Water As One Resource: How Groundwater Interacts with Lakes and Streams

Water As One Resource: How Groundwater Interacts with Lakes and Streams


Good morning everyone. Well I’m going to
give a brief talk here about how groundwater interacts with lakes and
streams and so I’ll be covering some ground water basics, maybe basic for
many people but not everyone, a little bit about stream hydrogeology and then I’ll
finish up with how pumping affects pumping from wells affects streams and
lakes. I wanted to point out a couple of good publications, both from the US
Geological Survey, one of these is called groundwater and surface water a single
resource, and this makes the point that groundwater and surface water are really
quite intimately related and really need to be managed and understood as a single
resource and you can download this book at this website, and then a second
one is this recent publication circular 1376 from the US Geological Survey about
groundwater and streamflow depletion by wells and understanding and managing the
effects of groundwater and much of the information I think we’ll all be
presenting today is in these two these two books. So to start out what is
groundwater? Well that’s groundwater is is any of the water filling the pores
cracks fractures and other interstities of in geologic materials below the
land’s surface. We usually think of groundwater as being in the saturated
zone. So the saturated zone is the zone of the earth where all the pores and
spaces in the rocks or soil or sand and gravel are filled with water and the top
of that is called the water table. Above the water table we have the unsaturated
zone where the pores are only partially filled with water although there is
water in that in that area. We need to talk about a little bit about the water
cycle, remembering that all water begins as precipitation, snow or rain falling on
the land surface percolating or infiltrating into the ground. Much of
that water runs off becomes runoff and much of that is transpires or
evapotranspires back to the atmosphere. A smaller percentage percolates into the
ground and eventually comes crosses the water table and becomes part of
the groundwater flow system. Ground water moves through the groundwater flow
system through aquifers. Aquifers are geologic units that are permeable enough
to transport and transmit and store significant quantities of groundwater.
Some of this groundwater is eventually going to discharge to a usually a
surface water body like a lake or stream or wetland, some of it gets intercepted
and discharges into wells. Groundwater moves in three dimensions. This this
diagram is is most relevant for humid climates where the water table is a
subdued representation of the topography but we have higher water levels under
hills and lower water levels near the stream. If we start taking that apart, we
see the ground water moves laterally from these divides on the tops of water
table to the low points in the streams. It also moves vertically in flow paths
that that go down and then back up to discharge to a surface water point and
the divides in the groundwater flow system are called groundwater divides. So
groundwater moves in three dimensions. So let’s get to some streams. Groundwater is
important for streams and lakes because it often sustains streams, lakes and
wetlands and so the groundwater flows through the groundwater flow system, ends
up discharging to a surface water body. The slide, the picture on the right shows
that there’s often a zone called the hyporheic zone right under a stream or
lake where there’s a lot of biochemical and geochemical reactions go on. So a lot
of that’s a very active biological and chemical zone, a lot of transformations
and chemistry happen right there. I don’t have more time to talk about that today
but just be aware that there’s a lot of in addition to this flow there’s a lot
of chemical and biological interactions that go on between groundwater and
surface water. Streams can be gaining and losing, meaning that as streams flow
along in the upper left here, this is called a gaining stream because water
ground water is flowing into the stream and maintaining its flow and and really
increasing its flow. In contrast the stream on the lower left is a we call
that a losing stream because you can see the water table is basically below the
level of the stream or the river and so water leaks out of the stream into the
groundwater system and so that stream eventually loses water. And the diagram
on the right is just a plot of stream flow versus distance down the stream
showing if you went through a losing reach how you would get a change a
slight decrease in stream flow as you went through that losing part of the
stream or where maybe a well near the stream is causing it to be a losing
stream by pulling water out of it. Sometimes streams and lakes can become
disconnected from the groundwater system meaning that there’s a unsaturated zone –
the water table drops below the bottom of the surface water body and so we have
an unsaturated zone there. Often there’s a mound in the water table below that as
shown in the upper left diagram and there’s still water moving from the
stream to the groundwater system but it’s moving in the in the unsaturated
zone in an unsaturated way. The lower diagram shows where the system has
become completely disconnected. In fact the stream has become dry. So here’s a
here’s a very and this could be probably more common in that in the more arid
parts of the country. A little bit about lakes. Lakes are often, particularly here
in Wisconsin and humid parts of the country, outcrops of the water table.
In other words they’re the low point in the landscape and that’s where the water
table appears at the surface. Lakes can are dynamic systems. There’s a lot of
ground water exchange between lakes and the groundwater systems so there’s
usually three different possibilities as shown on the right upper, we have a lake
that’s that’s a where groundwater is discharging into the lake all the time
all around it. Second or middle diagram water is leaving the lake and the third
is what we call a flow-through lake water is groundwater is
flowing into one side of the lake and out the other side of the lake and some
of the some groundwater actually passes beneath the lake, and all these we see
all these situations pretty commonly in groundwater systems. A little bit about
