Hi, my name is Michael Gooseff and I'm an Associate Professor in the Department of Civil
and Environmental Engineering at Colorado State University.
I've developed this presentation on groundwater and the Ogallala Aquifer.
We will talk a little bit about the basics of groundwater.
Then we'll talk about how we apply our knowledge of groundwater and groundwater management
to the Ogallala Aquifer which is one of the largest aquifers in the world.
It's in the central part of the United States.
Groundwater is an abundant resource.
Roughly 1.7 billion people around the world rely on aquifers for water source for municipal
use.
From this graphic from the US Geological Survey you can see there's an estimated 1,000,000
cubic miles of groundwater in the Earth's crust whereas there's only 30,000 cubic miles
of water stored in lakes and 300 cubic miles stored in the world's streams.
One advantage to groundwater resources is they're relatively still or moving very slowly
compared to the world's streams which can go through, of course, floods and droughts
rapidly changing the amount of water at any one place at any one time.
Groundwater occurs, as you would expect, in the subsurface.
It occurs below ground, hence where it's often found is invisible to us as the surface.
This is a schematic looking at just a cross section of surface water, maybe that's a lake
or a stream on the right-hand side.
Underneath this hill slope you can see there are two layers indicated here, an unsaturated
zone near the surface where we have some water stuck between particles of soil, rocks and
so forth, but not a whole lot.
The majority is air for the most part.
Then, there's a dashed blue line indicated a water table.
Below that water table we expect that the pores are mostly filled with water and we
call that the saturated zone.
That saturated zone is where we find groundwater and we call that area an aquifer.
The definition of an aquifer is a body of saturated rock through which water can easily
move.
At the surface, it is of course bounded at the surface by the land itself, we can have
the water table rise up to the land surface in which case we'll have that water ponding
or springs coming out near the surface.
Oftentimes it's depressed below the land surface.
That water table then defines the upper boundary of where our groundwater is stored.
An aquifer at depth might be bounded by bedrock or a harder layer or clay layer at depth.
If we look a little closer at what's going on in the ground, we see that the subsurface
is essentially a mix of rock or soil particles, some gaps in between those particles that
contain both some air and some water.
You can see on the left-hand side, we have more fractured material where we've got conduits
in between the particles.
On the right-hand side we have a more gravelly set-up where the pores are in between the
particles touching each other.
One of the things that happens is that inside these crevices the way that we get unsaturated
zones is that there's surface tension which is a property of the liquid water itself that
will stick water into tight little gaps in between these soil particles.
If we try to drain that water via gravity, we end up leaving a little bit behind in these
tight locations.
Where does groundwater come from originally?
Is it just generated in the ground?
No.
In general, groundwater comes from precipitation that infiltrates through the land surface
and reaches some depth to collect and form this saturated zone in the aquifer.
On the left-hand side you can see this sort of very simple diagram of the water cycle
where storm clouds are forming precipitation, that precipitation hits the ground and if
it comes too fast, the rain is applied faster than the ground can take it up, then there
may be some runoff that occurs.
Alternatively, a lot of that water is going to soak into the surface soils, work its way
into sediments, and reach the water table and contribute to the water stored in the
aquifer.
On the right-hand side you can now see a different approach to trying to provide water into an
aquifer.
On the extreme right side, we can have natural recharge from streams or lakes.
If the water table on the left-hand side as we see is depressed in elevation, it's at
a lower elevation than a stream or lake in which the aquifer is in contact, we can have
water move from the surface water into the ground.
In the middle here, toward the right-hand side, we have an injection pipe that is noted
where, essentially, water is being forced into the subsurface.
This might be a strategy that is required to try to store water in the subsurface for
later times in the year or something along those lines.
We use artificial recharge in a variety of different contexts to try to manage water
and sometimes the constituents of that water.
I alluded to the fact that water bodies can often act as boundaries to aquifers in the
previous slide.
Here's another example of that.
This is an indication of a stream moving through the center of this landscape.
We can see that the water table is generally at a lower elevation on both sides of the
stream in the subsurface.
What we would expect then is that water would leave the stream and work its way toward the
water table, contributing to groundwater in the local aquifer here.
But, we can also have the opposite.
We can have water tables that are fairly high compared to the elevation of water in a stream
channel or a lake or an ocean, in which case we expect that the flow of water is from the
groundwater or from the aquifer toward the surface water body.
Streams, lakes, oceans, other surface water bodies, can act as both sinks and sources
of groundwater.
Let's look at a simple example of how we figure out the shape of the water table and the direction
of groundwater flow.
In panel A here, you can see a bunch of circles that are shown.
These are meant to represent the locations of wells in the real world.
Each one of these numbers next to each one of those wells is the notation of the water
table elevation as observed in each one of these wells.
This is interpreted in feet above sea level.
You can see in the top right-hand corner, 152.31 feet above sea level and in the lower
left, almost in the lower left corner, 121.34.
