WEBVTT

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

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Some of the vocabulary associated with
them.

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Let's talk about groundwater flow.

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

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

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

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It flows away from areas where it enters
aquifers.

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Two places where water exits aquifers.

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

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exits aquifers are referred to 
as discharge areas.

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

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

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

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flow paths are fairly short.

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

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

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

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or even more.

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

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

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which is incredible.

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Okay so that-that would definitely be
exceptional.

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

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

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guy, Henry Darcy.

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

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

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movement of water through sand?

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

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

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

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this topic.

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

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Abraham Lincoln.

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

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Dijon in 1840.

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

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distribution systems.

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

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

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really important.

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

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

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um, got it's water distribution system.

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

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

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

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

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

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

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

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

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

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

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

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

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

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collect data, and then figure out
relationships.

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

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

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flow and out flow end,

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

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affected flow.

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

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relative to some arbitrary data.

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

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

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the experiment, because it's more
convenient.

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

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

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

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Okay, so that would be elevation head.

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

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

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difference in hydraulic head measured
between the manometers,

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

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

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Okay, how about column area?

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How would this affect flow?

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

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

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

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

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proportional to the area of sand 
available.

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Lastly, what about the length of the
tube.

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

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

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

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

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Okay, that's a pretty steep change in

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

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

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

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as steep.

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

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

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inversely proportional to length.

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

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

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

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

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to the length of the column.

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

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So this means inversely proportional,
because 1 over L.

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

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

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

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get, okay.

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

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equation, and that is Darcy's Law.

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So it really is not that complicated.

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

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slides, this just combines them all into
one equation.

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

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What the heck is 'K'?

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Well, 'K' is hydraulic conductivity.

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So let me explain this a little bit.

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

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

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

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

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Aquitards have low hydraulic conductivity
they're relatively impermeable.

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Water does not flow through them very
easily.

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

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It combines permeability with properties
of the fluid.

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

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through a porous meeting.

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

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gave you?

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

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

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

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

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

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

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

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left to the right.

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Towards well B.

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Because hydraulic head, uh, decreases
in that direction.

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

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

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

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

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changes over time, okay.

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How do we use this?

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How is this useful?

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

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water and irrigation water, this 
information is extremely important,

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because it helps us manage critical
water sources more effectively.

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

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

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able to manage them effectively.

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

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lecture, is shown here.

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It is the high planes aquifer.

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

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

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in the future.

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

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next lecture.

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Thank you for your attention.