Earth in Action - basic aspects of groundwater flow

Okay now that you know a little bit about
ground water systems.

Some of the vocabulary associated with
them.

Let's talk about groundwater flow.

Groundwater is usually not stagnant,
okay, it's usually moving.

It isn't moving fast like a stream but it
is usually moving,

and typical flow rates would be on the
order of about a half a meter per day.

It flows away from areas where it enters
aquifers.

Two places where water exits aquifers.

We call places where water enters an aquifer
recharge areas and places where water

exits aquifers are referred to
as discharge areas.

So you can say that water flows from
recharge areas to discharge areas,

and that's what's shown on the slide.

The blue lines depict groundwater flow
paths and you can see that some of these

flow paths are fairly short.

The shortest ones, groundwater can flow
along in a matter of days.

Others, however, are quite long, uh,
water can be isolated from the, um,

from the surface and aquitards and
aquifers for thousands of years,

or even more.

Some research, um, recently identified
some groundwater that was at least as old

as 1.5 billion years old, okay, and been
isolated in the subsurface for that long,

which is incredible.

Okay so that-that would definitely be
exceptional.

So what controls the movement of rocks and
sediment, well the equation that we use to

describe the flow of water through coarse
materials like sand was, uh, defined by this

guy, Henry Darcy.

And the equation that he used, or that he
defined, is known as Darcy's Law.

So at some point he must have asked a
question, what the heck controls the

movement of water through sand?

And I imagine that we've all asked that
question time or two.

Although, he probably would've said it
in French, cause he was, um, he was

actually a French engineer, uh, just to
give you a little historical context on

this topic.

Uh, he was an engineer and he lived 'bout
the same period of time as

Abraham Lincoln.

During his life he was very famous for
bringing a water distribution system to

Dijon in 1840.

Okay this was a big deal because very
few places at the time had water

distribution systems.

Wasn't, it was also at this time that few
places had sewer systems, okay.

So if you were living in a city, a good
reliable source of clean water was

really important.

Um, for reference, Paris didn't have,
oops, Paris didn't have a water

distribution system until 1865, a full
20 years after the little city of Dijon

um, got it's water distribution system.

It wasn't until very late into his life, 2
years before he died, that he carried

out the experiments that perhaps he is
best known for today.

He performed the experiments in hospital,
that might seem like, y'know a really

odd choice, at the time there probably
weren't as many places where, uh, that

were, y'know, convenient for setting
up experiments.

The experiment that he performed, uh, he
published results in his report in 1856,

and uh, they were just described in a
couple of pages in the very back of the

report, I think one of the appendices
for the, um, for the report.

Uh, so it was very much just an
afterthought, but it turned out to be a

very, uh, a really useful important piece
of work.

So what we're gonna do now is walk through
some aspects of Darcy's experiment.

This will help illustrate some basic controls on
the movement of water through porous

medium, and also the process of science,
how you can apply some basic reasoning,

collect data, and then figure out
relationships.

So what Darcy did was, uh, set up a tube
that contained some sand, and that

tube had some smaller tubes in it called
manometers poking into the, near the in

flow and out flow end,

and he'd float water through the sand tube
and, uh, observed how different variables

affected flow.

Hydraulic head is the name given to the
height at water rises and the manometers

relative to some arbitrary data.

In natural environments we would typically
pick sea level to be the datum.

In a lab experiment you might just pick
the bench top where you're carrying out

the experiment, because it's more
convenient.

Hydraulic head is the sum of two
components, the height that water rises in

the tube as a result of water pressure, we
call that pressure head, and then the

height that the water has because of how
far the tube is above the datum.

Okay, so that would be elevation head.

What Darcy observed is that water always
flows from high head to low head, okay,

and that the rate of flow through that
sand tube is proportional to the

difference in hydraulic head measured
between the manometers,

in other words, Delta H, uh, the
difference between this-this hydraulic

head and this hydraulic head,
is Delta H right here.

Okay, how about column area?

How would this affect flow?

