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.