we're considering sand/gravel particles that are perfectly spherical, while this obviously doesn't occur in real life. But along these theoretical lines, gravel and sand would have the exact same porosities. See this image. Although the larger sediment does have larger pores, the smaller sediment has many more of them. If you look at the image, the small sediment all the way on the right would look exactly the same as the larger sediment on the left if you just "zoomed in" a little. Thus all four images have the exact same porosities.

But yeah, it's obviously not like that in real life, as differently sized sediment do have different porosities. My gut explanation for this is that there are variations in size and shape within the category of "sand" or "gravel," which affects their porosities. Hence the porosity of one sample of sand will differ from another sample slightly, but we can still make general comparisons by looking at their average porosities.


Edit:

By the way...are you using sand in your model? I can't stand the stuff. It's messy, it gets everything, sticks to everything, clogs everything...

Yeah, I've been trying to use sand, but I've had pretty bad experiences with it so far... XD What would you suggest as an alternate?

[Epicfail]

Thanks, also, there was a concept called like, Legrate or something? what is that? and how is it really spelled?

[FueL]

Leachate?

Leachate is any water that drains from landfills and is probably contaminated as a result. They might ask you to show how to prevent leachate from contaminating the groundwater supply.

[TYG]

Well, first, theoretically we're considering sand/gravel particles that are perfectly spherical, while this obviously doesn't occur in real life. But along these theoretical lines, gravel and sand would have the exact same porosities. See this image. Although the larger sediment does have larger pores, the smaller sediment has many more of them. If you look at the image, the small sediment all the way on the right would look exactly the same as the larger sediment on the left if you just "zoomed in" a little. Thus all four images have the exact same porosities.

But yeah, it's obviously not like that in real life, as differently sized sediment do have different porosities. My gut explanation for this is that there are variations in size and shape within the category of "sand" or "gravel," which affects their porosities. Hence the porosity of one sample of sand will differ from another sample slightly, but we can still make general comparisons by looking at their average porosities.

So since the particle shape varies between each sample, you can't really say that gravel is more porous than sand as a general statement?

By the way...are you using sand in your model? I can't stand the stuff. It's messy, it gets everything, sticks to everything, clogs everything...

Nope, no sand. Every time water is added, the sand layer kind of implodes and sinks, so when the layers are tilted, it evens out.

[FueL]

So since the particle shape varies between each sample, you can't really say that gravel is more porous than sand as a general statement?

If you're going to make any general statement, it would be that sand is more porous than gravel. But in some cases, gravel can be more porous than sand. When talking about porosity we're concerned about the norm, not every single case.