Well this is really-really straight
forward, basically if you were to put a

partition in the column such that the area
available to flow, uh, was divided into

the flow rate, uh, through each half of
the sand tube, would be equal to one half

of the total flow, okay, so from this,
Darcy concluded that the rate of flow is

proportional to the area of sand
available.

Lastly, what about the length of the
tube.

Well Darcy figured out that if you keep
the head difference between each end of

the tubes the same but you lengthen the
tube, it effects the flow rate, okay.

Uh, so imagine, in fact it causes flow
rate to decrease, imagine that this sand

tube is one foot long, and that the
difference in head is 6 inches.

Okay, that's a pretty steep change in

hydraulic head over the-over the length of
that tube, right.

And so water would want to flow through
that fairly quickly.

But if the tube were 10 feet long that 6
inch drop over 10 feet isn't nearly

as steep.

In that case the water wouldn't move
through as fast, okay.

So from this we can conclude that, um, the
length, the flow through the tube is

inversely proportional to length.

In other words, if all else is the same,
as length increases, flow decreases.

Okay, so these 3 lines just summarized
what we just discussed flow is directly

proportional to the, uh, difference in
hydraulic head between each end of the

tube, it's proportional to the area of the
column, and it's inversely proportional

to the length of the column.

Okay, this symbol right here is, uh,
means proportional, right.

So this means inversely proportional,
because 1 over L.

So if we put this together, okay, this is
what we get.

Now we can replace this proportional sign,
with a sign, with an equal sign, by adding

a constant of proportionality, 'K',
and when we do that, this is what we

get, okay.

And if we just rearrange this a little
bit, we end up with this form of the

equation, and that is Darcy's Law.

So it really is not that complicated.

If you understood each part, each of the
parts that we went through in the previous

slides, this just combines them all into
one equation.

Um, one of the things that might seem a
little mysterious is this 'K'.

What the heck is 'K'?

Well, 'K' is hydraulic conductivity.

So let me explain this a little bit.

Um, basically you can think of it as,
y'know, basically the same thing as

permeability, it's a little bit different,
but think about it as permeability.

Aquifers have high hydraulic conductivity,
um, because they can transmit water

relatively easily, water can flow through
them, they're fairly permeable, right.

Aquitards have low hydraulic conductivity
they're relatively impermeable.

Water does not flow through them very
easily.

In reality, hydraulic conductivity is a
little bit more then permeability.

It combines permeability with properties
of the fluid.

But in any case just think about it as a
measure of how easily water can flow

through a porous meeting.

Okay, so how do geoscientists use Darcy's
Law and this information that I just

gave you?

Essentially you can think of wells, uh, as
the same thing as those manometers.

They fill the same roll as the manometers
I showed you in Darcy's experiment.

So we can go out and measure the elevation
of water in a well, and that tells us the

hydraulic head, where that water, where
that well is open and an aquifer.

Okay, so, so we can use these measurements
to help determine what direction ground

water is flowing in an aquifer, okay, so
in the, uh, in the illustration shown here,

this simple illustration, we would conclude
that ground water is flowing from the

left to the right.

Towards well B.

Because hydraulic head, uh, decreases
in that direction.

Remember ground water always flows from
high head to low head, okay.

And we can use this to help understand
flow rates, by assessing ground water

flow and combining it with, uh,
knowledge of geological properties of

aquifers we can evaluate how much water is
available in aquifers and how that

changes over time, okay.

How do we use this?

How is this useful?

Well given the extent to which we rely
on ground water as a source of drinking

water and irrigation water, this
information is extremely important,

because it helps us manage critical
water sources more effectively.

This is a huge, uh, this is a big deal.

It's really important, these water sources
are extremely important so we need to be

able to manage them effectively.

One of the examples, um, that I'm
going to focus on in the le-in the next

lecture, is shown here.

It is the high planes aquifer.

Okay, an aquifer that plays a vital role
in sustaining human populations in the

Great Plains, and an aquifer that's up
against some serious challenges

in the future.

So I look forward to talking about that
with you in more detail in the

next lecture.

Thank you for your attention.