Have you ever wondered
where your drinking water comes from?
A third of us get our drinking water
from underground.
It comes from special rocks
called aquifers.
Now these rocks hold water
very much like a sponge,
but not all rocks can do this.
There are two important things
that a rock needs to be an aquifer.
It needs to have spaces
in that rock to store water,
and it needs to be able
to let water flow through it.
These two things are called porosity,
which means it’s got
space to hold the water,
and permeability,
which means that water can
move through the rock.
I’ve got three rocks here that
have different properties,
and we’ll test out whether or not
they can hold the water,
and let that water pass through.
First of all, I’ve got a sandstone,
and I’m going to tip some water
onto the sandstone and
see what happens.
Now, it’s sitting on the surface
to begin with,
but as we’re waiting here,
it’s starting to slowly sink in,
and I know this because
it’s not on the surface like a bubble –
it’s starting to look a little bit wet,
and the bubble is sinking and lowering.
The second rock is basalt,
with lots of holes in it;
lots of bubbles in it,
and this is a volcanic rock.
You can guess what’s going
to happen here,
I’m going to pore some water,
and it’s sinking into those holes.
The third rock is a mudstone,
and if I pour some water on this,
that just sits there,
so that’s not getting absorbed,
that’s sitting on the top of the rock;
it’s not sinking in,
so it’s not permeable
and it’s not got the porosity
to hold water.
So we know that this first one
is both porous and permeable.
And it looks like
this second one is as well,
but we don’t know if it will
pass all the way through because
we don’t know if all of those bubbles
connect to one another.
And the third one, the mudstone,
it’s just sat on top,
so we can just brush it off the surface,
it isn’t sinking into the rock,
so that’s not porous
and it’s not permeable.
You can experiment with this yourself
even if you don’t have these rocks
because there are different things
that have these properties that
you might find in your kitchen.
This sandstone here is made up
of lots of individual grains of rock
that are stuck together
and have gaps in between.
And this can be very closely represented
by a jar of chocolate balls
that have spaces in between them,
so that’s going to be our sandstone.
Now we’ve got a rock
with lots of holes in it,
but we’re not sure if those holes
connect to one another
to make it permeable,
and that can be represented
by a bubbly chocolate bar.
And lastly, we’ve got our mudstone,
where water just sat on the top
and we can represent that by
a solid chocolate bar.
So we want to know if we can
use our sandstone as an aquifer,
so I’m going to put some milk
in between our grains
and we can see quite easily
that there is definitely porosity;
there are spaces between these grains
to hold the liquid,
and we’ll see if it’s permeable…
Yes, that’s permeable,
we can get the liquid out of the rocks
and it’s flowing through them.
The second one has holes in,
so it may be able to hold the water,
what about permeability:
can we get
the liquid out of this aquifer?
No.
So we know that that might be porous,
because it’s holding the water
within the spaces,
but it’s not actually permeable.
However, if there were
lots of cracks in this rock type,
it might be that the liquid
and the water in an aquifer
could still flow between them.
Finally is our last chocolate bar.
I probably don’t need to try
to show you what would happen
if you try and drink the milk
through the chocolate there –
it’s not porous,
and it’s not permeable,
and that’s very similar to our mudstone.
So have a go yourself using our
non-geological variations instead.