Those are our extremes.
All the other wells read somewhere in between.
We can take those elevations, that distribution in space and try to predict the contours or
the shape of the water table is.
If we draw in these contours, you can see that we have the 150 foot, 140, 130, and 120
foot contours that come in panel B here.
That gives us the shape of the water table.
We expect that this is much like the topography of the landscape, if you looked at a topographic
map.
We are going from high elevation water table in the right upper corner to a much lower
elevation, a drop of thirty feet or so as we get to the lower left-hand corner.
What that means is that we expect that groundwater is flowing from the upper right to the lower
left.
Our groundwater flow lines are going to flow or be in the direction that is perpendicular
to these contours.
In general, groundwater flows from locations of high water tables to locations of lower
tables.
The flow lines that we would expect to draw would come in in panel C.
How do we use groundwater as a resource?
We have to pump it if we want to use it in a variety of places that are away from the
natural discharge locations.
Natural discharge locations could be places like we see in the interactions with surface
water bodies as we talked about earlier, but it can also be in seeps and springs that occur
in a variety of places.
Often, what we need to do is pump water to a place where we want it because those are
very far from seeps and springs or surface water bodies.
Let's look at this example of two wells adjacent to two homes that might be using these wells
for domestic water use.
You can see that the well on the left is a deep well that goes down through a couple
of layers.
The upper layer is a sand and gravel aquifer, then we have what's called a low-permeability
till which means that we end up with water not moving very fast through very tight materials.
Then we're into this low-yield fractured bedrock aquifer.
The well on the left is pulling water from a very deep location compared to the well
on the right.
The well on the right is shallower, it stays above the low-permeability till.
The water that is being accessed by the well on the right is from that near-surface aquifer
of sand and gravel.
One of the things that you can see is that the water table becomes deformed around that
well on the right.
The reason that we have a deformation in the water table is simply because we are pulling
water through the well and causing a local depression of the water table.
That influence slowly changes as we move away from the well.
Therefore, we end up with something called the cone of depression.
The cone of depression is to say, to indicate, that the water table would be at a higher
elevation if we didn't have this well in place that was pulling from a deeper location.
Let's go back to the well on the left.
You'll notice that there is no cone of depression in that well.
The reason for that is because there is a disconnect between that surficial aquifer,
the sand and gravel aquifer, and the one from which that well is drawing water which is
below the low-permeability till.
There's not a strong communication between the suction of water very deep and that very
surficial aquifer near the surface.
What if we have more than well and they're both in the same aquifer?
Essentially, we can think of the cone of depression of each one of those wells adding together.
In the top panel, what you can see is an indication of the influence of each one of those wells
by themselves.
But in reality, when we add those two effects together, we end up with something like the
profile in the lower panel which indicates that our cone of depression is a combined
cone of depression.
We end up coming from the left to the right with Well A, we have sort of that natural
draw down, it's actually exacerbated a bit by the fact that Well B is so close.
Between Well A and Well B we have a very depressed water table because both of htose wells are
sucking water from the bottom of those wells.
Then, we have a slow elevation change as we move back toward a more natural water table
toward the right of Well B.
We can imagine that having fields and fields of wells could provide a lot of depression
of the water table.
It could be in very complex configurations dictated by the spatial distribution of wells
at the surface.
We've been talking a lot about water table and where water comes from to contribute to
an aquifer, but when we put all this together, the storage in an aquifer, the amount of water
that we expect is available for use by pumping is dependent on how much water comes in and
how much water is pumped out.
We can think of this as a balance between recharge of water to the water table, to the
subsurface from precipitation, and withdraw from pumping.
If we think of it as a big underground storage location of water, then the volume of what's
stored is growing and shrinking based on the balance of these two, inputs and outputs,
essentially.
In this figure from a recent paper that was published in Nature, we see a comparison between
the aquifer areas and the groundwater footprints for use of water from those aquifers.
If we have a high or large aquifer footprint, but a small aquifer area, the authors are
indicating here that there is a significant groundwater stress compared to the reverse.
On the left-hand side, you can see this color scale indicates groundwater stress being the
ratio of the groundwater footprint, where it's being used, compared to the aquifer area.
For example, we see the western Mexico aquifer area and groundwater footprint, groundwater
footprint is quite a bit larger than the aquifer area, and therefore it has a high groundwater
stress with this red indication here.
The High Plains Aquifer, also known as the Ogallala Aquifer, that we'll talk about more
in a few minutes, is indicated in the second arrow here.
We see that it has somewhere between five and ten as the ratio of the groundwater footprint
to the actual area of the aquifer underneath.
Then, you can see some of these other ratios worldwide.
What we see on the upper right-hand side of this figure is a distribution of the many
aquifers that were evaluated and their groundwater stress levels from left being low to right
being very high, greater than twenty.