base flow. Groundwater sustains streams like this trout stream shown on the
lower left by maintaining base flow. The groundwater discharge is the reason
that streams flow cool and steadily even during the hot parts of the year.
Here and here in Wisconsin we have a lot of great trout streams particularly in
the south west part of our state that flow all the time even in the middle
of the summer when it hasn’t rained for for days or weeks and that’s because
there’s a lot of groundwater discharge. Here’s a, the upper left here we have a
stream hydrograph with lots of peaks corresponding to various storms and so
forth but you can see that the base flow the black line is is what is being
sustained by groundwater. We can do things with with this sort of
information making a base flow index of looking at streams that have higher
percentage it’s a base flow versus this transient storm flow and try to map that
out. Here’s a map of Wisconsin and the bluer areas have higher total base flow
and then when we overlay that with where our trout streams are you can see how
important something like groundwater and base flow are to our trout fishing
industry. So it’s because the trout streams correspond to whether we have
these high base flow conditions. Now a little bit about how pumping
groundwater can affect streams. Pumping from an aquifer that’s near a stream or
a surface water body can cause two things: drawdown or lowering of water
levels near the well and a reduction of groundwater flow to nearby features, a
reduction of this natural discharge, and here is a fairly famous diagram just
showing a stream and an aquifer and a recharge area up here, ground water flowing
to the stream, and we put a well in there and the well has a little cone of
depression around it and you can see by the what the arrows are doing that that
well starts to diminish the flow to the stream and if we increase the pumping
from q1 to q2 here we even can reverse flow. So a
well near stream always is going to have some sort of impact on the stream and
and the amount of that impact will depend on the distance from the stream
and the amount of pumping from the well and so forth. I think Bill Alley will talk
a little bit more about that. Now a little bit about the water budget.
Remember that a water budget is kind of like a bank balance and everything has
to balance – all the inflows and outflows of water and hydrogeologists use these
days use mostly computer models to do this water budget accounting. In many
parts of the of the world when we think of a water budget deficit, that deficit
is going to be expressed as a decreased flow to our surface water features
lakes streams and wetlands because you don’t get something for nothing in the
groundwater world. You, anytime we pump a well and take water out of the ground
that water is water that would have ended up somewhere else usually in a
lake stream or wetland. And a scientist named Seward who’s a South African,
has illustrated this rather well I think with the concept of of the looking at
groundwater is sort of a bucket here where you have, think of this as an aquifer,
this diagram on the left, is full of water and we often see this with
aquifers. We have a, people have a whole lot of water in their aquifer and and
yet a little bit of pumping can can have a deleterious effect on ecological
resources and this is because not all that water is is really equally
available. Some of the water the very deep water is often what’s called
physically unabstractable here or it’s just hard or too expensive or
physically too difficult to get to. So even though the waters there, it’s hard
to use. And then we have water that’s maybe in the middle of the system here
that that we can pump out for use for human use or for agricultural use and
that’s water that can be consumed. But right at the top we have what’s often
called the ecological reserve. This is the water that would have been the first
water to discharge to a lake stream or wetland or spring and this and if we go
to the right and we start pumping that, we can see that
that’s the first water that gets depleted and so the first thing we see
if we start depleting an aquifer is decreased discharge to these surface
water features and that’s and that’s a can be a troubling thing. So there are a
few misconceptions and this is from that USGS circular about groundwater and
streamflow depletion. First the misconception is the total development
of groundwater resources from an aquifer system is safe or sustainable at rates
to the average rate of recharge. There was an old almost a myth
of that as long as you didn’t use more water than was recharged you were okay, but the problem
with that idea is that it doesn’t account for the water that’s needed for
to sustain ecology or for what’s called environmental services – the water that
sustains its base flow. So that needs to be accounted for. The second
misconception is that depletion is depending on the rate and direction of
water movement in the aquifer. That’s that’s really not true. It’s more the
decline in head or the decline in pressure in the aquifer is going to go
out in all directions. Third, depletion stops when pumping ceases. That’s not
true either and that’s because of the storage properties that I think Bill is
going to talk about in a minute, and finally if we have a confining unit or
an aquitard in our system there was some thought that pumping
below that aquitard is not going to it’s going to eliminate any any groundwater
depletion and that’s not completely true either. That’s, you may get some delay
in the depletion but it’s not going to eliminate it. And then so finally to
finish up here my main points today are that groundwater and surface water are
well connected and should be thought of as a single resource. We can’t really
manage them differently. All water comes from somewhere which seems obvious but
we need to remember that the water balance is critical for management
decisions and anything anytime you perturb the water balance you’re
changing something. Groundwater discharge becomes a base flow and sustains the
flow and streams and then lakes, and pumping from wells near streams can
reduce stream flow and these impacts depend on pumping rates and the local
hydrologic hydrogeologic setting. I think Bill and Thomas
will talk more about this. And that’s it for me Charlotte.

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